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Pilot-Scale Assessment of a Mobile Off-Grid Membrane Contactor System for the Recovery of Cyanide from Gold Processing Wastewater
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  • Vincent Hammer
    Vincent Hammer
    Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
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  • David C. Vuono
    David C. Vuono
    Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
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  • Francisco D. Alejo-Zapata
    Francisco D. Alejo-Zapata
    Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
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    Orcidhttps://orcid.org/0000-0003-3151-9211
  • Julia Zea
    Julia Zea
    Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
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  • Héctor G. Bolaños-Sosa
    Héctor G. Bolaños-Sosa
    Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
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  • Carlos A. Zevallos-Rojas
    Carlos A. Zevallos-Rojas
    Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
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  • Linda A. Figueroa
    Linda A. Figueroa
    Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
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    Orcidhttps://orcid.org/0000-0003-3588-6705
  • Christopher Bellona
    Christopher Bellona
    Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
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    Orcidhttps://orcid.org/0000-0003-3366-3559
  • Johan Vanneste*
    Johan Vanneste
    Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-2888-3584
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ACS ES&T Water

Cite this: ACS EST Water 2024, 4, 12, 5811–5820
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https://doi.org/10.1021/acsestwater.4c00797
Published November 11, 2024

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Abstract

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A pilot-scale membrane contactor was constructed to determine the effectiveness of cyanide recovery from real gold processing effluents from the United States and the Arequipa region of Peru. The system was designed to operate off-grid using a mobile solar array to enable implementation independently of power-grid availability. Although a 98% recovery of free cyanide was obtained during experiments with the U.S. process effluents, effluents from the Arequipa region posed a larger challenge. Free cyanide recovery from the first Arequipan facility’s effluent (Aq1) yielded only a 45% cyanide recovery, while the effluent of the second Arequipan facility (Aq2) showed a free cyanide recovery of 81%. Precipitation of minerals was observed throughout experimentation with both effluents and likely contributed to the overall lower recovery. Maintaining a feed solution pH of >7 limited precipitations, albeit at a lower mass transfer rate. However, a subsequent pH drop in the feed solution to 5 yielded an improved cyanide recovery rate of 95% for the Aq2 effluent. Economic analysis revealed that operational and CAPEX costs for cyanide recovery in each case were lower than those for the purchase of new cyanide. However, pretreatment and staged pH adjustment may be required for the efficient recovery of cyanide by using membrane contactors.

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Synopsis

A pilot-scale membrane contactor was constructed to recover cyanide from complex gold processing effluents in Arequipa and assess its economic feasibility.

1. Introduction

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The gold mining industry of Arequipa, Peru, is a significant economic contributor and has undergone many historical changes, evolving from placer mining and mercury amalgamation to gold cyanidation as the most common method in the present day. (1,2) This shift facilitated the extraction of low-grade gold ores, expanding the gold industry and increasing its profitability. (2) While gold cyanidation offers efficient advantages over historically employed practices, the production of cyanide-laden effluents and tailings poses an environmental threat that needs to be addressed. (3,4) Current cyanide treatment methods such as alkaline chlorination and UV-enhanced oxidation are used by gold processing facilities to reduce cyanide concentrations in effluent waters to levels suitable for discharge or storage adhering to the international cyanide management code. (5,6) These commonly implemented cyanide treatment technologies are costly, offer no direct return on investment, and produce secondary contaminants such as cyanate, thiocyanate, and ammonia that may require further treatment. (6,7) A more sustainable and cost-effective solution lies in cyanide recovery and reuse. Technologies such as acidification–volatilization–reabsorption (AVR) and the Green Gold ReCyn process have achieved large-scale adoptions; however, high capital, operating, and maintenance costs limit widespread utilization. (8) Alternative cyanide recovery methods such as sulfidization–acidification–recycling–thickening (SART) and ion exchange demonstrate potential; however, few large-scale implementations have been observed. (9,10) Membrane contactors for cyanide recovery have been successfully demonstrated for treating industrial wastewater and mining effluents at the lab-scale. (11−13)
Membrane contactors utilize a porous and hydrophobic membrane to facilitate the transfer of HCN gas from acidified process water to an alkaline capture solution for subsequent reuse. (11) This transfer is driven by the vapor pressure difference created between the two separated liquid phases. Membrane contactors demonstrate cost-effectiveness through the use of low-cost materials and efficient usage of chemicals, and past studies have shown the efficacy of membrane contactors for cyanide recovery at bench- and small-scale applications. (11,14) Additionally, the high packing density of membranes facilitates process capacity and scalability, compared to AVR and SART, and technoeconomic analysis reveals that membrane contactors are competitive with other cyanide recovery technologies. (11,14) Pilot-scale cyanide membrane contactor studies have only been conducted for pesticide manufacturing wastewaters but have not been implemented in any pilot- or large-scale study in the gold processing industry. (15)
Volatilization has been identified as a major loss of cyanide in gold cyanidation facilities in South America, especially Arequipa. (16,17) Cyanide recovery addresses both environmental concerns and economic losses of cyanide volatilization by effectively treating and reusing cyanide after gold extraction. An ideal application of a membrane contactor for a cyanide recycling system would be immediately after the last adsorption tank to minimize cyanide volatilization in the tailings pond. However, due to the high solid content of the effluent, this is likely not possible. Another potential point of application is the recovery of free cyanide from the tailings pond after sedimentation has occurred. Lower amounts of solids in the tailings pond water would allow the use of filters to pretreat the water. Losses due to volatilization and tailings might still occur prior to treatment with a cyanide recycling system, but rapid treatment after sedimentation would keep these losses minimal. Implementation of such a system could decrease the amount of new cyanide that must be added during the milling step. Confirmation of an economic model predicting the profitability of the process using these types of mining water would solidify cyanide recovery as a viable alternative to regular cyanide treatment or nontreatment.
This study expands on successful bench-scale demonstrations of cyanide recovery from real mining effluents to evaluate and optimize cyanide recovery using a pilot-scale membrane contactor in the unique conditions encountered by Arequipa’s gold processing operations. (11,12,14) Gold cyanidation effluents in Arequipa are generally not discharged because of extensive tailings storage facility (TSF) pond water reuse. Driven by extensive reuse practices, effluents experience a concentrating effect, increasing conductivities and constituent concentrations. (17) Notably, these effluents exhibit high concentrations of total cyanide (free cyanide, weak acid dissociable (WAD), and strong acid-dissociable (SAD) cyanide complexes) due to high sodium cyanide dosing during gold cyanidation and the absence of active cyanide treatment methods. (17) The presence of high WAD cyanide complexes, SAD cyanide complexes, and metals within the concentrated effluents might influence the performance of gold cyanidation when recycled. (18) The removal of free cyanide from gold cyanidation effluents could benefit the reusability of cyanide by effectively removing undesirable constituents from the recycled solution. However, some metal–cyanide complexes and other constituents may precipitate under the low pH conditions (<pH 7) required for cyanide volatilization, potentially affecting cyanide mass transfer rates and recovery or creating operational challenges. (7,12,19)
To comprehensively assess the applicability of membrane contactors for cyanide recovery under various operating conditions, this study employed a range of water sources. The investigation utilized synthetic water solutions that were formulated to represent ideal operating conditions. Additionally, real mining effluents were introduced to evaluate the performance under more realistic conditions. The real effluents included one sample sourced from a mine in the United States (US) and two samples obtained from Arequipa, Peru. Operational parameters of the flow rate and cyanide concentration for the U.S. effluent were examined experimentally to identify process limitations. Experimental results were used for comparing the performance of the pilot-scale membrane contactor with that of a lab-scale system treating the same effluents. (14) Building upon these findings, a technoeconomic model was developed to assess the economic viability of membrane contactors under the specific conditions of the analyzed effluents and operating conditions.

2. Materials and Methods

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2.1. Evaluated Water Types

In total, five different waters, two synthetic and three mining effluents, were evaluated in this study. Synthetic water solutions were prepared in Colorado and Arequipa to create a performance baseline and validate the membrane integrity between transport and experimentation. Synthetic water solutions were prepared using tap water with a pH of 11.5 (using NaOH) and spiked with 195 mg/L of NaCN to achieve nominal free cyanide concentrations of ∼100 mg/L. Barren heap leaching solution (US) was obtained from a U.S. gold mine. Throughout storage, the concentration of free cyanide in barren water was reduced to below the detection levels. Therefore, NaCN was added to achieve nominal free cyanide concentrations of 100 and 1000 mg/L prior to experiments (US1 and US2, respectively). Barren solutions were obtained from two gold cyanidation plants in Arequipa, Peru. Site 1 (Aq1) is a medium-scale processing plant with an ore capacity of 350 t/day, which utilizes a carbon in pulp and electrowinning process, whereas Arequipa site 2 (Aq2) is a large-scale processing facility with an ore capacity of 1730 t/day that uses the Merril–Crowe process. After arrival at the research facility, the pH of the water was adjusted to 11.5 to avoid volatilization of free cyanide. Cyanide concentrations were measured prior to experimentation, and no additional NaCN was added to adjust cyanide concentrations. Relevant water characteristics at the time of arrival are presented in Table 1.
Table 1. Composition of Real Mining Wastewaters from the United States and Arequipa
 USAq1Aq2
pHa9.569.3411.34
component (mg/L)   
CN–44.48631532
WAD─CN– 267.3 ± 109.0573.3 ± 134.0
SAD─CN– 468.3 ± 106.7489.0 ± 213.1
total organic C95.8 ± 3.76688.8 ± 22.1681.8 ± 1.4
Cl–121.8 ± 2.94277.6 ± 4.668.3 ± 0.9
NO3–863.7 ± 66.46168.3 ± 2.735.3 ± 0.6
NO2–32.5 ± 2.41BDLb83.6 ± 0.3
SO42–698.1 ± 38.854931.6 ± 14.92537.4 ± 14.0
PO42–BDLbBDLbBDLb
Ca197.7 ± 0.7453.6 ± 1.4626.0 ± 2.4
S107.2 ± 2.621438.0 ± 18.21602.6 ± 16.2
Na86.9 ± 0.672085.5 ± 8.82091.5 ± 9.6
Sr10.6 ± 0.161.9 ± 0.33.9 ± 0
K9.3 ± 0.4733.4 ± 0.7193.4 ± 2.7
Si4.1 ± 0.165.0 ± 0.211.0 ± 0.2
Mo2.6 ± 0.050.7 ± 07.6 ± 0.1
Mg1.2 ± 0.020.8 ± 0.10.5 ± 0.0
Fe0.15 ± 076.0 ± 0.50.1 ± 0
Cu0.14 ± 0252.8 ± 3.1119.8 ± 0.6
Al0.1 ± 0BDLb0.2 ± 0.0
As0.064 ± 013.3 ± 0.20.3 ± 0
ZnBDLb58.6 ± 0.7530.4 ± 3.1
a

Measured pH before adjustment to 11.5.

b

Below the detection limit.

2.2. Sample Analysis

Free cyanide concentrations in the field were measured manually with a Hach Test ‘N Tube (TNT) 862 using the colorimetric pyridine barbituric acid method and a Hach DR 1900 field spectrometer. Total cyanide and weak acid-dissociable cyanide complex (WAD) concentrations were determined using an FS3700 automated chemistry analyzer from the OI Analytical Flow Water with 330092CT and 330090CT cartridges. The methods used for the analysis were ASTM D 7286-08 for total cyanide and ASTM 7511 for WAD. Elemental analysis of the waters was performed using inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Avio-500, PerkinElmer, Fremont, CA). Inorganic anions in the water samples were measured by ion chromatography (IC) (Dionex 1500, Sunnyvale, CA). Total organic carbon (TOC) was measured by using a TOC analyzer (Shimadzu, Columbia, MD). Precipitation samples were analyzed using a Hitachi TM-1000 Tabletop scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS).

2.3. Pilot-Scale Treatment System

The pilot-scale membrane contactor consists of several components. It is powered by a 3300 W solar array to enable off-grid and sustainable operation. Lightweight aluminum construction allows for the transportation of the solar array to remote locations. The solar system is further equipped with three 3.8 kWh lithium iron phosphate batteries to ensure treatment can continue when no sunlight is available.
The membrane contactor is designed to operate as a batch-style treatment system with interchangeable feed and distillate tanks for more flexible field operations (Figure 1). For the experiments done with this system, two 31 L tanks and two 220 L tanks were used. The feed tank was additionally connected to a 20 and 0.45 μm cartridge filter to stop any precipitants from entering the membrane module. Water from both tanks is circulated using rotary vane pumps. pH sensors and an automatic dosing system regulate and maintain the desired pH levels for cyanide transfer by dosing 5 M NaOH and 3.6 M sulfuric acid. Ultrafiltration was added as an optional pretreatment method to remove solids from entering the membrane contactor.

Figure 1

Figure 1. Pilot-scale membrane contactor: (1) ultrafiltration membrane, (2) feed solution pump, (3) distillate solution pump, (4) 20 and 0.45 μm cartridge filter, (5) UV lamp, (6) polypropylene (PP) membrane module, (7) sampling port, (8) solar array, and (9) mining effluent storage. Image of (8) and (9) was taken at a location in Arequipa.

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All chemicals used during experimentation in Colorado were obtained from Sigma-Aldrich (St. Louis, MO) and Rocky Mountain Reagents Inc. (Golden, CO). However, the chemicals used in Arequipa were of unknown manufacturer and purity grade.
The system employed a commercially available 3M Liqui-Cel EXF-8 × 20″ Industrial Series Membrane Contactor, with a polypropylene hollow fiber membrane. Membrane-specific parameters such as the number of fibers, thickness of membrane, and membrane surface area are considered proprietary information by the manufacturer and are not disclosed. However, for the purposes of calculating the overall mass transfer coefficient (K), a similar fiber packing density was assumed as the smaller 2.5 × 8″ 1.5 m2 module from the same manufacturer, which yields an estimated membrane surface area of 40 m2.

2.4. Data Analysis and Modeling

The overall mass transfer coefficient (K) was used as the performance metric to compare the cyanide transfer of the membrane contactor at different operational parameters and effluent compositions in this study. K is required for the calculation of cyanide flux and the required membrane size for the economic analysis (eqs 2, S2, and S3). K, expressed in m/h, was calculated from the linear regression of the natural logarithm of the normalized feed concentration plotted against the membrane area- and feed volume-normalized time axis using eq 1
K=VAtln([HCN]f,0[HCN]f)
(1)
where V is the volume of the feed (m3), A is the membrane area (m2), t is the time (h), [HCN]f,0 is the initial feed hydrogen cyanide concentration, and [HCN]f is the feed hydrogen cyanide concentration at time t. (20) Given variable cyanide concentrations across the analyzed effluents, the cyanide flux was used as a measurement of mass transfer. Cyanide flux (g/(m2 h)) was calculated as
JCN−=K([HCN]f−[HCN]d)
(2)
where [HCN]f and [HCN]d are the concentrations of volatile hydrogen cyanide (g/m3) in the feed and distillate, respectively. (20) As HACH TNT 862 vials measure only the dissolved cyanide [CN–], the total free cyanide concentration (CNfree) was calculated from the measured cyanide ion (CN–) concentration and measured pH using the Henderson–Hasselbach equation
[HCN]=[CN−]10pH−pKa
(3)
For calculations using eq 3, the pKa of hydrogen cyanide was assumed to be 9.21 at 20 °C. (7)
A sequential mass balance model was performed using the calculated K and measured pH values to model the cyanide concentrations in the system. Initial cyanide flux was calculated using eqs 1–3 based on the starting feed cyanide concentration and pH. In subsequent time steps, cyanide concentrations were calculated based on the measured cyanide removal and the assumption that the removed cyanide was completely transferred to the distillate. From these concentrations, a new flux was calculated and further iterated for each time step. The resulting data were used for the development of the cost model, which is dependent on the calculated flux and K values.

2.5. Economic Analysis

The economic model developed in this study was used to evaluate the economic viability of cyanide recovery using effluents from three mining and gold cyanidation facilities (see Table 1). Two cases of low (100 mg/L─CN–) and high (1000 mg/L─CN–) cyanide concentrations were considered for the U.S. effluent, whereas one case was considered for Aq1 and two cases for Aq2. The U.S. effluent scenarios assumed a large-scale processing facility with a representative effluent flow rate of 200 m3/h. Considering the Aq1 effluent originated from a medium-scale gold cyanidation facility in the Arequipa region, a representative effluent flow rate of 17.5 m3/h was assumed based on the ore processing capacity and previous research. (17) Lastly, an effluent flow rate of 86.5 m3/h was assumed for Aq2 based on the ore processing capacity of the facility. The modeled treatment system for all cases is a batch-style treatment system designed to sufficiently treat the effluent flows. If residual cyanide concentrations in the feed exceeded 50 mg/L following the cyanide removal process, a secondary treatment step in the form of chemical oxidation (SO2 + air) was assumed. This treatment was to reduce the remaining feed cyanide concentration to 50 mg/L in accordance with the international cyanide management code. (5) Given that most gold cyanidation facilities already possess infrastructure for traditional cyanide treatment, only operational costs for SO2 and air were considered.
Formulas and relevant parameters used in the economic analysis are summarized in Table S2 of the Supporting Information.

2.6. Synthetic Effluent Integrity Tests

Experiments to confirm the functionality and condition of the membrane contactor were carried out using tap water spiked with sodium cyanide. Owing to the limited availability of deionized (DI) water at the Arequipa field location, tap water was chosen as the primary solvent for baseline experiments. All baseline experiments were carried out under the same parameters with a nominal feed cyanide concentration of 100 mg/L CN–, a membrane flow rate of 10 L/min, a pH of 6 in the feed solution and 11.5 in the distillate solution, and using 30 L feed and distillate tanks. These differences in membrane performance can likely be explained by differences between baseline experiments carried out in Colorado and Arequipa and differences in the tap water composition, NaCN purity, H2SO42– purity, and NaOH purity. Furthermore, it is known that temperature influences the mass transfer resistances of cyanide recovery. (14,21) While experiments in Colorado took place in a temperature-controlled setting, those in Arequipa were performed outdoors without direct temperature control (Figure 1). Even small differences in temperature, water quality (e.g., turbidity), and chemical purity could have a cumulative effect to explain the observed differences in K values between the two locations. (12,17,19) While the system’s functionality was demonstrated in both the Colorado and the Arequipa synthetic water trials, observed performance variations necessitated the inclusion of baseline experiments between real mining effluent experiments. This approach served as a consistency check, ensuring that no membrane performance decline or damage had occurred during the experiments with real mining effluents or transportation.

3. Results and Discussion

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3.1. Cyanide Recovery from U.S. Mining Effluents

To evaluate the effect of system flow rates on treatment efficacy, U.S. process effluents were utilized for a series of experiments with varying flow rates using a feed volume of 30 L. Previous studies have shown increases in K values with increasing flow rates. (11,14,21) These results were replicated using the pilot-scale system: higher flow rates and the resulting liquid turbulence improve mass transfer and K (Figure 2) and subsequently result in shorter treatment times. Our results further indicate that the flow rate had a stronger impact on the membrane performance of low cyanide concentration waters than that of high cyanide concentration waters. The impact of higher cyanide concentrations on mass transfer coefficient (K) values appeared less pronounced at flow rates of 5 and 10 L/min. However, a more substantial difference in K values was observed at a flow rate of 15 LPM.

Figure 2

Figure 2. Influence of the flow rate on membrane coefficients using U.S. effluents at a CN– concentration of 100 and 1000 mg/L. Feed and distillate pH were 6 and 11.5, respectively.

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A previous study using a lab-scale membrane contactor showed a decrease in K with increasing cyanide concentrations. Experiments using the U.S. effluents at cyanide concentrations of 100 and 1000 mg/L showed a slight decrease in K with the higher concentration. (14) Cyanide flux increased proportionally, demonstrating that the membrane contactor maintained a consistent recovery across a range of cyanide concentrations (Figure 3). The calculated initial cyanide flux increased from 0.79 g/(m2 h) at 100 mg/L to 7.3 g/(m2 h) at 1000 mg/L. The near-constant K across the two cyanide concentrations indicates that the membrane was not saturated with cyanide during the experiments, and no increase in mass transfer resistance was noted. This suggests potential for further optimization as the membrane contactor’s maximum capacity for cyanide transfer (i.e., the maximum flux) was not saturated under the tested concentrations and operating conditions. A much stronger drop in the K value was observed with ammonia recovery with membrane contactors (33%) where the ammonia flux maxed out at 25 g/(m2 h) at a feed concentration of 2 g/L NH4+. (22)

Figure 3

Figure 3. Effect of the initial cyanide concentration on the overall mass transfer coefficient and initial cyanide flux using U.S. effluent at a flow rate of 10 L/min.

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Two additional experiments were conducted, utilizing larger 220 L tanks and a 10 L/min flow rate. These experiments were carried out at two feed cyanide concentrations of 143 and 580 mg/L to investigate the system’s response across an industrially relevant concentration range. Results show a near-complete recovery of free cyanide after both experiments, with 98% for 143 mg/L and 99% for 580 mg/L (Figure 4B). In contrast to the trends observed in previous studies, (14) where a decrease in K values with an increasing free cyanide concentration was identified, the experiment using water with an initial free cyanide concentration of 580 mg/L exhibited a similar K (0.0088 m/h) compared to the 143 mg/L solution (0.0077 m/h) (Figure 4A). The comparable mass transfer coefficients obtained from both large- and smaller-volume (Figure 3) experiments suggest that the membrane performance remains consistent across different scales of treatment volume and cyanide concentrations (average K of 0.0079 m/h ± 0.00032). This finding further supports the scalability of membrane contactors for the cyanide treatment and allows for the effective treatment of effluents from gold cyanidation facilities of varying capacities using this pilot-scale system. However, even more important is the fact that treatment times to achieve similar cyanide recoveries are almost independent of feed cyanide concentrations. This is because the cyanide flux scales with the feed concentration, but at higher concentrations also proportionally more cyanide needs to be removed, which leads to a similar treatment time at different cyanide concentrations. If a certain final feed concentration needs to be reached instead of a specific recovery, higher feed concentrations will obviously lead to higher treatment times. Baseline experiments using synthetic water resulted in an overall mass transfer coefficient of 0.0109 ± 0.0005 m/h throughout experimentation with the U.S. effluent.

Figure 4

Figure 4. Cyanide recovery using high-volume feed solutions (220 L) at 10 L/min showing K (A) and overall cyanide recovery (B). Feed and distillate pH were 6 and 11.5, respectively.

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3.2. Cyanide Recovery from Arequipa Mining Effluents

Prior to cyanide treatment using a membrane contactor, gold cyanidation effluents must undergo pH adjustment to enable cyanide transfer. This necessary step resulted in the formation of precipitates when applied to the effluents of aqueous Aq1 and Aq2. Both effluents are rich in copper and zinc concentrations (Table 1), which suggests that the resulting precipitation primarily consists of metal–cyanide complexes. Precipitation of metal–cyanide complexes at low pH values is well known and can even be an effective method of copper removal from gold cyanidation plants. (23,24) While precipitation of metal–cyanide complexes can be beneficial in some applications, it poses challenges for membrane-based cyanide recovery processes. The effects of precipitation from the effluents of Aq1 and Aq2 on the membrane performance are discussed in the following section.
Effluent received from Aq1 exhibited high turbidity upon arrival, necessitating pretreatment via ultrafiltration to avoid damage to the membrane contactor. To determine the effects of feedwater quality on membrane contactor performance, experiments with effluents from Arequipan gold processing plants were evaluated under baseline conditions (pH 6 feed, pH 11.5 distillate, and 10 L/min flow). These operating parameters resulted in a limited free cyanide recovery of 45% (Figure 5) and a correspondingly low K value of only 0.0016 m/h. This is almost 5 times lower than that with the U.S. mining effluent that only had a slightly higher feed concentration of 1000 mg/L compared to 863 mg/L (Figure 3). This suggests that effluents from Aq1 may not be suitable for treatment using membrane contactors at pH 6. An important reason for the lower performance is that, as the feed pH was lowered to 6, a red precipitation was observed. Accumulation of these precipitants within the cartridge filters increased the feed-side pressure and differential pressure across the membrane contactor. Pressure relief valves were installed beforehand to protect the membrane against pressure spikes; however, this increase remained within acceptable levels during experimentation. The amount of energy required to maintain the desired flow rate of 10 L/min also increased. These operational challenges potentially increase strain on pumps and necessitate more frequent cartridge filter replacements. Moreover, the cartridge filters probably were not able to remove all of the precipitates, hence the significantly lower mass transfer coefficient. (19,25)

Figure 5

Figure 5. Cyanide recovery using real mining effluents from Aq1 (pH 6 feed, pH 11.5 distillate, and 10 L/min flow).

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Mineral precipitation is expected to significantly impact membrane contactor performance and cyanide recovery. (19,25) Insoluble cyanide–metal complexes formed within the feed solution remove cyanide species from the liquid phase, potentially reducing the driving force for mass transfer across the membrane and affecting the overall cyanide recovery. Precipitate deposition at the entrance to the hollow fiber membranes obstructs fluid flow, increasing the cross-membrane pressure and the risk of causing membrane rupture. (26) Within the membrane module, precipitation can further lead to membrane fouling, (27) reducing its permeability and decreasing cyanide transfer. The resulting decline in cyanide recovery would necessitate more frequent cleaning or replacement of the membrane. (27) Furthermore, the formation of precipitates could have implications for downstream processing of these effluents, and the disposal of precipitate-laden wastewater potentially requires additional treatment to avoid environmental contamination. As a result, for this specific wastewater, operating at higher pH values of 7 or more to avoid precipitation is recommended. This will also decrease mass transfer rates and maximum recovery, but the impact could be less severe than that of precipitation, as is demonstrated below with the wastewater from Aq2. In general, a simple jar test can inform us at what pH precipitates start to form and the membrane contactor should be operated at a slightly higher pH.
The effluent used from Aq2 originated from a gold processing facility employing the Merrill–Crowe process. To achieve effective gold recovery through zinc dust cementation, the process fluid must be prefiltered extensively. As a result, unlike the effluent from site 1, this effluent allowed for direct processing in the membrane contactor without the need for additional ultrafiltration. However, the Merrill–Crowe process does add substantial amounts of zinc ions that can form insoluble cyanide complexes in the pH 5–7 range. (28) Indeed, precipitation was encountered during the experiment upon lowering the feed solution pH to 6. The extent of red precipitation observed with the Arequipa site 2 effluent was much lower compared with that from Aq1, with a resulting diminished degree of clogging within the cartridge filters observed. Consequently, the treatment process achieved a higher cyanide recovery of 81% with a K value of 0.0026, suggesting improved overall treatment performance compared to Aq1 (Figure 6). Despite a minor reduction in K during membrane integrity tests, experiments with the effluent of Aq2 did not show a clear declining trend in the K value.

Figure 6

Figure 6. Cyanide recovery using real mining effluents from the Merrill–Crowe process of Aq2. Three trials were performed for this effluent (pH 6 feed, pH 11.5 distillate, and 10 L/min flow), and membrane integrity was confirmed after each trial with a baseline experiment (pH 6 feed, pH 11.5 distillate, and 10 L/min flow).

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To circumvent precipitation issues observed at a feed pH of 6, an additional experiment was conducted at a feed pH of 7, which resulted in a lower K value of 0.0018 m/h compared to 0.0024 m/h at pH 6. Consequently, cyanide removal took longer at the higher pH due to the decreased mass transfer rate, reaching 81% recovery at 120 min instead of 90 min (Figure 7). However, this compromise minimized problems associated with precipitate formation and potential filter and membrane clogging. To maximize free cyanide removal, a 2-stage approach was implemented: following the initial recovery stage (pH 7), the feed pH was further reduced to 5 to recover any remaining free cyanide. By first removing free cyanide at a pH above which zinc cyanide precipitates do not form, the amount of precipitates that can form when dropping the pH below 7 afterward is minimized. The 2-stage pH adjustment process yielded indeed an improved recovery of 95%, achieving an almost complete recovery of the measured free cyanide. Lowering the pH to 5, although beneficial for capturing additional free cyanide, resulted in the formation of white precipitates, which could also negatively impact system performance (Figure S2).

Figure 7

Figure 7. Cyanide recovery using real mining effluents from the Merril–Crow process of Aq2 (feed pH 7, discharge pH 11.5, 10 L/min flow). Additional pH drop was performed after initial cyanide recovery had slowed. Subsequently, feed pH was lowered to pH 5 to achieve additional cyanide recovery.

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Samples of red and white precipitates, obtained from Aq2 effluent experiments at initial feed pH 6 and pH 7 with a second pH drop to 5 (henceforth called “2-stage pH drop”), respectively, were dried and subsequently analyzed through EDS. Unfortunately, standard EDS detectors have poor detectability of nitrogen species, so cyanide complex precipitates cannot be identified. (29) The primary elements in the initial Aq2 precipitant (feed pH 5) are Zn, C, and Na (Figure 8). The compositions of precipitants from Aq2 (2-stage pH drop) are notably different. Zn has increased, while O, Cu, and Ca emerged as new components. The observed increase in zinc and copper contents in the Aq2 (2-stage pH drop) precipitate suggests that the additional pH reduction increased the precipitation of metal species relative to other species. EDS is, however, not a quantitative technique, so it is possible that with Aq2 (2-stage pH drop), there may still have been less metal–cyanide precipitates by mass compared to Aq2, allowing a higher overall cyanide recovery. Jar tests using the Aq1 and Aq2 effluents at different pH values reveal similar amounts of precipitation between pH values 6 and 7 and the 2-stage pH drop for Aq2 and higher amounts of precipitation for Aq1 at pH 6 (Figure 8C). The 2-stage pH drop for Aq2 resulted in slightly lower precipitation mass compared to the single-stage pH adjustments to pH values 6 and 7. This suggests further potential benefits in terms of reduced filter clogging in addition to higher cyanide recovery. Furthermore, it appears that the process of cyanide recovery itself may influence the composition of precipitants as seen for Aq2 (2-stage pH drop). Overall, the sites from which Aq1 and Aq2 originated would have to expect an estimated 477 and 1470 kg of precipitants per day. Given the predominance of zinc in the precipitates, further investigation of the recovery of zinc as a byproduct of cyanide recovery could be considered. Further analysis is necessary to reveal the exact mechanisms behind the observed precipitations and to assess their influence on the gold cyanidation process.

Figure 8

Figure 8. Elemental composition of precipitates formed in the effluent of Aq2 at (A) pH 6 and (B) 2-stage pH drop first to pH 7 and then to 5. Data was normalized for elements ≥ Na. Panel (C) shows the precipitation mass of Aq1 and Aq2 effluents at relevant pH values.

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3.3. Economic Analysis

An economic analysis was performed to evaluate the cost-effectiveness of cyanide recovery for each of the analyzed gold cyanidation effluents. Additionally, free cyanide concentrations of 100 and 1000 mg/L were considered for the U.S. effluent (US1 and US2), while the 2-stage approach was considered for the Aq2 effluent (2-stage pH drop). Case-specific parameters, including processing capacity and treatment volume, were determined through direct communication with the facilities that provided effluents for this study and are listed in Tables S2 and S3. (14,17) Figure 9 presents the costs for the recovery of cyanide for each case. Recovery costs are expressed in U.S. dollars per kilogram ($/kg) of recovered cyanide (CN–) to facilitate comparison across each case and to the cost of new cyanide. For the cost of new cyanide, a value of $5/kg CN– was assumed consistent with past studies on cyanide recovery technologies. (14) However, cyanide costs can be highly volatile. Cyanide is synthesized through the Andrussow process from methane, oxygen, and ammonia; the latter in turn is derived from methane. As a result, increasing global natural gas prices will lead to an increase in the price of cyanide. Other chemical costs were obtained through vendor quotes, considering NaOH and H2SO4 as the primary choice for base and acid control according to present industry practices. Prefiltration, although required for Aq1, was not considered for this economic analysis to allow for a more consistent comparison of the recovery process costs of each case. Moreover, as demonstrated with Aq2, the pH adjustment strategy can greatly reduce precipitate formation and the overall cyanide recovery. Additional cyanide treatment costs were considered whenever residual cyanide remained in the feed solution after experimentation. Cyanide oxidation with SO2 and air was chosen as a representative traditional treatment method as it is a commonly implemented technology. Employing cyanide recovery also greatly reduces or eliminates the need for traditional cyanide treatment through oxidation, resulting in an additional estimated cost savings of $4/kg CN–. The value of recovered cyanide was estimated to be equivalent to new cyanide. The high pH of the distillate solution minimizes further cyanide losses during storage and facilitates the direct reuse of recovered cyanide in the gold cyanidation process.

Figure 9

Figure 9. Cost distribution of recovered cyanide for medium- and large-scale gold cyanidation facilities is explored in Section 3.2. The cost of new cyanide was estimated to be $5.0/kg CN in all cases based on industry quotes for gold processing plants of similar sizes. Additional treatment costs of a SO2/air cyanide oxidation system were applied whenever residual cyanide remained in the feed solution.

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Economic analysis revealed that of all investigated cases, Aq1 exhibited a nearly identical cost for recycled cyanide compared to the purchase of new cyanide. The highest cost for this scenario was identified as chemical dosing and additional treatment costs. Low cyanide flux with only a 45% cyanide recovery rate results in process inefficiencies that decrease the economic viability of cyanide contactors with waters from Aq1. Low flux and low recovery further increase the per-unit costs of acid dosing and membrane costs.
Results from all other cases indicate that cyanide recovery using membrane contactors is cost-effective compared to new cyanide. The Aq2 effluent incurred base dosing and additional treatment costs as the highest per-unit costs. Other costs, such as acid dosing and membrane CAPEX and replacement, are lower due to the amount of cyanide recovered. Additional treatment costs can be further reduced by implementing a 2-stage pH adjustment (Aq2 (2-stage-pH drop)) to increase cyanide recovery. Acid dosing constituted the primary cost for US1, driven by the substantial acid requirement to decrease feed solution pH relative to the low potential cyanide recovery, even if 99% was recovered due to the low feed concentrations of 100 mg/L. Membrane and pumping energy expenditures were likewise elevated compared to other scenarios. Despite incurring high per-unit costs, cyanide recovery in US1 remains economically viable compared to purchasing new cyanide. Lastly, US2 is the case with the lowest cost at only 27% of the cost of a new cyanide. High cyanide flux with high free cyanide concentrations in the feed allows for a very efficient transfer of cyanide and low per-unit costs. A sensitivity analysis was performed to evaluate the influence of the pump flow rate and feed cyanide concentration on the costs of cyanide recovery for the U.S. effluent. Similarly to results observed in the lab-scale study, (14) an increase in flow rate increased membrane performance (K) and cyanide flux (J). Improved cyanide flux allows for the use of smaller membrane areas, reducing CAPEX and membrane replacement costs. Costs through increased energy consumption through pumping are offset by these savings, indicating that operating the membrane contactor at higher flow rates is beneficial to the US1 effluent. Per-unit costs of cyanide recovery were reduced by 4.6% at 15 L/min compared to 10 L/min, whereas costs increased by 5.3% at 5 L/min (Figure S4). Furthermore, the sensitivity analysis demonstrated that the treatment of effluents with high cyanide concentrations and the resulting increase in cyanide flux generally benefits economic viability. While base dosing costs increased due to the rapid cyanide transfer, the reduced per-unit acid dosing, CAPEX, membrane replacement, and pumping costs offset this increase. A feed cyanide concentration of 600 mg/L was found as the optimal concentration, reducing per-unit costs by 60.1% compared with 100 mg/L but only 7.4% compared with 1000 mg/L (Figure S6).
Additional costs not directly considered in this economic analysis could significantly impact the overall economic viability of cyanide recovery, particularly for the effluent stream of Aq1. As the only effluent that requires pretreatment through ultrafiltration, overall operational expenses are expected to increase beyond the cost of cyanide recovery. However, much of past research has primarily focused on the intrinsic cost of the cyanide recovery process, allowing comparison between different cyanide recovery technologies and membrane contactors of previous studies (Table 2). While cyanide recovery using the pilot-scale membrane contactor is generally competitive with other cyanide recovery technologies, the system exhibited limitations when treating complex effluents like Aq1. Specifically, cyanide recovery using the US2, employing identical effluent and parameters to Hammer et al.’s case 1, achieved similar recovery costs. (14) This indicates that membrane contactors are a comparable alternative to AVR, SART, ion exchange, and the ReCyn process. Results of the 2-stage process for Aq2 further indicate that the treatment of complex effluents is economically viable through the implementation of staged feed pH reduction. This approach yielded comparable treatment costs to AVR and ReCyn and demonstrated better economic performance than ion exchange. AVR, SART, and the ReCyn process show lower recovery costs compared to Aq1, Aq2, and US1, suggesting that these methods might be more cost-effective compared to the membrane contactor used in this study. However, a more direct comparison between these technologies would necessitate the use of the same gold cyanidation effluent.
Table 2. Comparison of Lab- and Pilot-Scale Cyanide Recovery Technology Costs with Other Technologies
 methodcost of recycled cyanide ($/kg)cost of new cyanide ($/kg)recovery cost as % of new cyanide cost
this study (Aq1)membrane contactor (pilot)4.80596
this study (Aq2)membrane contactor (pilot)2.98560
this study (Aq2) (2-stage pH drop)membrane contactor (pilot)2.05541
this study (US1)membrane contactor (pilot)3.16563
this study (US2)membrane contactor (pilot)1.36527
Hammer et al. (14)membrane contactor (lab) (case 1)1.31–1.80526–36
Hammer et al. (14)membrane contactor (lab) (case 2)3.60–4.317.3449–59
Adams and Lloyd (30)AVR1.02–1.392.6439–53
Adams and Lloyd (30)SART0.60–0.732.6423–28
Fleming (31)ion exchange1.60–2.171.88–2.8257–115
Whittle and Pan (10)ReCyn2.50550

4. Conclusions

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Membrane contactors are a promising technology for the recovery of cyanide from diverse gold cyanidation effluents. This study successfully demonstrated cyanide recovery from complex effluents prevalent in the Arequipan gold processing industry using a pilot-scale membrane contactor. However, the treatment of such effluents has presented significant challenges. Turbidity of carbon in pulp effluents and precipitation of metal–cyanide complexes in the low pH feed solution reduce cyanide recovery and negatively affect membrane performance. These findings raise concerns about the economic viability of membrane contactors to recover cyanide from complex effluents. However, this study shows that a stepwise reduction in the feed pH can reduce precipitate formation and increase cyanide recovery, allowing for more cost-effective cyanide removal from complex effluents. Moreover, membrane contactors are a promising technology for cyanide recovery from cleaner effluents, such as those originating from the U.S. site or effluents from the Merril–Crowe process. These effluents exhibit higher cyanide recovery with higher membrane performance, leading to reduced operational costs. It was further discovered that similar treatment times can be expected independently of feed cyanide concentration. As such, membrane contactors, when optimized for effluent characteristics, are suitable candidates for cyanide treatment and are cost-competitive with other cyanide recovery technologies. Further research should confirm the long-term integrity of the membrane and investigate the treatment of complex gold cyanidation effluents and their precipitants.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestwater.4c00797.

  • Additional formulas, tables of assumed and calculated values, and sensitivity analysis figures used in the technoeconomic analysis (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Vincent Hammer - Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
    • David C. Vuono - Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
    • Francisco D. Alejo-Zapata - Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru;  Orcidhttps://orcid.org/0000-0003-3151-9211
    • Julia Zea - Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
    • Héctor G. Bolaños-Sosa - Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
    • Carlos A. Zevallos-Rojas - Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa, Arequipa 04000, Peru
    • Linda A. Figueroa - Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0003-3588-6705
    • Christopher Bellona - Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0003-3366-3559
  • Author Contributions

    CrediT: Vincent Hammer data curation, formal analysis, investigation, methodology, software, validation, visualization, writing - original draft; David Charles Vuono conceptualization, methodology, project administration, visualization, writing - review & editing; Francisco D. Alejo-Zapata funding acquisition, investigation, project administration, resources, supervision, validation, writing - review & editing; Julia Zea investigation, methodology, project administration, writing - review & editing; Carlos A. Zevallos-Rojas investigation, methodology, writing - review & editing; Hector G. Bolaños-Sosa investigation, resources; Linda A. Figueroa supervision, validation, writing - review & editing; Christopher Bellona project administration, supervision, writing - review & editing; Johan Vanneste conceptualization, funding acquisition, investigation, methodology, project administration, resources, software, supervision, validation, visualization, writing - original draft, writing - review & editing.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This project was funded by the Center for Mining Sustainability. This is a collaboration between the Universidad Nacional San Agustin and Colorado School of Mines.

References

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    Cyanide recovery by tailings washing and pond stripping
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    Minerals Engineering (2008), 21 (6), 501-508CODEN: MENGEB; ISSN:0892-6875. (Elsevier Ltd.)
    Modern gold producers are experiencing the operational dichotomy of minimizing wildlife exposure to toxic levels of cyanide in tailings and process waters, while decreasing operating costs for exploitation of lower-grade ores. The widely accepted tailings weak-acid dissociable (WAD) cyanide level of <50 mg/L is typically achieved by oxidn. of cyanide using reagents such as SO2/air, H2SO5 or H2O2. Consumption rates and costs of these processes can be substantial; moreover, destruction of the reagent that is often the single largest operating cost center for the process plant represents an addnl. economic downside. Tech. and economic aspects of an alternative process for achieving these eco-efficiency objectives with minimal addnl. hazardous chems. are considered and compared against other process alternatives for a selected scenario. The WPS (washing - pond stripping) process comprises tailings slurry washing using counter-current high-rate thickeners and pond or tank stripping of cyanide from the thickener overflow. Water balance issues are avoided by use of cyanide-stripped water along with any make-up water requirements in the washing stage. The cyanide-rich caustic scrub soln. is recycled to the leach. Metals and other species that may build up in plant process waters even in the absence of tailings thickening can be treated by a variety of conventional measures. The WPS process is based on proven unit operations and hence utilizes existing plant infrastructure and other resources wherever possible. Single tailings thickeners are used in some gold plants for the recycle of water, along with a portion of the cyanide and sol. gold losses; this equipment can be better utilized by washing with recycle water to raise recoveries to high levels. Moreover, removal of cyanide from the overflow water improves management of the water balance and reduces cyanide consumption in the mill. The existing process water pond or tank is augmented with a covering commonly used in the water treatment industry for biogas prodn. and odor removal. This low-pressure system provides a long retention time for the HCN stripping process, and the pos. effect of pond solar heating on the Henry's Law const. can also improve the thermodn. driving force for stripping. The covered pond eliminates wildlife deaths in this area of the site; by also venting the carbon-in-leach (CIL) tanks in-circuit cyanide losses are further minimized, resulting in lower lime consumption by operating at lower pH conditions. Scrubbing systems are increasingly being designed in to new and existing gold circuits for the removal of cyanide, ammonia, volatile org. compds. (VOCs) and mercury from carbon regeneration kiln and gold room off-gas streams. Moreover, the occupational health and safety issues regarding elevated HCN levels above leach tanks employing hypersaline waters may require venting and scrubbing systems to be introduced. The addnl. sol. gold that is recovered in the WPS process can be a significant economic driver along with the advantages of cyanide recovery over destruction. The environmental risk of ongoing cyanide tailings management is minimized, while providing economic benefits from the recovery of cyanide, water and gold.
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  • Abstract

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    Figure 1

    Figure 1. Pilot-scale membrane contactor: (1) ultrafiltration membrane, (2) feed solution pump, (3) distillate solution pump, (4) 20 and 0.45 μm cartridge filter, (5) UV lamp, (6) polypropylene (PP) membrane module, (7) sampling port, (8) solar array, and (9) mining effluent storage. Image of (8) and (9) was taken at a location in Arequipa.

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    Figure 2

    Figure 2. Influence of the flow rate on membrane coefficients using U.S. effluents at a CN– concentration of 100 and 1000 mg/L. Feed and distillate pH were 6 and 11.5, respectively.

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    Figure 3

    Figure 3. Effect of the initial cyanide concentration on the overall mass transfer coefficient and initial cyanide flux using U.S. effluent at a flow rate of 10 L/min.

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    Figure 4

    Figure 4. Cyanide recovery using high-volume feed solutions (220 L) at 10 L/min showing K (A) and overall cyanide recovery (B). Feed and distillate pH were 6 and 11.5, respectively.

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    Figure 5

    Figure 5. Cyanide recovery using real mining effluents from Aq1 (pH 6 feed, pH 11.5 distillate, and 10 L/min flow).

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    Figure 6

    Figure 6. Cyanide recovery using real mining effluents from the Merrill–Crowe process of Aq2. Three trials were performed for this effluent (pH 6 feed, pH 11.5 distillate, and 10 L/min flow), and membrane integrity was confirmed after each trial with a baseline experiment (pH 6 feed, pH 11.5 distillate, and 10 L/min flow).

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    Figure 7

    Figure 7. Cyanide recovery using real mining effluents from the Merril–Crow process of Aq2 (feed pH 7, discharge pH 11.5, 10 L/min flow). Additional pH drop was performed after initial cyanide recovery had slowed. Subsequently, feed pH was lowered to pH 5 to achieve additional cyanide recovery.

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    Figure 8

    Figure 8. Elemental composition of precipitates formed in the effluent of Aq2 at (A) pH 6 and (B) 2-stage pH drop first to pH 7 and then to 5. Data was normalized for elements ≥ Na. Panel (C) shows the precipitation mass of Aq1 and Aq2 effluents at relevant pH values.

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    Figure 9

    Figure 9. Cost distribution of recovered cyanide for medium- and large-scale gold cyanidation facilities is explored in Section 3.2. The cost of new cyanide was estimated to be $5.0/kg CN in all cases based on industry quotes for gold processing plants of similar sizes. Additional treatment costs of a SO2/air cyanide oxidation system were applied whenever residual cyanide remained in the feed solution.

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  • References

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      Cyanide has been widely used as an essential raw material in several industries including textile, plastics, paints, photog., electroplating, agriculture, food, medicine and mining/metallurgy. Because of its high affinity for gold and silver, cyanide is able to selectively leach these metals from ores. Cyanide and cyanide compds. in wastewater streams are regulated. Residues and wastewater streams contg. cyanide compds. have to be treated to reduce the concn. of total cyanide and free cyanide below the regulated limits. Natural degrdn. reactions can render cyanide non-toxic, resulting in carbon dioxide and nitrogen compds. These natural reactions have been utilized by the mining industry as the most common means of attenuating cyanide. However, the rate of natural degrdn. is largely dependent on environmental conditions and may not produce an effluent of desirable quality in all cases year round. Technologies that include chem., biol., electrochem. and photochem. methods have been developed to remove cyanide and cyanide compds. to below the regulated limits in wastewaters. This paper discusses com. available and emerging methods for removing cyanide from waste streams, particularly from tailings and tailings reclaim waters that are generated in the gold mining processes.
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      Minerals Engineering (2012), 25 (1), 1-13CODEN: MENGEB; ISSN:0892-6875. (Elsevier Ltd.)
      A review. Many gold producers are today processing gold ores contg. significant amt. of cyanide sol. copper. Typically, cyanide destruction is used to prevent the discharge of copper cyanide into tailings storage facilities. This imposes a significant financial cost to producers from the addnl. cyanide used to solubilize the copper and the cost of cyanide destruction reagents. Therefore, the recovery of copper as a valuable byproduct and the recycle of cyanide to the leach circuit have the potential for significant economic and environmental benefits. This includes enabling the treatment of gold ores with even higher sol. copper. Over the years, a variety of processes have been developed or proposed to recover the copper and/or cyanide including acidification based technologies such as AVR and SART, direct electrowinning, activated carbon, ion exchange resins, solvent extn., polychelating polymers, and membrane technologies. In this paper, these processes are critically reviewed and compared, with particular focus on the advantages and limitations, and the sepn. of copper from cyanide. Ultimately, there is no universal process soln. and the choice is highly dependent on the nature of the stream to be treated and integration with the whole processing plant.
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      Design and cost estimation of a gas-filled membrane absorption (GFMA) process as alternative for cyanide recovery in gold mining
      Estay, Humberto; Troncoso, Elizabeth; Romero, Julio
      Journal of Membrane Science (2014), 466 (), 253-264CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Cyanide recovery processes are currently an attractive technol. implemented in gold mining. The redn. of operational costs, along with minimization or avoidance of the use of expensive processes in destroying cyanide, are among the main reasons that might persuade the gold mining industry to adopt these technologies. The SART and AVR processes are the most frequently implemented in the past years, although both technologies have high requirements of footprint area, equipment sizes, and reagents consumption. The present work proposes a novel cyanide recovery technol. based on membrane absorption to recover cyanide in gold mining. This study reports the design and cost estn. for cyanide recovery by means of the gas-filled membrane absorption (GFMA) process, which was previously characterized by exptl. trials using a com. hollow fiber contactor, and the modeling of its mass transfer was also carried out. Furthermore, preliminary economic studies of the GFMA, AVR, and SART processes were done in order to evaluate the feasibility of the GFMA process. The results from this work suggest that the membrane absorption process could reduce the requirements of footprint area and energy consumption, yielding a net present value at least 35% higher than the AVR process and competitive with the SART process. Hence, the membrane absorption process here proposed is a viable and attractive alternative to current cyanide recovery processes, esp. under specific operation conditions, such as low copper concn. in solns. or limitations in flow configuration, where the implementation of the SART process becomes difficult.
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    12. 12
      Quilaqueo, M.; Seriche, G.; Valetto, S.; Barros, L.; Díaz-Quezada, S.; Ruby-Figueroa, R. An Experimental Study of Membrane Contactor Modules for Recovering Cyanide through a Gas Membrane Process. Membranes 2020, 10 (5), 105  DOI: 10.3390/membranes10050105
      12
      An experimental study of membrane contactor modules for recovering cyanide through a gas membrane process
      Quilaqueo, Michelle; Seriche, Gabriel; Valetto, Sicely; Barros, Lorena; Diaz-Quezada, Simon; Ruby-Figueroa, Rene; Troncoso, Elizabeth; Estay, Humberto
      Membranes (Basel, Switzerland) (2020), 10 (5), 105CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
      Cyanide is one of the main reagents used in gold mining that can be recovered to reduce operational costs. Gas membrane technol. is an attractive method for intensifying both the stripping and absorption processes of valuable compds., such as cyanide. However, scaling-up this technol. from lab. to industry is an unsolved challenge because it requires the improvement of the exptl. methodologies that replicate lab-scale results at a larger scale. With this purpose in mind, this study compares the performance of three different hollow fiber membrane contactor modules (1.7 x 5.5 Mini Module, 1.7 x 10 Mini Module, and 2.5 x 8 Extra Flow). These are used for recovering cyanide from aq. solns. at lab. scale, using identical operational conditions. For each exptl. set-up, mass-transfer correlations at the ranges of feed flows assayed were detd. The modules with the smallest and largest area of mass transfer reached similar cyanide recoveries (>95% at 60 min), which demonstrate the impact of module configuration on their operating performance. The results obtained here are limited for scaling-up the membrane module performance only because operating modules with the largest area results in a low Re no. This fact limits the extrapolation of results from the mass-transfer correlation.
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    13. 13
      Han, B.; Shen, Z.; Wickramasinghe, S. R. Cyanide removal from industrial wastewaters using gas membranes. J. Membr. Sci. 2005, 257 (1–2), 171– 181,  DOI: 10.1016/j.memsci.2004.06.064
      13
      Cyanide removal from industrial wastewaters using gas membranes
      Han, Binbing; Shen, Zhisong; Wickramasinghe, S. Ranil
      Journal of Membrane Science (2005), 257 (1-2), 171-181CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Results are presented for the removal of cyanide from four industrial wastewaters using hollow fiber gas membranes in a pilot plant. The plant was operated in batch mode using 1000 L of feed soln. The plant contained 10 hollow fiber modules with total membrane surface area of 180 m2. The strip stream consisted of 10% NaOH. The overall mass transfer coeff. for cyanide was detd. exptl. and found to agree well with predictions using empirical correlations. Both the feed and membrane mass transfer coeffs. contribute to the overall mass transfer coeff. The strip side mass transfer coeff. may be ignored. Real wastewaters often contain other volatile species. These volatile species will also transfer to the strip soln. If the volatile species does not react with the strip stream, all three individual mass transfer coeffs. contribute to the overall mass transfer coeff. Good agreement between the experientially detd. overall mass transfer coeff. and the predicted values was obtained for other volatile species. The osmolarity of a wastewater may be different to that of the strip soln. Consequently, H2O vapor transport due to osmotic distn. will occur from the lower to higher osmolarity soln. The effects of osmotic distn. should be considered when sizing the feed and strip tanks.
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    14. 14
      Hammer, V.; Vanneste, J.; Vuono, D. C.; Alejo-Zapata, F. D.; Polanco-Cornejo, H. G.; Zea, J. Membrane Contactors as a Cost-Effective Cyanide Recovery Technology for Sustainable Gold Mining. ACS ES&T Water 2023, 3, 1935,  DOI: 10.1021/acsestwater.3c00026
      There is no corresponding record for this reference.
    15. 15
      Shen, Z.; Han, B.; Wickramasinghe, R. Cyanide Removal from Wastewater Using Gas Membranes: Pilot–Scale Study. Water Environ. Res. 2004, 76 (1), 15– 22,  DOI: 10.2175/106143004X141537
      There is no corresponding record for this reference.
    16. 16
      Brüger, A.; Fafilek, G.; Restrepo, B. O. J.; Rojas-Mendoza, L. On the volatilisation and decomposition of cyanide contaminations from gold mining. Sci. Total Environ. 2018, 627, 1167– 1173,  DOI: 10.1016/j.scitotenv.2018.01.320
      There is no corresponding record for this reference.
    17. 17
      Hammer, V.; Vanneste, J.; Alejo-Zapata, F. D.; Zea, J.; Bolaños-Sosa, H. G.; Zevallos Rojas, C. A. Characterization of medium and small-scale gold processing operations, wastewaters, and tailings in the Arequipa region of Peru. Sci. Total Environ. 2024, 945, 174034  DOI: 10.1016/j.scitotenv.2024.174034
      There is no corresponding record for this reference.
    18. 18
      Bahrami, A.; Hosseini, M. R.; Razmi, K. An Investigation on Reusing Process Water in Gold Cyanidation. Mine Water Environ. 2007, 26 (3), 191– 194,  DOI: 10.1007/s10230-007-0001-9
      There is no corresponding record for this reference.
    19. 19
      Shen, Z.; Huang, J.; Qian, G. Recovery of cyanide from wastewater using gas-filled membrane absorption. Water Environ. Res. 1997, 69 (3), 363– 367,  DOI: 10.2175/106143097X125560
      There is no corresponding record for this reference.
    20. 20
      Huo, X.; Vanneste, J.; Cath, T. Y.; Strathmann, T. J. A hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery from nitrate-contaminated waste ion exchange brine. Water Res. 2020, 175, 115688  DOI: 10.1016/j.watres.2020.115688
      20
      A hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery from nitrate-contaminated waste ion exchange brine
      Huo, Xiangchen; Vanneste, Johan; Cath, Tzahi Y.; Strathmann, Timothy J.
      Water Research (2020), 175 (), 115688CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Ion exchange is widely used to treat nitrate-contaminated groundwater, but high salt usage for resin regeneration and management of waste brine residuals increase treatment costs and add environmental burdens. Development of palladium-based catalytic nitrate treatment systems for brine treatment and reuse has showed promising activity for nitrate redn. and selectivity towards the N2 over the alternative product ammonia, but this strategy overlooks the potential value of nitrogen resources. Here, we evaluated a hybrid catalytic hydrogenation/membrane distn. process for nitrogen resource recovery during treatment and reuse of nitrate-contaminated waste ion exchange brines. In the first step of the hybrid process, a Ru/C catalyst with high selectivity towards ammonia was found to be effective for nitrate hydrogenation under conditions representative of waste brines, including expected salt buildup that would occur with repeated brine reuse cycles. The apparent rate consts. normalized to metal mass (0.30 ± 0.03 mM min-1 g-1Ru under baseline condition) were comparable to the state-of-the-art bimetallic Pd catalyst. In the second stage of the hybrid process, membrane distn. was applied to recover the ammonia product from the brine matrix, capturing nitrogen as ammonium sulfate, a com. fertilizer product. Soln. pH significantly influenced the rate of ammonia mass transfer through the gas-permeable membrane by controlling the fraction of free ammonia species (NH3) present in the soln. The rate of ammonia recovery was not affected by increasing salt levels in the brine, indicating the feasibility of membrane distn. for recovering ammonia over repeated reuse cycles. Finally, high rates of nitrate hydrogenation (apparent rate const. 1.80 ± 0.04 mM min-1 g-1Ru) and ammonia recovery (overall mass transfer coeff. 0.20 m h-1) with the hybrid treatment process were demonstrated when treating a real waste ion exchange brine obtained from a drinking water utility. These findings introduce an innovative strategy for recycling waste ion exchange brine while simultaneously recovering potentially valuable nitrogen resources when treating contaminated groundwater.
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    21. 21
      Estay, H.; Ortiz, M.; Romero, J. A novel process based on gas filled membrane absorption to recover cyanide in gold mining. Hydrometallurgy 2013, 134–135, 166– 176,  DOI: 10.1016/j.hydromet.2013.02.012
      21
      A novel process based on gas filled membrane absorption to recover cyanide in gold mining
      Estay, Humberto; Ortiz, Miguel; Romero, Julio
      Hydrometallurgy (2013), 134-135 (), 166-176CODEN: HYDRDA; ISSN:0304-386X. (Elsevier B.V.)
      In the last ten years, the operational costs in the gold and silver cyanidation processes have increased, fundamentally, due to the treatment of ores which contains high cyanide sol. metals, such as zinc, copper, and nickel, among others, increasing the cyanide consumption along with the increase in cyanide price. Addnl., the high cyanide consumptions in gold operations have increased the cyanide contents in leach tailings, forcing the inclusion of processes to recover or eliminate cyanide. For these reasons, a membrane contactor operation to recover cyanide from gold mining, using gas filled membrane absorption is proposed in this study, performing lab. tests in order to det. the throughput of this process and the most relevant parameters, which affect the cyanide recovery. Gas Filled Membrane Absorption (GFMA) process can ext. cyanide, using an absorption soln. of NaOH. In this process, previously reported for wastewater treatment, a hydrophobic membrane with its pores filled with air separates an aq. stream contg. cyanide and receives a soln. with a high pH value. These aq. solns. cannot penetrate into the membrane pores, promoting the cyanide transfer from the cyanidation soln. to the receiving soln. The exptl. results show cyanide recoveries higher than 90% in extn. times of approx. 10 min, achieving an av. cyanide transfer rate of 0.01 kg m-2 h-1 and identifying the pH, feed flow rate and copper concn. as the most influential parameters on the performance of the process. Furthermore, a phenomenol. transport model was developed in order to explain the crit. steps of the cyanide transfer in the GFMA process. This model was validated by means of the comparison with the exptl. results of cyanide extn. with the presence of Cu(I) and Zn(II) ions, obtaining a statistical error lower than 10%. Thus, the gas filled membrane absorption process can recover cyanide, with similar performance to the current cyanide recovery processes, and could involve a smaller size of the equipment due to the high transfer surface area per vol. that membrane contactors have.
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    22. 22
      Schwiebert, A. M.; Bush, J. A.; Bellona, C.; Vanneste, J.; Cath, T. Y. Membrane Contactors for Ammonia Recovery from Anaerobic Digester Centrate: Pretreatment and Process Optimization. ACS ES&T Water 2024, 4, 3284,  DOI: 10.1021/acsestwater.4c00162
      There is no corresponding record for this reference.
    23. 23
      Alonso-González, O.; Nava-Alonso, F.; Uribe-Salas, A. Copper removal from cyanide solutions by acidification. Miner. Eng. 2009, 22 (4), 324– 329,  DOI: 10.1016/j.mineng.2008.09.004
      There is no corresponding record for this reference.
    24. 24
      Leaver, E. S.; Woolf, J. A. Copper and Zinc in Cyanidation Sulphide-Acid Precipitation, Technical Paper 494; US Department of Commerce, 1931.
      There is no corresponding record for this reference.
    25. 25
      Kenfield, C. F.; Qin, R.; Semmens, M. J.; Cussler, E. L. Cyanide recovery across hollow fiber gas membranes. Environ. Sci. Technol. 1988, 22 (10), 1151– 1155,  DOI: 10.1021/es00175a003
      There is no corresponding record for this reference.
    26. 26
      Prasad, R.; Sirkar, K. K. Hollow fiber solvent extraction of pharmaceutical products: A case study. J. Membr. Sci. 1989, 47 (3), 235– 259,  DOI: 10.1016/S0376-7388(00)83078-1
      26
      Hollow fiber solvent extraction of pharmaceutical products: a case study
      Prasad, R.; Sirkar, K. K.
      Journal of Membrane Science (1989), 47 (3), 235-59CODEN: JMESDO; ISSN:0376-7388.
      Dispersion-free solvent extn. using Celgard microporous hydrophobic hollow fibers and flat membranes was utilized for extn./purifn. of fermn.-based pharmaceutical products. Extn. as well as back extn. was studied using a pH swing procedure. Problems of emulsion formation, inherent in current processes, were avoided to obtain stable dispersion-free operation. Very high solute recoveries and mass transfer rates were obtained in the hollow fiber devices. Modular plant design using a series-parallel arrangement of this type of extractors and cost of existing dispersion-based devices indicate that these novel devices can compete effectively with com. extractors.
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    27. 27
      Han, B.; Shen, Z.; Wickramasinghe, S. R. Fouling and Cleaning of Gas-Filled Membranes for Cyanide Removal. Sep. Sci. Technol. 2005, 40 (6), 1169– 1189,  DOI: 10.1081/SS-200053315
      There is no corresponding record for this reference.
    28. 28
      Hampton, A. P. Cyanide Recovery in CCD-Merrill Crowe Circuit: Pilot Testwork’ of Cyanisorb Process at Nerco Delamar Silver Mine [Internet]. [cited 2024 Aug 8]. Available from: https://www.idahogeology.org/Uploads/Data/MineDocs/BO0100_015.pdf.
      There is no corresponding record for this reference.
    29. 29
      Buck, E. Electron Microscopy Characterization of Suspended Solids from Hanford Tank 241-AP-105 Direct Feed Waste [Internet], 2017 Nov [cited 2024 Aug 8] p. PNNL--27065, 1598866. Report No.: PNNL--27065, 1598866. Available from: https://www.osti.gov/servlets/purl/1598866/.
      There is no corresponding record for this reference.
    30. 30
      Adams, M.; Lloyd, V. Cyanide recovery by tailings washing and pond stripping. Miner. Eng. 2008, 21 (6), 501– 508,  DOI: 10.1016/j.mineng.2008.02.015
      30
      Cyanide recovery by tailings washing and pond stripping
      Adams, Mike; Lloyd, Vincent
      Minerals Engineering (2008), 21 (6), 501-508CODEN: MENGEB; ISSN:0892-6875. (Elsevier Ltd.)
      Modern gold producers are experiencing the operational dichotomy of minimizing wildlife exposure to toxic levels of cyanide in tailings and process waters, while decreasing operating costs for exploitation of lower-grade ores. The widely accepted tailings weak-acid dissociable (WAD) cyanide level of <50 mg/L is typically achieved by oxidn. of cyanide using reagents such as SO2/air, H2SO5 or H2O2. Consumption rates and costs of these processes can be substantial; moreover, destruction of the reagent that is often the single largest operating cost center for the process plant represents an addnl. economic downside. Tech. and economic aspects of an alternative process for achieving these eco-efficiency objectives with minimal addnl. hazardous chems. are considered and compared against other process alternatives for a selected scenario. The WPS (washing - pond stripping) process comprises tailings slurry washing using counter-current high-rate thickeners and pond or tank stripping of cyanide from the thickener overflow. Water balance issues are avoided by use of cyanide-stripped water along with any make-up water requirements in the washing stage. The cyanide-rich caustic scrub soln. is recycled to the leach. Metals and other species that may build up in plant process waters even in the absence of tailings thickening can be treated by a variety of conventional measures. The WPS process is based on proven unit operations and hence utilizes existing plant infrastructure and other resources wherever possible. Single tailings thickeners are used in some gold plants for the recycle of water, along with a portion of the cyanide and sol. gold losses; this equipment can be better utilized by washing with recycle water to raise recoveries to high levels. Moreover, removal of cyanide from the overflow water improves management of the water balance and reduces cyanide consumption in the mill. The existing process water pond or tank is augmented with a covering commonly used in the water treatment industry for biogas prodn. and odor removal. This low-pressure system provides a long retention time for the HCN stripping process, and the pos. effect of pond solar heating on the Henry's Law const. can also improve the thermodn. driving force for stripping. The covered pond eliminates wildlife deaths in this area of the site; by also venting the carbon-in-leach (CIL) tanks in-circuit cyanide losses are further minimized, resulting in lower lime consumption by operating at lower pH conditions. Scrubbing systems are increasingly being designed in to new and existing gold circuits for the removal of cyanide, ammonia, volatile org. compds. (VOCs) and mercury from carbon regeneration kiln and gold room off-gas streams. Moreover, the occupational health and safety issues regarding elevated HCN levels above leach tanks employing hypersaline waters may require venting and scrubbing systems to be introduced. The addnl. sol. gold that is recovered in the WPS process can be a significant economic driver along with the advantages of cyanide recovery over destruction. The environmental risk of ongoing cyanide tailings management is minimized, while providing economic benefits from the recovery of cyanide, water and gold.
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    31. 31
      Fleming, C. A. Cyanide Recovery. In Gold Ore Processing [Internet]; Elsevier; 2016 [cited 2024 Jul 25]. pp 647– 661. Available from: https://linkinghub.elsevier.com/retrieve/pii/B9780444636584000360.
      There is no corresponding record for this reference.

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