RZOLV Environmental Reprocessing of Legacy Leach Pads and Tailings

Duane Nelson
6 days ago
10 min read

Updated: 5 days ago

A Two-Stage Non-Cyanide Hydrometallurgical Approach for Reprocessing Legacy Cyanide Leach Pads and Tailings - Alkaline electrochemical cyanide reduction followed by low-pH RZOLV recovery of residual precious metals

Reza Kafaei, Hanif Jafari, and Duane Nelson

RZOLV Technologies Inc. Technical discussion draft, May 2026

Abstract

Legacy cyanide tailings, spent heap-leach pads, and previously leached precious-metal residues constitute a combined environmental liability and residual-metal resource. These materials can contain residual free cyanide, weak-acid-dissociable cyanide, strong metal-cyanide complexes, entrained process solution, dissolved metals, and unrecovered gold or silver. This paper reformulates a proposed RZOLV-based flowsheet as a scientific process hypothesis: residual cyanide risk is first reduced in an alkaline electrochemical prewash, and only after analytical verification is the material transitioned into a low-pH RZOLV leach environment for non-cyanide recovery of residual precious metals. The approach intentionally separates cyanide destruction chemistry from gold dissolution chemistry. Electrochemical oxidation is proposed under alkaline conditions to limit hydrogen cyanide generation while converting free and weakly complexed cyanide toward cyanate and downstream nitrogen species. The subsequent RZOLV stage is operated at approximately pH 1.5-2.5 under controlled oxidation-reduction potential to dissolve solution-accessible residual gold and stabilize it as a soluble complex, followed by activated carbon adsorption as the base-case recovery route. The paper defines the conceptual flowsheet, process-control rationale, validation endpoints, environmental measurements, and principal risks that must be resolved through representative bench and pilot testing. Predicted performance ranges are treated as hypotheses, not demonstrated commercial results, and should be validated feed-by-feed through cyanide speciation, hydrogen cyanide monitoring, leach kinetics, reagent consumption, recovery testing, toxicity assessment, and mass-balance closure.

Keywords

Legacy tailings; spent heap-leach pads; cyanide destruction; weak-acid-dissociable cyanide; electrochemical oxidation; non-cyanide gold leaching; RZOLV; activated carbon; mine closure; hydrometallurgy

1. Introduction

Cyanidation remains the dominant industrial method for gold and silver extraction because of its metallurgical effectiveness, operational familiarity, and compatibility with activated carbon adsorption, elution, and electrowinning. Its long-term use has also produced a large inventory of legacy cyanide-impacted materials, including tailings, spent heap-leach pads, ponds, process residues, and entrained solutions. These materials can remain subject to closure obligations because residual free cyanide, WAD cyanide, metal-cyanide complexes, soluble metals, acid-base behavior, and leachable constituents must be managed over time.

At the same time, many such residues contain residual precious metals. Remaining gold can be stranded for reasons that include incomplete solution contact, permeability constraints, copper interference, ore heterogeneity, short leach duration, poor liberation, or historical operating limitations. Reprocessing these materials with new cyanide may recover additional value, but it can extend the cyanide-management burden, introduce further detoxification requirements, increase permitting friction, and complicate closure planning.

This paper evaluates a conceptual two-stage alternative. The first stage is an alkaline electrochemical prewash intended to reduce residual cyanide risk before any acidification. The second stage is a controlled low-pH RZOLV leach intended to recover residual precious metals without adding new cyanide. The central hypothesis is that a staged, monitored, and closed-loop flowsheet may recover a portion of the remaining metal value while improving the environmental and closure profile of cyanide-impacted residues.

2. Process Hypothesis and Scientific Rationale

The proposed process is based on a strict separation of two incompatible objectives. Cyanide destruction or reduction should occur under alkaline conditions, where cyanide remains primarily as CN- and hydrogen cyanide gas risk is minimized. Gold dissolution by RZOLV is then conducted only after residual cyanide has been analytically reduced to a site-specific acceptable threshold and the material has been rinsed or drained.

The RZOLV stage is positioned as a controlled aqueous redox-complexation system rather than a simple reagent substitution. The leach is operated within a defined acidic pH-ORP window, approximately pH 1.5-2.5, where oxidation supports gold dissolution and ligand complexation stabilizes dissolved gold. This approach requires process control rather than maximum oxidation. Excessive ORP, uncontrolled acidification, or unmanaged impurity build-up could increase reagent consumption, reduce selectivity, or create safety and water-treatment challenges.

Figure 1. Conceptual two-stage flowsheet: alkaline electrochemical cyanide reduction followed by controlled low-pH RZOLV leaching, downstream recovery, recycle, washing, neutralization, and closure validation.

3. Conceptual Flowsheet

The flowsheet can be represented as ten linked unit operations: feed characterization, alkaline electrochemical prewash, prewash solution treatment/recycle, rinse and cyanide verification, controlled acidification, RZOLV leaching, downstream gold recovery, solution recycle or reconditioning, residue washing and neutralization, and final environmental validation. Each operation is governed by analytical endpoints rather than by residence time alone.

Stage	Primary objective	Critical measurements / controls
Feed characterization	Quantify recoverable value, cyanide inventory, impurity profile, and environmental risk.	Au, Ag, Cu, Fe, As, Zn, Pb, Hg; free/WAD/total cyanide; pH; alkalinity/acidity; moisture; mineralogy; leachability; permeability.
Alkaline electrochemical prewash	Reduce residual free and WAD cyanide before any low-pH processing.	pH >10-10.5; ORP; electrode performance; energy use; HCN monitoring; cyanate, ammonia, nitrate, nitrite, thiocyanate; dissolved metals.
Rinse and verification	Remove treated cyanide-bearing pore solution and confirm readiness for acidification.	Free/WAD/total cyanide; pH; ORP; HCN gas; dissolved metals; nitrogen species.
Controlled acidification	Transition material to the RZOLV operating window without generating material HCN risk.	Stepwise acid addition; pH 1.5-2.5 target; HCN monitoring; metal release; acid consumption.
RZOLV leaching	Dissolve residual solution-accessible gold and potentially silver without adding cyanide.	pH, ORP, reagent dose, oxidant moderation, time-series Au/Ag assays, ligand residuals, degradation products, impurities.
Activated carbon recovery	Recover dissolved gold from pregnant RZOLV solution using familiar gold-plant technology.	Carbon loading rate/capacity, barren solution assays, impurity co-loading, fouling, elution efficiency, EW/refining behavior.
Recycle and environmental treatment	Minimize discharge and manage reagent, metals, nitrogen species, and residual solution chemistry.	pH/ORP reconditioning, bleed treatment, precipitation/filtration/resin options, residue wash, neutralization, toxicity and leachability tests.

4. Proposed Experimental Program

Because performance is expected to be feed-specific, the proposed flowsheet should be evaluated through a staged experimental program. The program should begin with baseline characterization and proceed only after cyanide endpoints and safety controls have been demonstrated. The preferred experimental design is sequential: cyanide characterization, alkaline electrochemical treatment, rinse and pH-transition testing, RZOLV leaching, carbon recovery, solution recycle, residue neutralization, and environmental validation.

Phase	Tests	Expected output
1. Baseline characterization	Head assays; free, WAD, and total cyanide; pH, alkalinity, acidity; mineralogy; particle size; permeability; moisture; TCLP/SPLP or site-specific leachability.	Metal value, cyanide speciation, acid/base behavior, gold accessibility, and baseline environmental profile.
2. Electrochemical prewash	Alkaline wash kinetics; electrode screening; current density/energy evaluation; ORP profile; cyanide destruction kinetics; HCN monitoring; nitrogen-species tracking.	Cyanide reduction rate, best electrode configuration, byproduct profile, energy intensity, and safety basis.
3. Rinse and acidification safety	Rinse efficiency; stepwise pH reduction; HCN monitoring; metals released during pH transition.	Confirmation that low-pH treatment can be initiated only after cyanide risk is controlled.
4. RZOLV leaching	pH 1.5/2.0/2.5 comparison; ORP mapping; dose response; time-series extraction; residue assays; ligand residual/degradation analysis.	Leach kinetics, final extraction, reagent efficiency, and operating-window definition.
5. Recovery testing	Carbon loading; resin screening as backup; elution; electrowinning; impurity deportment.	Commercial recovery pathway, gold accounting, product quality, and carbon/resin/EW design inputs.
6. Environmental validation	Residue washing; neutralization; leachability; acute toxicity where relevant; neutralized water chemistry; full mass balance.	Closure pathway, discharge/reuse suitability, toxicity reduction, and environmental credibility.

5. Predicted Performance Metrics and Reporting Framework

No site-specific experimental results are asserted in this paper. The following ranges are hypothesis-level targets derived from the conceptual design and must be verified using representative residues. Results should be reported as mass-balanced analytical outcomes, with clear separation between cyanide reduction, precious-metal extraction, downstream recovery, and final residue stability.

Parameter	Hypothesis-level target / expected observation	Validation requirement
Free cyanide	90-99%+ reduction under optimized alkaline electrochemical prewash conditions.	Free cyanide analysis before/after treatment, including pore-solution and wash-water basis.
WAD cyanide	75-95%+ reduction, depending on complexed metals and residue history.	WAD cyanide speciation and endpoint confirmation before acidification.
Total cyanide	More variable; strong metal-cyanide complexes may persist.	Total cyanide, metal-cyanide speciation where possible, and polishing requirement.
HCN gas risk	Controlled by alkaline pH during cyanide treatment and by verification before acidification.	Continuous or frequent HCN monitoring during wash and pH-transition tests.
Residual gold recovery	Approximately 40-85% of remaining recoverable gold in favorable solution-accessible materials; lower in refractory or encapsulated residues.	Head/residue assays, solution assays, carbon assays, and reconciled mass balance.
Silver recovery	Potentially recoverable where silver is soluble under the RZOLV conditions.	Time-series Ag assays and residue deportment.
Carbon recovery	Activated carbon is the base-case recovery pathway for dissolved gold.	Loading rate, capacity, selectivity, fouling, elution, and repeated-cycle performance.
Solution recycle	Early testing target of 2-5 cycles with potential improvement after optimization.	Locked-cycle leach/recovery/recycle testing, impurity build-up, reagent makeup, and bleed management.
Residue closure profile	Potential reduction in cyanide inventory, recoverable metal inventory, and mobile metal risk after wash and neutralization.	Neutralized residue pH, leachability, toxicity, and long-term geochemical assessment.

Figure 2. Conceptual carbon-based recovery circuit: pregnant solution collection, carbon adsorption, loaded carbon handling, elution/electrowinning, recovered gold product, barren solution storage, and recycle/reconditioning.

6. Activated Carbon as the Base-Case Recovery Route

Activated carbon is proposed as the base-case downstream recovery route because it connects non-cyanide RZOLV dissolution to familiar gold-plant operating practice. The conceptual circuit comprises pregnant solution clarification, carbon adsorption in columns or contact tanks, loaded carbon handling, elution, electrowinning or refining, barren solution monitoring, and solution recycle or reconditioning.

Carbon adoption should not be assumed without testwork. Each residue and process solution should be evaluated for loading kinetics, ultimate capacity, impurity co-loading, organic or sulfur-species fouling, elution requirements, carbon regeneration or replacement frequency, and performance during repeated solution recycle. Resin or direct electrowinning may remain secondary options, particularly if carbon is inhibited by site-specific impurities, but activated carbon provides the strongest initial bridge to conventional heap-leach and gold-room infrastructure.

7. Risk Controls

The principal technical risk is unsafe acidification of cyanide-bearing material. The flowsheet therefore requires alkaline cyanide treatment and analytical verification before pH is lowered into the RZOLV operating window. Additional risks include incomplete cyanide destruction, persistence of strong metal-cyanide complexes, cyanate or ammonia byproduct formation, copper and iron build-up, RZOLV ligand degradation at improper ORP, recovery-media fouling, and residue leachability after treatment. Each risk has an associated control: endpoint-based cyanide testing, HCN monitoring, nitrogen-species analysis, impurity bleed or treatment, pH-ORP window control, carbon/resin cycle testing, residue washing, neutralization, and independent environmental validation.

8. Environmental and Closure Implications

If validated, the proposed process could reposition certain legacy cyanide residues from passive closure liabilities into controlled recovery-and-remediation opportunities. The environmental thesis is not that low-pH processing is inherently benign, but that a staged and monitored process may reduce residual cyanide risk, avoid the addition of new cyanide, recover stranded precious-metal value that could offset remediation costs, reduce residual recoverable metal inventory, and support a better-defined closure pathway through washing, neutralization, water treatment, and toxicity/leachability confirmation.

The strongest candidates are spent pads or tailings in which residual gold remains solution-accessible and historical cyanidation was limited by solution contact, permeability, copper interference, short leach duration, or operational inefficiency. Poor candidates include strongly refractory residues, materials with high acid-generation potential unless separately managed, or residues in which residual value is physically inaccessible without additional liberation or oxidation pretreatment.

9. Conclusions

A staged RZOLV environmental reprocessing flowsheet is scientifically plausible as a process hypothesis for selected legacy cyanide leach pads and tailings. The proposed sequence is: characterize the residue; reduce residual cyanide under alkaline electrochemical conditions; verify cyanide endpoints and HCN safety; rinse or drain; acidify under control to the RZOLV operating window; leach residual precious metals without adding cyanide; recover dissolved gold using activated carbon as the base-case route; recycle or treat solutions; wash, neutralize, and validate residues.

The concept's potential value is the combination of risk reduction and value recovery. It may reduce cyanide inventory before low-pH processing, avoid renewed cyanide use, recover a portion of stranded residual precious metals, and improve closure economics. However, all predicted benefits are site-specific and remain contingent on representative testwork. The critical proof points are cyanide destruction efficiency, HCN control during pH transition, RZOLV leach kinetics, reagent consumption, carbon recovery, recycle stability, residue leachability, toxicity reduction, and a closed mass balance.

Risk	Primary control
HCN generation during pH reduction	Maintain alkaline cyanide destruction first; verify free and WAD cyanide before acidification; use gas monitoring and engineered ventilation.
Incomplete cyanide destruction	Operate to analytical endpoints, not fixed treatment time; include polishing or extended wash treatment when required.
Persistent metal-cyanide complexes	Track total cyanide and dissolved metals; evaluate additional oxidation, precipitation, or water-treatment steps.
Byproduct formation	Monitor cyanate, ammonia, nitrate, nitrite, thiocyanate, total nitrogen, and toxicity.
RZOLV reagent consumption	Maintain pH-ORP window; monitor copper, iron, manganese, sulfur species, and oxidant demand; optimize dose and recycle.
Carbon fouling or poor loading	Conduct carbon loading, elution, regeneration, and recycle tests; evaluate resin or EW alternatives if required.
Residue leachability	Wash, neutralize, and validate using site-specific leachability and toxicity methods.

Regulatory and Technical Disclaimer

This paper is a technical discussion draft prepared from a conceptual RZOLV legacy-retreatment report. It does not constitute a feasibility study, preliminary economic assessment, engineering design, environmental impact assessment, permit application, closure plan, legal opinion, investment recommendation, or guarantee of technical or commercial success. Predicted ranges and target outcomes are illustrative process hypotheses only. RZOLV should not be characterized as a stand-alone cyanide-destruction reagent unless representative testwork confirms cyanide destruction kinetics, reaction pathways, residual cyanide species, byproducts, and safety performance under defined operating conditions. Any acidification of cyanide-bearing material requires analytical confirmation of residual cyanide status, HCN monitoring, containment, worker-safety procedures, regulatory review, and site-specific engineering controls.

References

International Cyanide Management Institute. International Cyanide Management Code for the Manufacture, Transport, and Use of Cyanide in the Production of Gold and Silver.
International Cyanide Management Institute. Mining Guidance: International Cyanide Management Code. June 2021.
U.S. Environmental Protection Agency. Treatment of Cyanide Heap Leaches and Tailings. Technical report.
Botz, M. M. Overview of Cyanide Treatment Methods. Mining Environmental Management, 2001.
Young, C. A., and Jordan, T. S. Cyanide Remediation: Current and Past Technologies. Proceedings of the 10th Annual Conference on Hazardous Waste Research.
Mudder, T. I., Botz, M. M., and Smith, A. Chemistry and Treatment of Cyanidation Wastes. Mining Journal Books.
Kitis, M., Akcil, A., Karakaya, E., and Yigit, N. O. Destruction of cyanide by hydrogen peroxide in tailings slurries from low-bearing sulphidic gold ores. Minerals Engineering, 2005.
Akcil, A. Destruction of cyanide in gold mill effluents: biological versus chemical treatments.
Comninellis, C., and Chen, G. Electrochemistry for the Environment. Springer.
Martínez-Huitle, C. A., and Ferro, S. Electrochemical oxidation of pollutants and boron-doped diamond electrodes.
Marsden, J., and House, I. The Chemistry of Gold Extraction. Society for Mining, Metallurgy & Exploration.
Adams, M. D., ed. Gold Ore Processing: Project Development and Operations. Elsevier.
van Zyl, D., Hutchison, I., and Kiel, J. Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects. SME.
Lottermoser, B. G. Mine Wastes: Characterization, Treatment and Environmental Impacts. Springer.
INAP. Global Acid Rock Drainage Guide. International Network for Acid Prevention.
APHA/AWWA/WEF. Standard Methods for the Examination of Water and Wastewater.
ASTM D2036. Standard Test Methods for Cyanides in Water.
U.S. EPA Method 1311. Toxicity Characteristic Leaching Procedure.
U.S. EPA Method 1312. Synthetic Precipitation Leaching Procedure.
Fleming, C. A. Literature on activated carbon adsorption in gold recovery, including CIP, CIL, and CIC circuits.
Davidson, R. J. Literature on gold adsorption onto activated carbon and factors controlling loading, selectivity, and elution.
AARL and Zadra elution process literature for stripping gold from activated carbon.
Aylmore, M. G. Alternative lixiviants to cyanide for leaching gold ores. In Gold Ore Processing.
Hilson, G., and Monhemius, A. J. Alternatives to cyanide in the gold mining industry: what prospects for the future? Journal of Cleaner Production.
Groenewald, T. Literature on thiourea leaching of gold, including chemistry, kinetics, and reagent consumption considerations.
Hiskey, J. B. Literature on thiourea and alternative lixiviants for gold extraction.
RZOLV Technologies Inc. RZOLV chemistry positioning materials and site-specific testwork reports, subject to appropriate technical verification.