Authors Gerald Whittle and Jason Pan stated “The ReCYN process by GreenGold Technologies Pte Ltd is a world-leading approach in cyanide recovery, metal recovery and tailings detoxification. Based on an innovative resin-bead absorbant, ReCYN reduces cyanide consumption by 50%, capturing free cyanide from the plant tailings and recycling it back into the leach circuit while recovering metal complexes and making them available for sale. In the process, ReCYN detoxifies the tailings stream and guarantees 100%-compliant clean water discharge. ReCYN is efficient, economical and environmentally friendly. It is the only cost-saving innovation of its kind that is generating results today on an industrial scale.”

The Indonesian GreenGold subsidiary is doing ongoing engineering for the mine in relation to the technology. Read the full article…..

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There is no simple answer, but there is an answer.

The experience of the last 30 years has shown that any project can be accommodated using the RECYN Process for cyanide recovery and tailings detox. 

There are three parts to the evaluation process: free cyanide recovery, detoxification, and metal recovery:

• Cyanide recovery is simply an economical consideration, although often it will overlap with detox requirements.  The two commercial applications in Indonesia are based on free cyanide recovery; but they find this also satisfies detox requirements, resulting in substantial cost savings and recovery benefits. The capital cost is easily justified and usually results in payback in less than one year.  Economics are more favorable for larger projects and higher cyanide levels.
• Detox is a more complex evaluation because it involves detailed solution chemistry and compliance requirements. The same adsorption plant can be used for detox, but a metal recovery plant must be added. The evaluation generally splits into two areas, compliance for tailings dams and compliance for river discharge.  Either level can be accommodated with the RECYN process. The cyanide code level of <50ppm CN WAD for discharge to tailings dams is easily achieved, usually just by free cyanide recovery. For river discharge, the levels are more country specific, but can easily be accommodated with the RECYN process, usually by including a metal recovery stage.
• Metal recovery is required for the detox of tailings, but can be assessed on a stand-alone economic basis if there is no detox requirement.

A preliminary evaluation requires a few basic details to be inputted to an evaluation model that gives capital and operating costs and cost-benefit information. The assumption is that no capital benefit is derived from existing plant equipment.

The GreenGold website has a template for a preliminary assessment.  

Cyanide

Gold/silver ores rely on a good solubility for both metals, which is favored by high cyanide levels, particularly for silver.  Often cyanide levels are restricted to keep down consumption and detox costs, but this can also reduce metal recovery levels.  By eliminating detox costs cyanide levels can be increased to better suit metal recovery economics.

The effort to find a replacement for cyanide has persisted for over 100 years. The hope has been to find a less problematic solvent, which is more publically acceptable.

No acceptable alternative has been found, failing due to being less efficient, less economic, or not environmentally better. It’s often a combination of all three deficiencies.

We can more easily accept the superior efficiency of cyanide if we solve the environmental concern and turn its non-selectivity to an advantage.

An adjunct to the non-selective “problem” of cyanide is the search for a selective adsorbent.

Carbon is partially selective and has a fairly complex elution process.  The most common alternative is a gold-specific resin, but this is also not so applicable to gold/silver ores, or if copper is to be simultaneously recovered.

Resin is the best alternative if preg-robbing ores are processed. For most gold circuits, carbon is currently considered the best choice.  It has a proven performance record and robust treatment methods.

Having chosen a non-selective solvent and partially selective adsorbent we are left with the challenge of cleaning up what is left solubilized in the tailings.  Here the answer is more obvious, a non-selective resin.

The RECYN process not only absorbs all the solubilized metals, but has the added advantage of recovering free cyanide for reuse.

Cyanide was first used in the 19th Century and has not been replaced in the 21st Century. More than 50 years ago, resin was proposed as an adsorbent to reduce free and metal complexed cyanides from
gold plant tailings.

The author of this paper has been involved in the development of the resin technology for the last 30 years, initially with the Vitrokele process. This is a classic case of the challenges of technology commercialization. The answer has been persistence.

Vitrokele

A serious attempt at commercialization of a resin based system was undertaken by Tallon and Signet Engineering in the 80’s and early 90’s. The technology, known as Vitrokele, was focused on the replacement of carbon for metal recovery.

A major effort was applied to continuous piloting of the technology at mine site locations in Australia and Canada.  The results gave confidence in the process and chemistry prior to commercial application.

A total of four commercial applications were constructed over a five year period:

• The technology was first applied very successfully at Connemara in Zimbabwe on a gold heap leach project.  It was successful because the ore did not have any significantly soluble base metals.
• The second commercial application was at Avocet’s Penjom project in Malaysia.  Penjom had encountered significant preg-robbing ore, causing a dramatic reduction in gold recovery in their CIL circuit. The addition of kerosene overcame the pre-robbing but contaminated the carbon. Resin was not affected by kerosene and was therefore a good substitute for carbon. Signet converted the CIL circuit to an RIL circuit using Vitrokele resin. The resin proved impervious to kerosene, but not to copper. The copper occupied gold sites and substantially reduced the resin activity.  Penjom then changed to a gold-specific resin having been convinced that resin was a practical viable alternative.
• The third commercial project for Vitrokele was the MayDay project for Jason Mining.  This was a small vat leach project that also had high soluble copper levels. Many aspects of the technology were proven at this project, but copper again proved to be the downfall. There was an inadequate copper removal circuit, even though this was a standard part of the technology, proven at pilot plant level.  Again the focus was on gold recovery. The focus on metal recovery consequently had a major negative impact on Vitrokele technology.
• The fourth and final attempt at commercial application of the technology was in 1998 at the Mirah Gold/Silver Project in Indonesia.  In this instance the focus of the technology was reversed and cyanide recovery and detoxification became the main objective.  Carbon was retained as the primary metal recovery process. The introduction of a cyanide recovery process made the project economically viable, as it was previously marginal. Unfortunately for the technology, the Mirah Project was not commissioned as scheduled due to the 1998 Asian Financial Crisis. It was two months away from commissioning, but then sat in the Borneo jungle for the next 14 years.

These factors, combined with the untimely passing of two of the main protagonists of the technology, resulted in the Vitrokele technology being abandoned.

Alternatives

For the past 15 years, other research groups have persisted with resin based variants of the Vitrokele process, but no commercial process has eventuated. There has been a similar focus on finding alternatives to cyanide, in the hope they will be more environmentally acceptable.  Again nothing has emerged as a serious contender.

So what is the answer?

Simply accept cyanide as the superior solvent and address the concerns: Hence, the RECYN process.

The reason alternative solvents are widely sought is simply due to the negative connotations of cyanide in the public arena. The historical basis of the poor reputation is not helped by several catastrophic tailings dam failures in recent years.

This paper describes a process that can help change the negative perception associated with the use of cyanide and at the same time substantially improve the economics of its use. Known as the RECYN Process, it is a resin based technology that allows for the economic recovery of cyanide from gold/silver plant tailings. The process is also used to recover dissolved base metals and gold/silver cyanide complexes. The combination of recovery steps overcomes any need to further detoxify the tailings, changing a net cost to a profit. Gold projects incorporating the RECYN Process can reduce the purchase and shipment of cyanide by 50%, reduce the cost of cyanide usage and produce a fully decontaminated tailings from the process plant. These advantages, both economic and environmental, can help change the negativity associated with cyanide.

The core technology is not new, having been first published in South Africa in the 1950’s by Eric Goldblatt. It was reinvented in the late 80’s by a Canadian group called Tallon, and known as the Vitrokele Process. Several commercial operations based on gold recovery were constructed with varying success, until a combination of circumstances occurred in the mid 90’s to once again put the technology into hibernation.

Over the last twenty years there have been several attempts to duplicate the process with small modifications, but nothing commercial has resulted. Green Gold (GG) has successfully operated a commercial scale plant for the recovery and recycle of cyanide on a continuous basis for over two years. An average of 1t/day of NaCN has been recycled at a cost of 50% of new cyanide. There has been no further detoxification of the tailings to meet environmental compliance levels. A second project has been operating for 4 months recycling 1.5t/day.

Following its success at the Mirah project, the RECYN Technology has now been applied to a second project: the reborn Mt. Muro project which is also located in Central Kalimantan. This project has a similar mineralization to Mirah being high silver, low sulphidation, epithermal, narrow vein ore body.

Many of the lessons learned with the Mirah RECYN plant have been applied at Mt. Muro, which has a 50% larger throughput than Mirah, recycling 1.5t NaCN per day.

A note for the record: Part of the plant modification at Mt. Muro entailed converting the recovery circuit from Merrill-Crowe to carbon, in spite of +100g/t silver levels.  Another myth overturned.

Engineering companies are entrusted with the task of designing and constructing process plants reflecting the best available technologies.  In this capacity they have an advantage in persuading mining companies to implement new technology to enhance project economics.  Historically the risk was arguably borne by the engineering company, especially when undertaking lump sum projects with performance guarantees.  This was the case with the Signet’s Mirah gold/silver project.

Depressed gold prices in the early 90’s were not favourable for the Mirah Project, which was relatively low grade (2.3g/t Au), high silver (70g/t Ag), low Reserve (250koz) and high strip ratio (15:1).

The feasibility study showed a marginal project, which on balance was not attractive for funding.

By introducing a cyanide recovery process, along with economic detoxification, allowed the Mirah project to get across the line and become an economic proposition.

As mentioned above, the project was progressed to within two months of commissioning when it succumbed to the 1998 Asian Financial crisis.

Following a recovery of the gold price in mid-2000s the project was revisited.  Luckily, 10 years in the Borneo jungle had not had much effect on the plant and equipment.  Possibly a testament to the quality of construction and equipment selection.

The Mirah Gold/Silver project was finally commissioned in 2012, but the cyanide recovery section was not brought online until 2015.  Again, this demonstrates the focus on production over new technology.

The reason was the tailings were initially being discharged to a temporary storage facility and were not subject to the normal environmental compliance.  However when this requirement changed a destruction step was introduced using SMBS.  This proved expensive and destroyed not only the cyanide, but also the steel work.  The incentive to commission the resin plant became imperative.

The cyanide recycle plant was commissioned in early 2015 and has operated continuously ever since, recovering on average 1t/d of NaCN direct to the leach circuit.

No detox has been necessary since commissioning the cyanide recovery plant and the mine regularly have fishing competitions in the TSF discharge pond.  During a recent site visit a senior government official took a drink of the untreated TSF water to demonstrate his confidence.  It should be noted that no mine personnel followed his lead.

The Mirah project has an average level of soluble copper around 25-50ppm.  The compliance level for discharge to the TSF is <50ppm WAD Cyanide.  Although a metal recovery section is included as part of the resin plant, it has not been utilized after commissioning due to the low soluble copper levels.  Removal of free cyanide is sufficient to meet compliance levels.  A programme to recommission the metals section is now underway to test the base metal recovery efficiency with different eluants.

A major benefit of recycling cyanide has been the ability to increase levels in the leach to 1000ppm NaCN resulting in a substantial increase in silver recovery, up to 90%  Gold recovery averages 97%.

Classic advice states that a high silver ore requires a Merrill-Crowe recovery circuit.  This is attributed to the much higher carbon treatment levels for the high metal content.  Signet Engineering built several process plants, including the Mirah Project, using carbon for metal recovery where silver levels were over 100g/t.  The application has proven to be very successful and has shown increased silver recovery where preg-robbing occurs.