What Should Be Considered When Operating a Cyanide Leaching Plant Safely and Effectively?

The safe operation of a cyanide leaching plant to protect people and the environment and control the extraction process involves more than filling tanks. All aspects of delivering, storing and transferring cyanide and its control in the extraction process, including monitoring, maintenance, emergency procedures and training of personnel, must be clearly assigned to competent individuals at the site. The controls will be set by the site team, who are familiar with the ore, the process chemistry, the equipment and site requirements. This article does not describe how to use cyanide or attempt to explain why CIL recovery may be low; it sets out the operating framework.

Safe operation of a cyanide leaching plant and process control are interconnected. Any change in the condition of plant equipment or in the feed must be assessed against the site-approved procedures before any change is made to the plant settings.

What Makes a Cyanide Leaching Plant Safe to Operate?

How should a site manage cyanide-related hazards?

The full lifecycle of cyanide at a site should be managed from receipt to storage through to use in process, residue management and closure. Written procedures should identify who can use the reagent, what checks are required before use, how abnormal situations are reported and who is leading the emergency response. The International Cyanide Management Code is a recognized standard for safe transport and use of cyanide solutions, worker protection, training, emergency response and site operating controls. These principles should be translated into site-specific procedures as opposed to a number of generic checklists.

Controlling access to a site and communication with workers are as important as having the right equipment. Workers should receive appropriate instruction, clearly labeled equipment, up-to-date contact information, and be able to stop or isolate work when conditions are not covered by approved procedures. Emergency drills should test the site’s alarm, response team and communication systems. Unlike other incidents where workers may attempt to provide ad-hoc rescue or treatment, cyanide-related incidents are best managed by following a site plan that directs workers to trained responders and established medical treatment.

Responsibilities for response to an incident should be clearly visible in the cyanide leaching plant’s shift handovers, access controls and current emergency contacts, rather than be left to someone’s informal knowledge.

Why do layout and control procedures affect safe performance?

A gold leaching plant should be designed in a manner to allow for easy inspection. However, reagent use areas should be restricted to authorized personnel. Other items such as transfer points, drains, sumps, ventilation, instrumentation and emergency equipment should be considered in the overall plant design and reviewed accordingly. A safe operating procedure should detail how to acknowledge an alarm, check an instrument, and deal with a process deviation. Specific limits must be included in approved site documentation and reflect the actual process and relevant legislation.

Supporting controls through planned maintenance of the gold processing plant is essential. This includes scheduled inspection of pumps, valves, pipes, agitators, instruments, containment and other protective systems. By keeping track of repeat leaks, unusual readings or other deterioration of plant items before they cause an exposure or process shutdown, gold leach circuit maintenance is not just about keeping items of plant operational; it is about ensuring that the gold leach circuit is being operated within the defined limits and that adequate protective measures are in place.

Monitor whether measuring equipment is adequately calibrated and check whether isolation procedures have been updated to correspond with the current installed equipment following maintenance or modifications.

How Do Feed Preparation and Leaching Equipment Support Stable Operation?

What should be checked before slurry enters a leaching tank?

Feed to the leach circuit needs to be of consistent quality and therefore it is important that the preparation section is controlled to a degree to ensure that representative samples can be taken to assess changes in ore type, size, moisture, etc. and that the operating team has a clear understanding of the relevant testwork and process instructions to enable them to adjust reagent addition and other equipment settings as required. A small-scale gold leaching plant still needs to have adequate controls for its throughput and number of staff and reagent handling and emergency action plans are not optional just because it is a small plant.

In cyanide-based gold plants, processing of gold-bearing material typically involves grinding prior to leaching processing, and use of agitated tanks for slurry/reagent contact. If adsorption is included in the chosen process option, then carbon in slurry is separated using screens and the loaded carbon then taken to recovery section in the plant. These are process descriptions at plant stage, not a typical list of equipment to be purchased. Number and arrangement of tanks, plus other slurry-related parameters and control values, all require testwork and subsequent engineering for a specific ore type.

Gold circuits continue to adsorb on carbon after cyanide leaching, followed by desorption of loaded carbon, for example by electrolysis, and subsequent regeneration of the adsorption carbon and treatment of the residue from the leaching section. Thus, there are interfaces in a cyanide leaching plant which need to be defined for smooth operating handovers, but which must be kept separate from the production control interfaces in order to have suitable exposure controls.

Hongji Mine Machinery's Gold CIL/CIP Process provides a useful reference for these process interfaces. The described CIP flowsheet includes leaching pulp preparation, cyanide leaching, activated carbon adsorption, loaded-carbon desorption, gold recovery, carbon recycling and leached-pulp treatment. The solution also identifies ball mills, thickeners, leaching tanks and mixing tanks as relevant equipment. These stages highlight the importance of checking equipment condition, slurry transfer arrangements and maintenance access throughout the circuit.

What does leaching tank condition contribute?

Leaching tanks are designed to mix and support a leaching process; therefore they must also be mechanically robust and withstand the site conditions under which they operate. An inspection plan must cover the tank, tank lining where fitted, the agitator, drive, access, connections to other plant and instrumentation. The frequency for the inspections and their acceptance criteria should be set by suitably qualified people using the relevant design and site risk assessment and NOT off some frequency that has been copied from another mine.

Service compatibility must also be considered when selecting tanks for Leaching. Leaching tanks may be agitated using mechanical or air agitation, but there are many different processes, slurries and environments in which a gold leaching plant may be operating. Therefore, prior to any expansion or refurbishment of a gold leaching plant, the process basis, materials compatibility, required agitation, tank layout, isolation points and maintenance access must be confirmed. In this way, the plant can be operated productively and safely intervened with as necessary for inspection and maintenance.

Process-specific verification of equipment at a cyanide leaching plant, such as verification of the leaching tanks, verification of the transfer lines, verification of seals and verification of controls, as opposed to verification of generic labels.

Inspection Item	What to Check	Why Check This
The tank	Tank shell free of deformation, corrosion, leakage and cracks; welds and foundation in good condition	Tank failure can release cyanide slurry, causing personnel exposure and environmental contamination
Tank lining (where fitted)	Lining free of detachment, blistering, wear and perforation; joints intact	The lining isolates the slurry from the steel shell; damage accelerates corrosion and can lead to leakage
The agitator	Impeller free of wear, bending and material build-up; secure connection between impeller and shaft	Loss of agitation causes solids settling and lower leach recovery, and can lead to shaft seizure
The drive	Motor and gearbox noise and temperature normal; belts and couplings tight and free of abnormal noise; lubricant level correct	Drive failure stops agitation, and abnormal vibration can damage the tank structure
Access	Platforms, ladders, guards and manholes complete, clear and non-slip	Required for safe inspection and maintenance, and serves as the escape and rescue route in an emergency
Connections to other plant	Inlet and outlet pipes, valves, flanges and seals free of leaks; supports and flexible connections in sound condition	Connections are common leak points and form the key isolation interfaces between process sections
Instrumentation	Level, temperature and density instruments reading correctly; alarms and interlocks functional	Instrument readings are the basis for operating decisions and alarm response; failed instrumentation hides process deviations

How Can Hongji Mine Machinery Support Relevant Plant Selection?

Which equipment is relevant to the operating discussion?

At Hongji Mine Machinery, we supply gold-processing equipment, including leaching tanks and associated plant equipment. Our process discussions cover slurry preparation and agitation, leaching, and subsequent slurry handling. The project team should select equipment based on ore testwork, process requirements, material compatibility, plant layout, and site safety controls. Product descriptions do not replace a site-specific hazard review.

Case Study: 150t/d Gold CIL Processing Plant in Peru

• Case Background: A 150 t/d gold CIL processing plant built in Peru, covering the full leaching, adsorption and gold recovery circuit.
• Ore Type: Quartz vein gold ore.
• Capacity: Designed throughput of 150 tons per day.
• Key Equipment: Crushing equipment, ball mills, agitation leaching tanks, activated carbon adsorption equipment, carbon screens and a loaded-carbon desorption system.
• Installation and Commissioning: Installation and commissioning support was included to confirm that the installed plant matched the process design before operation began.
• Operator Training: Operator training and maintenance support were provided so that the site team could run and maintain the circuit in line with its operating requirements.
• Project Considerations: Equipment was selected and customized based on ore characteristics and processing capacity, illustrating the importance of matching equipment selection with operational requirements.

What should buyers verify before choosing a package?

In establishing a plant package, buyers should request their supplier to utilize information such as ore type, process test results, designed plant throughput, water and power supply information, as well as other site constraints to establish appropriate plant equipment and process interfaces. The final design should be scrutinized by qualified engineering and safety personnel. Condition assessments on existing plant circuits can establish whether individual items of plant require replacement or whether a wider review of the process and/or appropriate safeguards is required.

FAQ

Q: What is the first priority when operating a cyanide leaching plant?

A: A site-specific cyanide management system is established and followed by all personnel at a site, including handling by trained individuals, monitoring, maintenance, emergency response plans and site responsibility. Process conditions to be set and operated in accordance with approved design and operating practices.

Q: Does a smaller gold leaching plant need the same safety planning?

A small-scale gold leaching plant requires the same documented controls and trained people for these controls as a large plant. Although the equipment and the corresponding procedures might differ from one site to the other, assessment of reagent handling and exposure is required, as well as maintenance and emergency response.

Q: Can changing a leaching tank improve plant safety?

A: It may address a specific equipment limitation, but replacement alone does not establish safe operation. Review the tank duty, material compatibility, agitation, connections, isolation, monitoring, and access as part of the full cyanide leaching plant assessment. Hongji Mine Machinery recommends that buyers verify these requirements with the project’s process and safety specialists.