San Albino Project — Process Design Report (Report No. 300)

This review details the process design for the 250 tonne per day gold recovery plant at the San Albino Project, based on the technical report .

The San Albino processing plant is designed as a 250 tonne per day carbon-in-leach (CIL) gold recovery plant. The process design is based on a combination of a gravity circuit and CIL technology. The flow sheet encompasses primary, secondary, and tertiary crushing, followed by ball milling, a gravity recovery stage, a six-tank CIL circuit, and subsequent gold extraction from loaded carbon via acid washing, stripping, and electro-winning. Tailings are dewatered using drum filters, treated for cyanide destruction using the INCO system, and then transported to a dry stack area.

Critical Data

Overview

The San Albino processing plant is a conventional gold recovery facility designed to treat mineralized material at a nominal rate of 250 tonnes per day. The process design incorporates primary, secondary, and tertiary crushing stages to reduce the ore to a suitable feed size for the ball mill. The milling circuit is paired with a gravity recovery unit for the capture of free gold, and the resulting slurry is processed through a CIL circuit with an approximate 30-hour residence time. The design includes reagent preparation areas for cyanide and caustic soda, a carbon stripping and electro-winning section for gold recovery, and a tailings treatment system based on the INCO cyanide destruction method.

Key Process Stages

The plant design begins with mineralized feed receiving, where run of mine material is dumped into a hopper fitted with a stationary grizzly. A variable speed apron feeder draws material at a nominal rate of 50 tonnes per hour into the primary jaw crusher, producing a minus 100 mm product. The crushed material is screened, with oversize recirculated and undersize transferred to a crushed material stockpile with 7 days of storage capacity.

Secondary and tertiary crushing stages are implemented to further reduce the material. The secondary jaw crusher discharges into a screening station that classifies material for the tertiary cone crusher. Material sized to -25 mm is fed to the tertiary crusher, which reduces it to a P80 of 6.3 mm for the mill feed hopper.

The milling and gravity circuit uses a variable speed belt feeder drawing material at a nominal 10.4 tonnes per hour into the ball mill. The mill discharge, with a target grind size of P80 70 microns, is fed to a cyclone feed pump box where lime is added. A pump feeds the discharge to two operating hydro cyclones. The underflow flows by gravity to the gravity recovery circuit, where a set of Knelson concentrators provides two steps for free gold recovery. The cyclone overflow is cleaned of trash by a vibrating screen before entering thickener tanks. Thickener overflow is recycled to the mill, and underflow feeds the CIL circuit.

The CIL circuit consists of six tanks providing approximately 30 hours of residence time, receiving slurry with 40% to 45% solids. The tanks are installed on a step down elevation to allow gravity flow. Carbon is transferred every two days, and tanks 1 and 2 include provisions for pH control using milk of lime or caustic soda. Instrumentation is included to monitor gold concentration, free cyanide, pH, ionic strength, organic compounds, temperature, carbon particle size, and mixing efficiency.

Gold extraction from loaded carbon begins with acid washing to remove calcium and other inorganic contaminants. After neutralization and alkalinity adjustment, the carbon is stripped in a vessel using a heated solution. The stripped carbon is water-washed and returned to the CIL circuit. The pregnant solution is processed through sludge type electro-winning cells, where the majority of gold is washed to the cell bottom. The gravity concentrate and gold sludge filter cake are smelted with borax in a refinery furnace.

Additional Interesting Data and Summary

The plant includes a cyanide preparation station using process water and a metering system for delivery. A caustic soda station prepares a diluted solution for pH control. The design includes a planned program for activated carbon make-up to replace carbon losses as fines. The plant does not include a carbon regeneration circuit, based on an assumption of minimum loss of carbon absorption capacity in normal operation, with an estimated one-year active life for the carbon.

For tailings treatment, the slurry is dewatered by two stages of drum filtration with an intermediate re-pulper tank. The filter cake, at approximately 8% to 12% moisture, is transported to a dry stack area. The INCO system using copper catalyzed SO2/air with sodium metabisulphite is specified for cyanide destruction. Alternate methods using ferrous sulphate were noted as possible for future evaluation. Hydrogen peroxide was not considered for cyanide destruction due to handling and storage difficulties, poor performance on slurry, and the process design treating filter cake.

Key Processes

• Crushing and screening
• Ball milling
• Gravity gold recovery
• Carbon-in-leach
• Carbon stripping
• Electro-winning and smelting
• Cyanide destruction
• Tailings dry stacking

Source: San Albino Project , Process Design Report (Report No. 300), Report Date Unknown.

Project website: San Albino Project, Process Design Report (Report No. 300)