Cyanide Leaching: How do you determine the optimal parameters for temperature, time, and concentration?
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Gold Ore Leaching Process Tests
Grinding Fineness Test
The exposure of monomer gold or bare gold surface is a prerequisite for cyanide leaching or new non-toxic leaching methods. Increasing the grinding fineness appropriately can enhance the leaching rate. However, over-grinding not only raises milling costs but also increases the likelihood of leachable impurities entering the leach solution, leading to the loss of cyanide or leaching agents and dissolved gold. To determine the appropriate grinding fineness, a grinding fineness test must be conducted first.
Pretreatment Agent Selection Test
Gold ore leaching often requires pretreatment agent selection tests. Common agents like calcium peroxide, sodium hypochlorite, sodium peroxide, hydrogen peroxide, citric acid, and lead nitrate are compared with conventional methods where no pretreatment agent is used, aiming to determine if pretreatment is necessary. Calcium peroxide, sodium hypochlorite, and sodium peroxide are stable and widely used multifunctional inorganic peroxides, characterized by prolonged oxygen release, which helps improve gold leaching rates in leach slurry. Hydrogen peroxide and citric acid supply sufficient oxygen during the leaching process as the main oxygen-generating agents. Lead nitrate’s lead ions (in appropriate amounts) can destroy the passivation film on gold during cyanide leaching, speeding up gold dissolution, reducing cyanidation time, and increasing the leaching rate.
Protective Alkali and Lime Dosage Test
To stabilize the sodium cyanide solution or non-toxic leaching agents and minimize chemical losses, a suitable amount of alkali must be added to the leach to maintain a certain slurry alkalinity. Within a certain range, as alkali concentration increases, the gold leaching rate remains constant while the leaching agent dosage decreases accordingly. However, excessive alkalinity slows gold dissolution and reduces the leaching rate, necessitating determining the optimal alkali dosage and slurry pH. In tests and production, widely available and low-cost lime is usually used as the leaching protective alkali. This helps determine the specific dosage needed for practical production.
Leaching Agent Dosage Test
In the gold leaching process, the leaching agent dosage is directly proportional to the gold leaching rate within a certain range. However, excessively high dosages not only raise production costs but also have little impact on further increasing the leaching rate. Therefore, based on the grinding fineness test, a leaching agent dosage test is conducted to determine the optimal dosage, further lowering agent consumption and production costs.
Leaching Time Test
To achieve high leaching rates, extending leaching time is a common practice, allowing complete gold dissolution and maximizing leaching efficiency. As leaching time increases, the gold leaching rate gradually rises until it stabilizes. However, prolonged leaching time also dissolves and accumulates other impurities in the slurry, hindering gold dissolution. A leaching time test is conducted to determine the optimal duration.
Slurry Concentration Test
During leaching, the slurry concentration directly affects the gold leaching rate and speed. Higher concentrations result in higher viscosity and lower fluidity, reducing both the gold leaching rate and speed. Conversely, too low a concentration increases leaching efficiency but also necessitates larger equipment and higher investment, while proportionally increasing reagent dosages and production costs. A slurry concentration test is conducted to determine the optimal leach slurry concentration.
Activated Carbon Pretreatment Test
For the carbon-in-leach (CIL) method, hard and wear-resistant activated carbon must be used to avoid fine carbon particles entering the tailings due to abrasion during stirring, leading to gold loss and reduced recovery rates. The test typically uses coconut shell activated carbon with a particle size of 6-40 mesh. The pretreatment conditions involve a water-to-carbon ratio of 5:1, stirring for 4 hours at 1700 RPM. The carbon is then screened using 6-mesh and 16-mesh sieves, removing fine particles below 16 mesh. The selected carbon (6-16 mesh) is used for carbon leaching and adsorption tests.
Base Carbon Density Test
In gold ore leaching tests, 6-16 mesh coconut shell activated carbon is usually selected to adsorb and recover dissolved gold, yielding gold-loaded carbon, which is then subjected to mature carbon desorption and electrowinning to produce finished gold. The base carbon density directly impacts adsorption efficiency. A base carbon density test is conducted to determine the optimal density.
Carbon Adsorption Time Test
To determine the appropriate carbon leaching (adsorption) time and minimize wear on gold-loaded carbon, a pre-leaching and carbon leaching (adsorption) time test is needed after determining the total leaching time.
Comprehensive Carbon Leaching Process Test
To verify the stability of the carbon leaching process and the reproducibility of test results, a comprehensive parallel test of the entire carbon leaching process is conducted. After determining the optimal conditions in the above nine tests, the final integrated validation test is performed. This completes a full-scale test study for carbon slurry leaching in gold ore processing. Depending on actual production needs, additional tests may include tailings (barren solution) recycling trials or measuring carbon leaching residue settling rates.
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