Adsorbing gold and heavy metals

Activated carbon adsorption gold is a classic technology in gold extraction, especially in the treatment of low-grade gold ore. It relies on a special adsorption mechanism to achieve efficient enrichment of gold ions. Due to its low cost and high efficiency, this technology is widely used in hydrometallurgical processes.

Case Details

The adsorption mechanisms of gold by coconut shell activated carbon can be categorized as follows:

1. In cyanide solutions, gold is adsorbed onto the inner surface of the activated carbon in the form of ion pairs or neutral molecules, or complexed ions combine with metal ions and precipitate in the pores of the activated carbon. The metal ions originate from the solution or the ash content of the activated carbon.

2. Au(CN)2- undergoes chemical decomposition, generating insoluble AuCN, which remains in the pores of the carbon.

3. Some gold-cyanide complexes are reduced to compounds containing gold atoms with oxidation states of 0 and 1.

4. Au(CN)2- or AuCN is reduced to metallic Au.

Coconut shell activated carbon is mainly used for gold extraction via heap leaching or carbon-in-pulp processes. It can effectively recover gold directly from cyanide-containing solutions, exhibiting good selectivity, strong adaptability, and reusability.

Whether using heap leaching or carbon-in-pulp (CIP) leaching, a key step in gold production is utilizing the highly selective adsorption of gold-containing compounds by activated carbon to achieve efficient, rapid, and low-cost gold enrichment.

The performance of activated carbon directly affects:

1) the gold refining production rate, i.e., the rate at which gold is adsorbed per unit time;

2) the reduction of waste in gold adsorption production.

The gold refining production rate is determined by two factors: the rate at which activated carbon adsorbs gold per unit weight per unit time (representative parameter R-value), and the capacity of activated carbon to adsorb gold per unit weight (representative parameter K-value).

Once the mine is designed and constructed, the number and total volume of activated carbon adsorption tanks are fixed. Using activated carbon with a higher gold adsorption capacity obviously extends the activated carbon regeneration cycle, thus contributing to a higher gold production rate.

The impact of activated carbon with a higher R-value on increasing the gold production rate is even more direct and obvious. The R-value test typically involves adding 1g of the activated carbon to 1 liter of a gold solution of a specific concentration (e.g., 10mg/L) to extract gold. The remaining concentration of the gold solution is tested every 10 minutes until the test is completed after 60 minutes. The R-value can then be calculated.

Generally, for activated carbon to have a higher K-value (total gold adsorption capacity) and a faster adsorption rate, it must have a higher specific surface area, or a higher iodine adsorption value. Unfortunately, a higher iodine adsorption value is achieved by activating and "drilling" more micropores on the order of 1nm in the carbon skeleton of the activated carbon. A side effect of creating more micropores is a decrease in the hardness of the activated carbon, resulting in more debris during the production of domestically produced activated carbon and potential gold loss. Assuming that each ton of activated carbon adsorbs on the order of 5kg of gold, for every 1% loss of debris, each ton of activated carbon that has completed adsorption will lose 50g of gold.

Generally, domestically produced activated carbon uses the Sleip furnace activation process. To achieve higher hardness, the iodine adsorption value is controlled at around 900-950 mg/g. While this increases hardness, it sacrifices the optimal adsorption capacity and rate.

The Swedish company Jacobi uses specific coconut shell varieties as raw materials and employs larger, more advanced rotary kilns (significantly different in quality from the small-diameter rotary kilns used domestically). They primarily produce the 4500 and G210AS types of coconut shell activated carbon, which boast excellent capacity and hardness. Their iodine adsorption values are 1000 and 1150 mg/g respectively, while their hardness values are both above 99.

Given the same K and R values, choosing a product with higher hardness is an effective way to reduce production costs and increase productivity.