The carbonaceous raw materials (coconut, peach kernel, coal, etc.) are activated in the presence of high humidity (800 to 1000 ° C) and a suitable oxidizing agent (steam, air, carbon dioxide) to become activated carbon. Activated carbon is a porous structure with a surface area of ​​600-1500 square meters per gram of carbon, which has strong adsorption capacity for certain substances.
The principle of adsorption of gold by activated carbon is inconclusive. A brief introduction is now made:
Gold is adsorbed from the alkaline cyanide solution by activated carbon, and Au(CN) 2 - does not undergo a chemical reaction, and the neutral complex remains on the carbon in the form of Me n+ [Au(CN) 2 ]n - .
If the activated carbon used contains 5% calcium oxide in the ash, the adsorption of gold is carried out as follows:
2KAu(CN) 2 +Ca(OH) 2 +2CO 2 =Ca[Au(CN) 2 ] 2 +2KHCO 3
If metal ions are not present, the adsorption of gold proceeds as follows:
KAu(CN) 2 +H 2 O+CO 2 = HAu(CN) 2 + KHCO 3
Gold is adsorbed in an acidic solution, and gold cyanide ions are precipitated in the pores of activated carbon with insoluble AuCN:
Au(CN) 2 - +H + = AuCN+HCN
The same is true for silver adsorption:
Ag(CN) 2 - +H + = AgCN+HCN
It is also believed that the adsorption of Ag(CN) 2 - by activated carbon is the principle of anion exchange, that is, the simple electrostatic interaction between positive and negative charges. However, some studies have found that the presence of a single anion (such as Cl - , I - , etc.) in the cyanide solution does not affect the adsorption capacity of carbon to Ag(CN) 2 - , thus denying the above purity. When a neutral organic molecule is present in the cyanide solution, the adsorption capacity of carbon to gold decreases, and it is considered that the adsorption state of carbon to gold is a neutral molecule. In an acidic solution, gold is adsorbed as HAu(CN) 2 ; in a neutral solution, it is adsorbed as a salt such as NaAu(CN) 2 , which is enriched on charcoal by the action of van der Waals force.
The intensity of gold adsorption depends on the nature of the electrolyte in the medium. In the presence of cationic Na + , Ca 2+ , etc., the gold cyanide complex cannot be adsorbed on the carbon. It is also believed that when gold is adsorbed on carbon as a dicyanohydrate complex Mn+[Au(CN) 2 ] - , when Mn + is an alkali metal ion, the complex of the ruthenium complex to charcoal It is stronger than when Mn + is an alkali metal. The adsorption order is:
Ca 2+ >Mg 2+ >H + >Li + >Na + >K +
K + , Na + , Ca 2+ in the adsorption medium is also beneficial to the adsorption of Ag(CN) 2 - .
The adsorption capacity of activated carbon for gold is greater than that for silver, because gold has an ionic radius greater than that of silver.
In summary, the insights of activated carbon on the adsorption of cyanide can be summarized into three categories:
That is, it is adsorbed in the state of Au(CN) 2 - , AuCN and Au. At present, there are many people who hold the opinion that Au(CN) 2 - is adsorbed.
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