Gold adsorption mechanism of activated carbon (3)

The types and properties of activated carbon in China are shown in Table 3. At present, apricot nuclear charcoal is widely used in China's carbon slurry process (coconut charcoal is widely used in foreign countries). For adsorption from solution of gold (charcoal column) the quality of coal char may be used.

2. Selection of activated carbon
The most important condition for selecting activated carbon from carbon slurry is that it has good adsorption properties for gold, and the second is that carbon particles must have strong wear resistance. Carbon with good adsorption characteristics has good selectivity to gold and silver , and large adsorption capacity and recovery rate; carbon with strong wear resistance can minimize grinding loss and reduce the loss of gold-loaded charcoal with slurry loss. The loss of gold. This is due to the carbon used in the carbon slurry process. In the production process, it is generally formulated into a carbon suspension, which is sprayed or hydraulically conveyed, compressed air or mechanically stirred, sieved, and the like. In particular, the quality of each batch of charcoal cannot be uniform. Some of the carbon particles and all the carbon particles and the edges and edges (because they are not spherical particles) have small mechanical strength, weak wear resistance and are most likely to wear. And they are the most active part of carbon, which has good adsorption performance and large adsorption capacity. Its wear will not only increase the loss of gold and silver, but also cause the whole batch of carbon adsorption performance to decline, causing fluctuations in operating indexes, and it is necessary to increase the amount of carbon input during the operation. To this end, in the industrial production, each batch of new charcoal should be screened to remove wood chips and impurities before use, and then stirred in a mechanical agitation tank together with the abrasive (the same gold-free waste rock as the ore). The carbon particles and corners of the mechanical strength are broken, so that the carbon particles are approximately spherical and sieved to remove the charcoal. In the production, attention should be paid to the wear index of charcoal in the operation and recycling of charcoal pulp, and the graded charcoal with good wear resistance should be selected as much as possible. The properties are generally evaluated by the following parameters: 1 adsorption capacity; 2 adsorption rate; 3 carbon strength; 4 carbon density; 5 carbon particle size; 6 carbon ash; 7 carbon water; [next]
The adsorption capacity and adsorption rate are indicators for characterizing the activity of carbon. The strength characterizes the ability of activated carbon to resist abrasion in the carbon slurry process loop. Density and ash are parameters related to activity. The particle size and the fraction under the sieve are process-related parameters. . The relationship between various parameters, especially the active parameters, is closely related to abrasion resistance, density, and ash.
The adsorption capacity refers to the maximum load capacity of activated carbon for gold. Different activated carbon adsorption capacities are different, and the amount of gold supported by the same activated carbon varies with the content of gold in the adsorption solution. In order to compare the gold carrying capacity of different types of activated carbon, a uniform standard must be established. The Calgon test method defines the adsorption capacity as the amount of gold on the char when it is brought into equilibrium with the 1 μg/g gold-containing equilibrium solution, and is expressed by the K value. South Africa is defined as the amount of gold on the char after 24 h of contact with charcoal and 1 μg/g gold-containing equilibrium solution. (Note: The gold-containing equilibrium solution means that during the adsorption process, the gold grade of the gold-containing solution does not change much, that is, the adsorption equilibrium concentration is equivalent to the concentration of the stock solution.)
The adsorption rate refers to the degree to which the activated carbon adsorbs gold in the solution. The adsorption rate of carbon to gold is closely related to the gold content of the solution, so it must be defined. The Calgon test method defines the adsorption rate as the relationship between the time plotted on the adsorption kinetics equation q=K·lg t and the adsorption capacity when the carbon is contacted with a gold solution of 5 μg/g, starting from the calibration time. The reciprocal R of the intercept x on the capacity coordinate indicates the adsorption speed, expressed as the R value (see Figure 9).

South Africa defines the adsorption rate as the percentage of carbon adsorption to gold during the agitation of carbon in a 10×10 ^-6 gold-containing solution for 60 min.
The strength characterizes the wear resistance of activated carbon. The strength of activated carbon depends on the nature of the raw materials for the production of carbon and the processing methods and ingredients. Generally, activated carbon produced from raw materials such as coconut shell and core is more wear-resistant.
When adsorbed by a fixed bed (adsorption column), the carbon wear is small and the strength requirement is not high. However, in the carbon slurry process, the wear loss of carbon is relatively large due to the collision and intense friction between the carbon particles, between the carbon particles and the mechanical components, and between the carbon particles and the pipes. Therefore, the strength must be emphasized when selecting activated carbon. [next]
The specific standard hardness value of carbon used in the United States indicates the wear resistance of activated carbon. In South Africa, the carbon wear loss rate measured after grinding for a period of time according to the specified strength indicates the anti-wear ability of carbon.
Density, particle size, ash, moisture and the fraction under the sieve are the physical and chemical properties of the activated carbon itself, which have a great influence on the activity of the carbon and its application. There are three representations of the density of activated carbon: packing density (bulk density), true density, and apparent density.
For the carbon slurry gold extraction process, among the above performance parameters, the most important performance parameters are only three, namely: adsorption speed, adsorption capacity and strength. D. MacArthur et al. proposed the following three technical indicators:
1 Balanced adsorption for 24 h in a gold-containing volume of 1 g/m ³ solution, the carbon loading capacity of the carbon should reach 25 g/kg;
2 in the gold-containing volume mass of 10g / m, stirring in the solution for 1h, the adsorption rate of carbon to gold should reach 60%;
3 Put the charcoal in the bottle and roll it on the rock machine for 24h, the wear rate should be less than 2%.
In view of the fact that the specific surface area of ​​the gold-filled activated carbon hole is generally 900~1 200 m ² /g (N ₂ , BET), when the adsorption operation capacity reaches 6 g Au per kg of carbon, there is only a gold atom on the specific surface area of ​​800 m ² . The occupation is less active, and the general commodity carbon is easy to meet the requirements of adsorption capacity. Therefore, the activated carbon selected for the carbon slurry method mainly considers its wear resistance, that is, the carbon is added to the finely ground slurry to be stirred and leached together. From the comparison, it is preferred to use a carbon product with good wear resistance.
However, activated carbon is a non-uniform product. There are close relationships between its various performance parameters. For example, the adsorption activity of activated carbon decreases as the strength increases. That is to say, the adsorption capacity and adsorption speed and strength of activated carbon are contradictory and mutually restrictive. Table 4 compares the strength and adsorption activity of three activated carbons.

Table 4 shows that the gold adsorption capacities of the three activated carbons are similar, and the C-type activated carbon has low abrasion resistance, but the adsorption rate is relatively high. The carbons of A and B have high abrasion resistance and relatively low adsorption speed.
Even with the same activated carbon, its strength and activity are not constant. After the activated carbon is subjected to intense abrasion in the process, the softer high-activity components on the carbon are gradually lost, while the harder low-active components remain, so as the carbon moves forward gradually in the process, The activity of carbon gradually decreases and the strength gradually increases. Table 5 shows the relationship between the strength and activity of activated carbon at various stages in the production of a carbon slurry plant.

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