The evaluation of the activity of carbon is basically based on the fact that the charcoal has a strength comparable to that of carbon which has been proven to have sufficient abrasion resistance in production. When selecting activated carbon, the anti-wear test should be done first, and the test conditions should be very strict. The ground carbon was then measured for its adsorption rate and adsorption capacity. The purpose of this is to bring the selected chars closer to the parameters upon which the char is selected after being put into use. When selecting or evaluating in a variety of charcoal, it is of course possible to select a char which has a higher activity after the anti-wear test.
The principle of selecting activated carbon is to ensure that the carbon has high activity in order to recover the gold most efficiently from the solution; and to ensure that the carbon has sufficient strength, and at the same time, the density and particle size of the carbon are taken into consideration. High activity is still required. While attaching importance to the strength of carbon, it is necessary to prevent the tendency to use hard carbon and to avoid a significant decrease in the activity of carbon.
Although modern carbon pulp plants are designed to separate and transport charcoal with fixed screens and plastic tubes instead of vibrating screens and steel pipes, some factories have also modified or coated rubber-like plastic materials on the tip of mechanically stirred impellers. It has been greatly improved in preventing carbon wear. However, when choosing activated carbon, it should pay attention to its wear resistance and minimize the loss of gold due to carbon abrasion.
Since activated carbon adsorbs gold from a cyanide solution containing a large amount of heavy metals and other impurity ions, another important indicator of carbon is the selective adsorption performance for gold and silver . Coco charcoal, which is widely used in the United States and South Africa, has good adsorption capacity and selectivity because of its large adsorption capacity and selectivity. In addition, the United States is also a trial to suppress charcoal wood, hay, peat and lignite, peat, South Africa also try to develop a pressed charcoal.
In addition to the imported coconut shell charcoal in China, most of the cyanide plants use domestic coconut shell charcoal and apricot nuclear charcoal. A variety of core charcoal has also been developed and tested. The Sichuan Institute of Forestry has also developed a kind of wood charcoal. Its strength can reach 86%~93.8%.
In 1980, Ivanova (JI.C.14saxosa) introduced an AY-50 type activated carbon which was synthesized by phenolic resin and activated at a high temperature in a vacuum of 1000 ° C or a CO 2 gas stream. This carbon has a pure surface and excellent anion exchange in the presence of air oxygen. It has strong reducing ability for heavy metal ions and simple gold, silver ions and complex ions, especially for gold and silver. Has excellent selectivity. When using AY-50 charcoal to test the cyanide solution containing gold, silver and associated impurities of copper , zinc , iron , nickel and cobalt , gold, silver and copper have just entered the carbon, and as the adsorption capacity increases, Impurities such as copper are gradually replaced by gold and silver, and the capacity of gold and silver in carbon is increasing. When the carbon finally reaches saturation, the distribution ratio of the total adsorption capacity is Au 80%, Ag 20%, and the gold + silver selection coefficient is equal to 1. When the AM-2 resin was used for the comparative experiment under the same conditions, when the resin finally reached saturation, the adsorption capacity of gold was 5.2 mg/g, accounting for 19% of the total capacity, and the selectivity was equal to 0.19. 75%, silver and nickel each account for 2% to 4%.
3. Adsorption mechanism The mechanism of adsorption of gold and silver by activated carbon has not been fully studied yet. The reason why activated carbon can be successfully applied in the gold and silver production industry is mainly due to the practice and experience accumulation of production. Practice has proved that activated carbon with small pore size (1.0~2.0nm in diameter) has unmatched good selectivity for gold adsorption. [next]
According to the discussion of various researchers, the adsorption of activated carbon mainly depends on its numerous internal pores and huge specific surface area, and its external surface area and oxidation state are less effective. The outer surface simply provides access to the internal cavity. The oxide of the surface layer is mainly to make the hydrophobic skeleton of the carbon hydrophilic, so that the activated carbon has affinity for various polar and non-polar compounds. The adsorption function of activated carbon occurs due to the unbalanced force of the carbon atoms constituting the surface of the pore wall. The larger the surface area of ​​these pore walls, the better the function of adsorbing substances.
The adsorption process of carbon includes physical adsorption and chemical adsorption. Physical adsorption is related to van der Waals force, which is a function between dipoles and reversible adsorption based on hydrogen bonding. Chemical adsorption is an irreversible adsorption combined with valence bonds of ionic bonds or covalent bonds. In most cases, the adsorption process of carbon is controlled by physical adsorption, and the process is exothermic reaction in adsorption kinetics. Activated carbon diffuses to cause molecules or ions to reach the inner surface of the pores of carbon, thus reaction time Determined by the length of the diffusion path, as shown in Figure 7, the gold shell coconut charcoal, 90% of the pore diameter is about 1.0nm, large volume (relatively) Au(CN) 2 - molecule (or complex ion) It is only possible to reach the inner surface of these micropores with only tortuous passages and a slow diffusion process.
The mechanism of adsorption of gold from cyanide or slurry by activated carbon is extremely important in industrial production. It is essential for improving the adsorption performance of carbon and enhancing the adsorption of carbon. To this end, many researchers have conducted long-term and extensive research on the mechanism of carbon adsorption of gold. However, there is no consistent understanding of the mechanism by which activated carbon adsorbs gold from cyanide solutions. Generally can be summarized into four forms of adsorption.
1) Adsorption by metal After the adsorption of gold from the gold chloride complex (AuCl 4 - ) solution by activated carbon, it is apparent that yellow gold is present on the surface of the carbon. It is concluded that the gold cyanide complex can also be reduced by carbon. This view holds that a reducing gas adsorbed on carbon, such as CO, can reduce gold.
In recent years, X-ray photoelectron spectroscopy (XPS) has been used to study the oxidation state of gold in adsorbed carbon, and the observed gold valence state is +0.3. Accordingly, it can be considered that there is a reduction effect upon carbon adsorption, although it is partial reduction. In addition, the gold is desorbed from the gold-loaded activated carbon, and the desorbent used is not cyanide because the cyanide solution is the best solvent for metal gold. This fact also supports the idea of ​​reducing adsorption.
However, the CO gas is introduced into the gold cyanide solution and no gold is reduced; moreover, from their reduction potential [relative to the calomel electrode, activated carbon: -0.14 V; AuCl 4 - : +0.8 V; AuBr 2 - :+ 0.7V; AuI 2 - : +0.3V; Au(CN) 2 - :-0.85V] to judge, carbon can reduce AuCl 4 - , AuBr 2 - , AuI 2 - , but can not be more negatively charged Au(CN) 2 - reduction. Therefore, it is believed that the mechanism of adsorption of gold cyanide complex by carbon reduction to metal gold is theoretically to be further studied.
2) Adsorption in the form of Au(CN) 2 - anion Gold is recognized by the adsorption of activated carbon in the form of Au(CN) 2 - anion. That is, the theory of anion exchange suggests that there is a positively charged lattice on the surface of carbon. These positively charged lattices are produced by the fact that the activated carbon is contacted with air at room temperature to form a surface oxide having a basic character, and the binding of such oxygen to the carbon is not strong. When charcoal interacts with water, it transfers to the solution and forms OH-ions, so that the carbon surface carries a positive charge: [next]
C+O 2 +2H 2 O=====C 2+ +2OH - +H 2 O 2
The electric double layer OH - ions in solution and Au (CN) 2 - exchange, i.e., having an anion exchanger properties, it can be said positively charged grid on charcoal solution of Au (CN) 2 - anion.
This mechanism explains the increase in the adsorption capacity of carbon as the acidity of the cyanide solution increases. Because at the lower pH, the above reaction equilibrium shifts to the right, producing more positive charge sites, it can adsorb more Au(CN) 2 - anions.
Also, the presence of oxygen in the cyanide solution is advantageous for adsorption. Studies have shown that the order of adsorption of carbon on the following ions is:
Au(CN) 2 - >Ag(CN) 2 - >CN -
The above-mentioned recognition encounters an unexplained problem that when a large amount of Cl - or ClO 4 - anion is present in the cyanidation solution, the adsorption capacity of Au(CN) 2 - is not lowered. Cl - anion, especially ClO 4 - anion, which is similar to Au(CN) 2 - and belongs to the large and weakly hydrated anion, which should be adsorbed by carbon with Au (CN) 2 - but this is not the case. When such a solution is adsorbed by the ion exchange resin, the presence of ClO 4 - significantly reduces the adsorption capacity of gold. From this point of view, the mechanism of adsorption in the form of Au(CN) 2 - is not completely convincing.
3) M n + [Au (CN) 2] n - ions are adsorbed This mechanism proposed is based on the fact form: a solution in the presence of cyanide anions (e.g., CI -, ClO 4 -, etc.), and even at a concentration up to 1.5 When mol/L, the adsorption capacity of gold is not lowered. However, when a neutral molecule (such as kerosene) is present in the solution, the amount of gold adsorbed is lowered.
The composition of the neutral molecule of carbon adsorption gold depends on the pH of the solution. In the acidic solution, gold is adsorbed by HAu(CN) 2 , and in a neutral or alkaline medium, gold is adsorbed as a salt. This kind of adsorption is enriched in charcoal by the effect of van der Waals force, the so-called "dispersion force".
It was found that after the charcoal adsorbed with NaAu(CN) 2 was burned, the sodium content in the obtained ash was insufficient to form NaAu(CN) 2 ; the KAu(CN) 2 solution adsorbed by pine charcoal and carbon charcoal contained a large amount of Acidic carbonate, and potassium ions remain in the solution; the presence of acid promotes the adsorption of gold, and the presence of salt (such as CaCl 2 ) can also increase the adsorption capacity of gold. It is found from the above that gold is adsorbed by carbon in the form of Mn + [Au(CN) 2 ] n - . When Mn+ is an alkali metal cation, the adsorption is not as strong as the alkaline earth metal cation, that is, the adsorption strength depends on the metal cation, and the order is: [next]
Ca 2+ >Mg 2+ >H + >Li + >Na + >K +
Thus, Ca 2+ in activated carbon ash and Ca 2+ and H + in solution may replace Na + and K + , such as:
2KAu(CN) 2 +Ca(OH) 2 +2CO 2 =====Ca[Au(CN) 2 ] 2 +2KHCO 3
In the formula, Ca(OH) 2 is caused by 50% CaO in the pine charcoal ash.
Click mechanism, gold to M n + [Au (CN) 2] n - ion pair or a neutral molecule is carbon adsorption, wherein the surface adsorption M n + is an alkaline earth metal cation rather than alkali metal ion adsorption, both carbon effect It can also be through precipitation in the pores, or a combination of the two.
4) Precipitation in the form of AuCN It was early thought that insoluble AuCN could be precipitated in the pores of carbon. The production of AuCN is the result of oxidation:
KAu(CN) 2 +0.5O 2 =====AuCN+KCNO
Some people think that it is the result of acid decomposition:
Au(CN) 2 - +H + =====AuCN+HCN
Tests have shown that the lower the pH of the solution, the greater the gold capacity adsorbed in the carbon:
pH | 1 | 2 | 3 | 6 | 12 |
Loading amount / [mg(Au )· g -1 (C)] | 200 | 160 | 120 | 80 | 60 |
In summary, the mechanism of adsorption of gold-cyanide complexes on activated carbon is still insufficient, and no matter which mechanism, it has its credible and unreliable components. Therefore, a combined adsorption mechanism has been proposed:
â‘ on the inner surface of the char or great micropores adsorb M n + [Au (CN) 2] n - or ions of neutral molecules, and then discharged M n +.
2Au(CN) 2 - chemically decomposes into insoluble AuCN, and AuCN remains in the micropores.
The 3AuCN moiety is reduced to a mixture of zero-valent and one-valent gold atoms (+0.3 valence).
The process of adsorbing gold and silver from cyanide slurry (or cyanide solution) is not carried out under ideal conditions, and it is inevitably affected by various impurity ions dissolved into the solution at the same time. The soluble sulfide in the raw material is dissolved in the alkaline liquid (even if the solution does not contain cyanide), and the elemental sulfur contaminated solution is decomposed. In the cyanide leaching solution, in addition to the gold and silver complexes, it also contains different concentrations of copper, nickel, lead , zinc, diamond, iron and other cyanide complexes, and their large amount will also affect the carbon to gold. Adsorption. If the concentrate is subjected to amalgamation before flotation, the mercury entering the concentrate will generate a neutral ion Hg(CN) 2 (a covalent compound) during cyanidation, which will directly interact with Au(CN) 2 - Competing for the adsorption grid on the carbon, it can even replace (displace) a number of adsorbed Au(CN) 2 from the gold-loaded charcoal. [next]
The gold-related polymers have so far been obtained as neutral compounds, or their cations contain 6, 9, 11 gold atoms, in which the average oxidation state of gold fluctuates between 0.2 and 0.33. Therefore, from the viewpoint of polymer adsorption, it is reasonable for carbon to strongly adsorb gold from the cyanide solution. This is because the polymer is generally highly insoluble in water. If two or more substances compete for the adsorption of carbon, the most difficult substance to dissolve in the medium will preferentially adsorb to the carbon. This is where carbon can strongly adsorb gold selectively.
Due to the "displacement" effect of the adsorption kinetics of activated carbon, activated carbon has better selectivity. That is, in the uncharged solution of carbon, the impurity ions (the same silver) adsorbed by the carbon are gradually displaced into the solution by the cyanide gold ions, and the saturated carbon is mainly loaded with gold. This situation is most evident in experiments such as Ivanova (л·C·Иванова). Table 7 shows an example of the adsorption of a cyanide solution containing various impurities in an adsorption column using AY-50 type phenolic carbon. It can be seen from the table that when a volume of solution is passed, the adsorption of carbon on gold is zero, while the adsorption of zinc and copper is the highest. Later, as the amount of solution passed increased, the amount of gold and silver adsorbed gradually increased, and the impurities were gradually displaced into the carbon. When the solution was 3435 volumes, the gold partition coefficient of the gold-loaded saturated carbon accounted for 80% and silver 20%. It is also consistent with the viewpoint of the partition coefficient of carbon-adsorbed organic compounds, that is, the low-solubility component has a higher partition coefficient when the carbon adsorption reaches saturation.
Table 7 Â Ay-50 Â Adsorption distribution ratio of activated carbon to gold, silver and impurities | |||||||
Cyanide solution throughput (specific volume) | Carbon adsorption capacity /% | ( Au+Ag ): Impurity | |||||
Au | Ag | Ni | Cu | Zn | Fe | ||
1 | 0 | 3 | 3 | 32 | 57 | 5 | 3:97 |
10 | 2 | 5 | 4 | 27 | 59 | 3 | 7:93 |
92 | 10 | 28 | twenty three | twenty four | 15 | 0 | 38 : 62 |
242 | 14 | 42 | 13 | 17 | 13 | 1 | 56:44 |
496 | twenty three | 60 | 12 | 3 | 2 | 0 | 83:17 |
953 | 33 | 55 | 11 | 1 | 0 | 0 | 88:12 |
2145 | 53 | 41 | 6 | 0 | 0 | 0 | 94:6 |
2512 | 55 | 39 | 6 | 0 | 0 | 0 | 94:6 |
2435 | 80 | 20 | 0 | 0 | 0 | 0 | 100:0 |
It can also be seen from Table 7 that the adsorption of carbon on silver begins to be higher than that of gold and rapidly rises to a maximum value, after which silver is gradually replaced by gold from carbon and eventually falls to one-third of the maximum adsorption capacity. If cyanide is introduced again, the adsorption rate of silver may continue to drop to a lower value, and may even drop to near zero. In order to effectively recover the dissolved silver from the cyanide solution, some cyanide plants have adopted measures such as increasing the carbon input to reduce the adsorption capacity of carbon or segmentation adsorption to improve the adsorption recovery rate of silver. Practice has proved to be feasible.
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