Resin adsorption mechanism (1)

1. Type and nature of resin
Over the years, the adsorption of gold cyanide solution for ion exchange resins made many studies indicated, the ion exchange resin to adsorb gold mainly: AM, AB-17, IRA -400,717 and other strongly basic anion exchange resin, Weakly basic anion exchange resins such as AH-18 and 704, mixed anion exchange resins such as AM-2Б and AП-2, and other synthetic resins.
Among these resins, the weakly basic anion exchange resin is better than the strongly basic anion exchange resin in terms of adsorption selectivity to gold, but the former has low strength and poor adsorption kinetics and desorption performance. The adsorption kinetic characteristics are preferably a strong basic anion exchange resin and a mixed anion exchange resin.
This is because the ion exchange resin is an insoluble solid three-dimensional polymer containing ionized groups which are dissociated in solution by a flexible polymer polymer interlaced. The ionized group is a resin matrix network skeleton formed by a resin cross-linking bond, a bridged polymer molecular hydrocarbon chain, and a fixed ion which is rigidly bonded to the skeleton and firmly attached to the skeleton, and is opposite to the fixed ion charge. The counter ion is composed (Figure 1). The counter ion in the resin is an ion that can exchange ions in the solution, and the resin can be classified into a cation exchange resin and an anion exchange resin according to the charge symbol of the counter ion. For example, R represents an ion exchange resin with a fixed ion, and A and B respectively represent exchanged ions in the resin phase and the aqueous phase, and the exchange reaction of the two-phase ion can be expressed as:

When the resin is immersed in the solution, its volume is increased by 1.5 to 2.0 times, which is the expansion property of the resin. Originally, an organic monomer (such as styrene) for synthesizing an ion exchange resin is hydrophobic and does not swell due to water absorption. However, since a hydrophilic active group is introduced into the matrix of the resin, after the resin is immersed in the solution, the aqueous solution infiltrates the active group along the channel of the molecular void and hydrates and swells. The swelling property of the ion exchange resin is expressed by the expansion coefficient K, which is the ratio of the expanded resin specific volume VH to the air-dried resin specific volume VC: [next]

The expansion coefficient of the anion exchange resin varies in the range of 2.0 to 3.0. The anion exchange resin produced and supplied by the user contains 50% to 56% of water.
Since the resin swells with water and returns to its original state after drying, this change causes the internal particles of the resin to move back and forth and generate internal stress, causing wear and damage of the resin. Therefore, it is not advisable to allow the resin to expand and dry frequently during the production process.
The selective adsorption of certain ions by the resin is an important property of ion exchange resins. Therefore, before each resin is selected, the test should be carried out to determine the order in which they selectively adsorb certain ions in the target production solution, so as to select the resin with the best effect.
The mechanical strength of the resin is of great significance in practical applications. Because the resin is subjected to various mediating effects such as media, load, adsorption equipment and ore friction, sieving impact, and dry, wet and cold, thermal changes, etc., the surface of the resin hydrocarbon substrate with low strength is easily destroyed. In particular, the resin used in the slurry adsorption process should have a certain mechanical strength.
The former Soviet Union is widely used in the production of AM-2Б mixed anion exchange resin because it has better adsorption and desorption properties, higher selectivity and mechanical strength than other resins.
The AM-2 quinone anion exchange resin is a bifunctional resin having a macroporous structure. The matrix of the resin consists of a copolymer of styrene and p-divinylbenzene treated with chloromethanol. Wherein, it contains 10% to 12% of p-divinylbenzene, thereby ensuring the mechanical strength required for the resin, and introducing a strong alkaline quaternary ammonium base of about 1:1 into the matrix of the anion exchange resin by amination reaction. A weakly basic tertiary amine base reactive group. Since there are two reactive groups in the resin, the selectivity to gold and the adsorption capacity are increased. For example, when R represents a resin matrix, the molecular formula can be expressed as:

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Since the resin has a macroporous structure, the ion diffusion rate in the resin can be increased, so that the total ion exchange rate is accelerated, and the resin dynamic characteristics are greatly improved. This resin has the following characteristics: a chloride ion exchange capacity of 3.2 mg (equivalent) / g, a particle size of 0.6 to 1.2 mm, a specific surface area of ​​32 m ² /g, a dry resin density of 0.42 g / cm ³ , and a commercial resin moisture content of 52%. 58%, the expansion coefficient in water is 2.7, the transportation and storage temperature is not lower than 5 °C, before the new resin is used, first wash it with 3~4 times the mass fraction of 0.5% HCl or H ₂ SO ₄ solution, remove the washing A foam produced by a process consisting of finely divided resin. The washing is preferably carried out simultaneously with sieve analysis (mesh opening 0.4 mm) to remove the fine resin. The addition of these fine-grained resins to the adsorption process causes gold to be lost with the tailings.
The AN-2 type resin is a mixed alkali anion exchange resin composed of a copolymer of a copolymer of p-divinylbenzene and 60% isooctane having a volume fraction of 12%, and has a particle size of 0.4 to 1.5 mm.
AH-18 resin is a weakly basic anion exchange resin developed by the Soviet Union with dimethylamine as the active group. It has good selectivity for gold adsorption, generally accounting for 50%~60% of the total adsorption capacity, but mechanical strength. Poor, and the recyclability of the resin is not good.
AB-17, IRA-400 and 717 strong basic anion exchange resins have high mechanical strength and good adsorption and desorption kinetics, but have poor selectivity for gold adsorption, generally only 18% of total adsorption capacity. %about. Table 1 lists the adsorption capacity of the IRA-400 resin for adsorbing metal ions such as gold, silver , and copper from the cyanide solution.

2. Resin exchange reaction
Industrially used ion exchange resins are synthetically synthesized. They are similar to plastic structures and are stable solid three-dimensional polymers in both acid and alkaline solutions, the composition of which contains ionizable groups that are dissociable in solution. The ionizable group is composed of a fixed ion strongly bonded to a polymer skeleton (resin matrix) of the resin and a counter ion opposite to the fixed ion charge. The counterion of the resin is the ion that exchanges with the ions in the solution.
The ion exchange process can be envisaged in the following steps: 1 the ions in the solution diffuse toward the surface of the resin particles; 2 the ions migrate to the inside of the resin particles; 3 the ion exchange reaction; 4 the exchanged counter ions from the inside of the resin particles Diffusion to the surface; 5 counter ions diffuse into the solution. Of these 5 steps, 1 and 5, 2, and 4 are the same, except that the ions are different and the direction of movement is reversed. Since the ion exchange process is a multi-step process, its overall speed (process exchange speed) is determined by the step that is the slowest.
Numerous studies have shown that the chemical reaction step 3 of the exchange is generally very fast, so it does not determine the total velocity of the ion exchange process, and the diffusion process is decisive in the ion exchange kinetics. Research data indicates that the ion exchange rate is related to the resin particle size. When the granularity is reduced, the exchange process speeds up. It can be seen that the rate of ion exchange is determined by the diffusion of ions in the resin particles or the rate of ion diffusion in the liquid immobile layer (liquid film) around the resin particles. The former is called the diffusion of the rubber layer, and the latter is called the diffusion of the film. Among them, the diffusion of the rubber layer is mostly slower than the diffusion of the film layer. Therefore, in the ion exchange process of recovering gold from the slurry, the exchange speed mainly depends on the diffusion of the gel layer of the ions. However, in the gold and silver desorption process of the gold-loaded resin, the ion exchange rate is probably controlled by the diffusion of the film layer because the process is carried out in a fixed bed of resin without agitation. At this time, the thickness of the film layer is large, and the difference in concentration of the interface solution inside and outside the film layer and the diffusion rate of ions are small. Although the temperature of the solution can be increased to accelerate the diffusion of the film layer, the liquid temperature is generally not more than 50 to 60 ° C due to poor thermal stability of the resin. Exceeding this temperature range damages the reactive groups of the resin and lowers the adsorption capacity of the resin.
According to the symbol of the counter ion charge in the ion exchange resin, it is classified into a cation exchange resin and an anion exchange resin. If R is a fixed ion in the ion exchange resin, the ion exchange reaction can be written as the following reaction formula:

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The ion exchange resins used in industry must meet the following two basic requirements:
1 Whether it is at room temperature or high temperature, it is insoluble in water or an aqueous solution of acid or alkali, which requires insolubility and chemical stability to ensure that the resin can be reused many times.
2 High mechanical strength with wear and impact resistance. For this reason, the resin matrix contains divinylbenzene having a mass fraction of 8% to 12%. The percentage of divinylbenzene is referred to as the "degree of crosslinking".
The resin is a regular pellet, and the particle size is selected from 0.2 to 1.2 mm.
An ion exchange resin containing only one form of a reactive group is referred to as a monofunctional resin, and a multifunctional resin containing several forms of reactive groups. A multifunctional anion exchange grease for the gold adsorption process. For example, the former Soviet Union AM-2B anion exchange resin is bifunctional, introducing a quaternary amine group and a tertiary amine group. The matrix consists of a copolymer of styrene and p-divinylbenzene treated with chloromethanol, cross-linking. Degree 10%~12%.
During the adsorption process, the cyanide complex anions of noble metals and impurities (Zn, Cu, Ni, Co, etc.) are adsorbed as follows: