Gold adsorption mechanism of activated carbon (2)

2) Activated carbon pore structure The pores of activated carbon are the space left between the basic microcrystals (amorphous part) after the carbon and carbon components of the carbon are consumed during the activation process. Although activated carbon is composed of a tiny carbon wafer similar to graphite , its wafer is only a few carbon atoms thick, and some carbon molecules make up many open pore walls. These open cavities are between about 0.8 and 200 nm in diameter. As long as the activation method is appropriate, a very large number of pores can be formed, and the total area of ​​the pore walls, that is, the so-called surface area, can generally reach 500 to 1,700 m ² /g, which is the main reason why activated carbon exhibits a large adsorption capacity. Activated carbon of the same surface area has a large difference in adsorption capacity, which is related to the shape and distribution of pores and also to surface chemistry.
It is difficult to obtain a consistent understanding of the shape of the pores. In general, a hypothetical cylindrical shape is often used. In addition, different research methods use different shapes, such as a bottle neck shape, a capillary shape with open ends, a capillary shape with one end occluded, a flat plate shape formed by two planes, a V shape and a conical shape. It is generally calculated that the pores are assumed to be cylindrical capillary shapes.
Dubinin divides the pore distribution of activated carbon into three series, which are divided according to the size of the pores:
Large holes: radius (1000~100000)×10 ^-10 m; transition (middle) holes: radius (20~1000)×10 ^-10 m; micro holes: radius <20×10 ^-10 m.
Due to the different types of activated carbon, the micropore volume is between 0.15 and 0.9 mL/g, which accounts for more than 95% of the total area of ​​activated carbon per unit weight. From this figure, activated carbon has a particularly developed microporous property compared to other adsorbents.
The volume of the transition hole is usually 0.02 to 0.1 mL/g, and the specific surface area does not exceed 5% of the total area. However, take special methods of activation, in particular activation conditions (prolonged activation time, slow rate of temperature increase, the use of chemical activation, such as grasping activated zinc or activated phosphoric acid, etc.) can be manufactured at a transition developed activated carbon pores. Its volume can reach 0.3~0.9mL/g, and the surface area can reach or exceed 200m ² /g.
The large pore volume is 0.2~0.5ml/g, and its surface area is small, generally not exceeding 0.5~2.0m ² /g.
The three pores of activated carbon have their own adsorption characteristics, while the pores that play a decisive role in adsorption are micropores. However, there are few micropores directly distributed on the outer surface of the activated carbon, and the transition pores are usually separated from the large pores, and then the micropores are separated from the transition pores, as shown in FIG. Therefore, the adsorbate should be adsorbed in the micropores and must pass through the macropores and transition holes. In addition, in the liquid phase adsorption, the adsorbate having a large molecular diameter hardly enters the micropores, and thus is adsorbed in the transition pores, so a certain degree of transition pores is necessary. The ratio of the surface area of ​​the macropores to the total surface area is small, and does not greatly affect the amount of adsorption, but when activated carbon is used as a catalyst carrier, its effect is important.

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The pore distribution has a large influence on the adsorption capacity due to the presence of molecular sieves. This is because a certain size of the adsorbate molecule cannot enter a pore smaller than its diameter, and how much molecule can be allowed to enter, according to the steric effect, is about 0.5 to 0.2 of the pore diameter. In addition, in the liquid phase adsorption, there is also a solvent effect of the adsorbate molecules, that is, the apparent molecular diameter of the adsorbate in the liquid phase becomes large, and the pores having a small diameter are often unable to enter.
Activated carbon is made of an organic material, such as trees, nut shell, fruit pits, and sugar, brown coal, bituminous coal, anthracite and the like, under an atmosphere of CO, CO _2, H ₂ O (the absence of air) was heated to 800 ~ 900 ℃, Activated to obtain activated carbon; approximately 20% of the char is vaporized during the activation process.
C+CO ₂ →2CO
C+H ₂ O=====CO+H ₂
The remaining carbon is penetrating through the microporous structure (see Figure 6). The pores are very developed, and most of them are open pores with a diameter of 0.5~2μm. Therefore, activated carbon has a large specific surface (400 to 1000 m ² /g). The activity of activated carbon is a huge effect produced by the combination of both the surface and the reactive groups present on the surface.

The activated carbon used for adsorbing gold from cyanide pulp is prepared by high-temperature heat activation method, that is, dehydration and carbonization of coconut shell or core in an inert gas of 500-600 ° C, and then water at 800-1100 ° C. Activation is carried out in steam, carbon dioxide, air or any mixture thereof, so that its microcrystalline structure predominates. The typical coconut shell charcoal thus produced has a pore size of about 1.0 nm, which accounts for about 90% of the total volume of the pore (Fig. 7).

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In short, the pore distribution is a physical factor that has a great influence on the adsorption of activated carbon. Since there are various problems in the method of measuring pores or surface areas, it is difficult to theoretically analyze the relationship between adsorption and pore distribution. In general, powdered activated carbon has many large pores, while granular activated carbon micropores are developed.
3) Composition of activated carbon element The adsorption characteristics of activated carbon are not only affected by its pore structure, but also by its chemical composition. The adsorption force of the very regular graphite surface is mainly caused by the dispersion force in the van der Waals force. The phenomenon that occurs is physical adsorption. The basic microcrystalline structure of carbon is irregular, which obviously changes the composition of the electron cloud on the carbon skeleton. As a result, unsaturated valence or unpaired electrons are generated, so activated carbon has strong adsorption to polar substances. . Another reason for the irregular structure of the basic structure is due to the presence of heteroatoms. A variety of heteroatoms or functional groups formed by heteroatoms are present in the basic structure of the carbon to "modify" the carbon surface, thereby altering the adsorption characteristics of the carbon.
In addition to carbon, activated carbon contains two other mixtures. One is a chemically bonded element, which is represented by oxygen or hydrogen. In the raw material, there is incomplete carbonization, which exists in the structure of activated carbon in a graphitized state, or forms a chemical bond on the surface during activation or exists as an oxide on the carbon surface due to oxygen or water vapor. . Another mixture is ash which constitutes the inorganic constituent of the activated carbon. The content and composition of ash vary with the type of activated carbon. The mass fraction of coconut shell activated carbon is about 3.5% ash, 0.1% of potassium, aluminum, silicon, sodium, iron oxide, a small amount of magnesium, calcium, boron, copper, silver, zinc, tin and traces of lithium, rubidium, strontium, lead and so on. The use of sugar can produce activated carbon with very low ash content. Activated carbon containing almost no ash can be made of polyvinyl chloride or phenolic resin, and its ash content can be up to 0.01% or less. Table 1 shows the elemental composition of several activated carbons.

It can be seen that in addition to the inorganic burning residue (ash) in activated carbon, carbon accounts for 90% to 94%, and oxygen and hydrogen account for most of the rest; except for special sulfur-containing carbon, activated carbon is hardly Sulfur is included; in addition, the results of the nitrogen analysis are only trace amounts. The proportion of ash in the activated carbon increases as the degree of carbonization and activation of the raw material increases.
The ash in the activated carbon can be removed by hydrofluoric acid, hydrochloric acid, nitric acid or mixed acid, but the hydrofluoric acid will change the pores of the carbon and catalyze the formation of oxide on the surface of the activated carbon. Similarly, hydrochloric acid also increases the surface oxide of carbon.
Ash is catalyzed by the activation of water vapor, and iron and other components show a strong catalytic effect in the reaction of carbon and carbon dioxide. Ash is often affected by adsorption, and the adsorption of some polar substances on activated carbon is increased by ash, because ash causes defects in the basic structure of activated carbon, while the defective part chemically adsorbs oxygen.
4) Activated carbon surface oxide The main factors affecting the adsorption or other properties of activated carbon are the presence of oxygen and hydrogen. These elements chemically combine with the carbon atoms to form an organic portion of the activated carbon that is different from the ash. According to the theory of solid surface inhomogeneity, oxygen or hydrogen and other heteroatoms in the carbon material are combined with the carbon atoms at the ends or lattice defects of the graphite crystallites. The atomic valence of these atoms varies depending on whether the surrounding carbon atoms are sufficiently saturated, so the reactivity is quite different.
In the study of surface oxides of activated carbon, Sohilow concluded that carbon monoxide exists in three states: surface oxides A, B, and C. Fig. 8 shows a model of a surface oxide. Oxide A is formed at a temperature of 700 ° C or higher, oxide B is formed at 300 ° C or higher, and reacts with CO ₂ to release CO at 700 ° C or higher. A portion of the oxide B is converted to the oxide C in the range of 300 to 850 °C. In general activated carbon, surface oxides are mostly present in the form of "B". Oxides A and B adsorb acid in liquid phase adsorption without adsorbing alkali, while oxide C adsorbs alkali when pressure is stable. Therefore, oxides A and B are basic oxides, and oxide C is an acidic oxide.

When carbonaceous materials are heated and carbonized, elements other than carbon, such as oxygen and hydrogen, are sequentially desorbed from the carbon structure, and various organic functional groups are formed on the surface of the carbon due to different heating or carbonization conditions, and organic functional groups have been detected on the surface of the activated carbon. There are: a carboxyl group, a phenol type hydroxyl group, a hydrazine type carbonyl group, an ether type, a peroxide, an ester, a fluorescein, a carbonic anhydride, a cyclic peroxide, and the like. [next]
The heat treatment of the carbides can ultimately be regarded as charcoal. The carbon bond sequentially transitions to the graphite crystal, and the middle product is non-graphite carbon. As the temperature is from low to high, elements such as oxygen and hydrogen successively leave. At this time, the form of the surface oxide is: at low temperature, it is rich in oxygen - COOH, then - OH is more, at high temperature - COOH is reduced, and C = 0 is increased.
5) Types and properties of activated carbon Activated carbon is a general term for a group of adsorbents. There are many types of activated carbon, which are mainly divided into three categories according to the source of raw materials: coal charcoal, husks and wood. There are thousands of types of activated carbon produced in various countries around the world, many of which are special carbons, and many are used for decolorization, deodorization and removal of harmful components of pharmaceuticals, sugar, monosodium glutamate, metallurgy, chemicals and environmental protection. . The special charcoal used for adsorbing gold and silver from cyanide leaching pulp started late. The best variety today is coconut shell charcoal, but there are dozens of grades in various countries. Followed by apricot kernel, olive nucleus, peach core and other nuclear charcoal and a variety of synthetic carbon.
Hole size is an important indicator of many properties of activated carbon. Since the pore diameter of carbon adsorption of iodine molecules is at least 1. Onm, it is similar to the diameter of the pores required for the adsorption of gold from cyanide solution by activated carbon, so the iodine value is an important indicator for measuring the activated carbon. And the gold charcoal must have a small pore diameter, a large pore volume and a high specific surface area. In addition, the high quality gold metal must have good wear resistance, can be subjected to shearing, compression, collision and other forces for many times and for a long time in a harsh operating environment to maintain structural integrity, and as small as possible Cut the loss. When selecting the type of activated carbon, carbon strength, adsorption speed, and adsorption capacity are mainly considered. It has strict requirements on its activity, pore size, surface area and pore volume. The selection principle depends on the technical application, economical and guaranteed.
Since there are dozens of grades of coconut shell charcoal produced in various countries today, the physical properties and chemical adsorption properties of coconut shell charcoal which can be used for extracting gold from the carbon slurry method (referred to as other types of charcoal) are listed in Table 2.

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