Carbon regeneration technology

The carbon slurry gold extraction plant mainly uses activated carbon as the carrier, and plays an important role in transportation from ore to finished gold in the gold extraction process. The desorption operation of gold-loaded carbon is also a category of carbon regeneration for activated carbon. However, the charcoal after desorption is not as high as the fresh activated carbon. In order to ensure good adsorption capacity when returning, the carbon must be washed by acid and alkali and regenerated at high temperature. It has the advantage of being able to remove carbon and adsorb it. Carbonates and other acid soluble precipitates and organic materials. During the thermal regeneration process, the steam reacts with the char to produce some gas. Since the gas escapes from the surface of the carbon, the carbon surface is removed, and the carbon is activated and regenerated to have the same activity as the fresh carbon. After the particle size screening, the qualified charcoal is returned to the adsorption operation for recycling.
1. The role of carbon regeneration
Activated carbon regeneration is a technical measure taken to reduce or lose the adsorption capacity when activated carbon adsorbs a large amount of impurities, and to remove the adsorbed impurities and restore the adsorption activity of the carbon. In the CIP process, carbon adsorption system is a complex system of a multi-component coexist in the system in addition to activated carbon selective adsorption of gold and silver, the various organic compounds (mainly lubricating oil, and volatile oil and flocculant Various chemical agents such as flotation reagents and base metal compounds (mainly CaCO ₃ , MgCO ₃ , Fe(OH) ₃ , SiO _{2 ,} etc.) also have strong adsorption capacity. These materials are difficult to remove in the desorption system. With the continuous recycling of activated carbon, such substances accumulate on the carbon, and a large amount of impurities are accumulated in the pores of the carbon, which reduces the surface of the available micropores, and even causes the pores to clog, thereby adsorbing the noble metal by the carbon. The activity is reduced or even lost.
The activity of the same activated carbon at different points in the process is different. As the retention time of activated carbon in the adsorption process is prolonged, the adsorption activity is gradually reduced. Figure 1 is a typical kinetic activity curve taken by taking a carbon sample from each point in the process, performing a kinetic activity analysis, and comparing it with the adsorption activity of fresh activated carbon.

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In all production processes using activated carbon in large quantities, the regeneration of activated carbon is an important part of it. In particular, charcoal regeneration has a strong vitality in the carbon extraction process, which is a popular key operation and plays a central role. As the number of times the activated carbon is recycled in the cyanide slurry increases, the degree of aging also increases. A semi-industrial test conducted by the US Bureau of Mines demonstrates that the absorbed charcoal can be reused 15 times. When it is not suitable for the process requirements, it is called loss of activity, and it is necessary to carry out the active treatment of recovering activated carbon, that is, carbon regeneration. Carbon regeneration is divided into two treatment processes, one is acid-base washing regeneration, and the other is high temperature activated regeneration.
The activated carbon of the regenerated activated carbon should be substantially or completely restored to the level of fresh activated carbon. The activated carbon used in each cycle must be subjected to an acid-base regeneration washing treatment, but it is not necessary to perform a high-temperature activation regeneration treatment. However, activated carbon which has not been subjected to acid-base regeneration or acid-base regeneration treatment contains carbonates and other acid-soluble precipitates, which cannot be solved in the thermal regeneration treatment process, and still affects the activity of the recycled carbon.
2. Carbon regeneration process
Activated carbon regeneration includes both pickling and heating regeneration. The domestic common carbon regeneration process is shown in Figure 2. The purpose of the acid-base washing is to remove the ruthenium metal oxide adsorbed on the activated carbon. This is to prevent the ruthenium metal oxide from catalyzing the combustion of carbon during heating regeneration before heating regeneration. For pickling, hydrochloric acid or nitric acid can be used. It can be done before the desorption operation or after the desorption operation. Domestic carbon slurry plants are mostly carried out after the desorption operation, usually using 3% HCl or 5% HNO ₃ and stirring and washing at 40 ° C for 30 min. The amount of washing liquid is generally 3 to 5 times the volume of carbon, which can remove about 90% of the calcium and magnesium precipitate on the carbon. However, the production of domestic and international practice shows that removing only part of the inorganic acid compounds adsorbed on the carbon, the iodine value and can only be restored value CTC activated carbon, inorganic ash carbon reduction, adsorption rate and adsorption capacity of the carbon to improve incomplete. The heating regeneration can remove the organic ash adsorbed on the carbon and can cause most of the inorganic ash to be thermally decomposed. After heating and regeneration, the adsorption capacity and adsorption rate of carbon are fully recovered, and the adsorption activity can reach or approach the new carbon level. Therefore, heating regeneration is the primary and necessary means of the carbon regeneration process.

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High temperature activation is to wash the carbon after repeated use for several times, then wash it with neutral water to neutrality, then place the wet carbon in a regenerative kiln (furnace) and heat it to 650 in a water vapor atmosphere insulated from air. ~800 ° C, 30 minutes thermal decomposition to remove organic matter and other substances in the carbon, restore carbon activity.
The principle of high-temperature activated regeneration of carbon is to dry the wet carbon with a high-temperature gas. During the heating process, the adsorbed organic matter is in the form of desorption, carbonization and oxidation by the action of distillation, desorption or thermal decomposition of water vapor according to its properties. The activated carbon is removed at the active point. 100~150°C is the drying zone temperature, 650~700°C is the carbonization process, and the oxidizing gas is mainly the superheated water vapor. No air or oxygen is expected to be present. The thermal regeneration process usually goes through the following steps. The basic principle is shown in Figure 3.

1 low temperature drying, below 200 ° C, volatile volatile substances volatilize.
2200~500 °C, the volatile adsorbent volatiles and the decomposition of unstable adsorbate, while releasing volatile components.
3500~700 °C, thermal dissociation of carbon surface deposits and pyrolysis of non-volatile adsorbate.
â‘£ When the 700 deg.] C, under the action of water vapor and carbon dioxide gas, the pyrolysis product are oxidized. Its chemical reaction is as follows:
C+H ₂ O→CO+H ₂
CO+H ₂ O→CO ₂ +H ₂
Temperature is an important condition for heating regeneration. As the temperature increases, the ash content of the activated carbon decreases, the wear loss increases, the anti-wear ability decreases, the adsorption speed increases, and the activity increases, as shown in Table 1. [next]

In the drying and carbonization stages, the temperature rise should not be too fast. Especially in the carbonization stage, if the temperature rises too fast, the activation of activated carbon will have a very bad effect. When activated at a temperature of 815 ° C or higher, the activated carbon itself is inevitably burned out, and this loss of ignition is also affected by the amount of water vapor and carbon dioxide gas immersed in the activated carbon. This loss accounts for most of the amount of activated carbon lost during high temperature heating regeneration. Therefore, in practice, the temperature in the drying and carbonization stages is generally controlled below 400-700 ° C, and the temperature in the activation stage is controlled at 650-800 ° C. In fact, due to the endothermic effect of water vapor and char, the actual temperature is generally around 750 °C.
At present, it is impossible to accurately specify the regeneration time. In practice, the residence time of the char in the regeneration kiln is generally controlled. The effect of residence time on the regeneration effect is shown in Table 2. The optimal residence time in production is determined by experiment and is usually controlled at 20~30min. Excessive regeneration time will not only increase the loss rate of charcoal, but also cause the recovered adsorption activity to be lost again.

When there is no water vapor, a severe oxidation reaction occurs, and the pores of the carbon are rapidly destroyed. When there is steam, the carbon pores slowly disappear at 700 °C. When held at 850 ° C for 30 min, the damage to the pores occurred rapidly. When there is superheated steam, the carbon porosity during regeneration at 650-700 °C is restored, even after 60 min, the damage to the pores is slight. Thermal regeneration is mainly to remove a variety of organic matter adsorbed on carbon. The low temperature of heat regeneration, the short residence time in the rotary kiln and the high moisture content of the desorbed carbon will reduce the effect of the thermally regenerated activated carbon.
The residence time of carbon thermal regeneration in the furnace or kiln is preferably 5~10min, and the prolonged time will not increase the activity of the carbon. It is very important to keep the temperature of the carbon in line with the temperature of the kiln to be the true temperature of the carbon regeneration, in order to achieve the purpose of carbon heat regeneration. The heating method is preferably external heating to reduce the carbon consumption during thermal regeneration. With the thermal regeneration of the granular carbon seven times, the carbon particle size is reduced by about 13%, and the actual particle size change is about 1.4%.
After pickling and thermal regeneration of the char, new high activity layers and pores can be formed, restoring or substantially returning to the level of fresh charcoal. After the rapid regeneration, the charcoal is removed by sieving after rapid cooling, and then immersed in distilled water for 12 hours, and then the carbon suspension is returned to the adsorption process.
3. The industrial equipment for regenerating granular carbon in the carbon heat regeneration device has a multi-layer bed furnace, a rotary kiln, a flow layer type and a moving layer type, and a vertical furnace. There are two types of heating methods: internal heating and external heating. Useful natural gas or coal gas heat, electric heating or the like the fuel. At present, China uses an electric external heating rotary kiln for carbon thermal regeneration, and also a distillation tank type reverberatory furnace. In foreign countries, carbon is added to the rotary kiln, most of which use spiral feeding, the effect is not good, and the carbon particles are easily broken.
At present, the regeneration technology developed includes the Semiex fluidized bed regenerator and the Lintur regeneration furnace. Kilns used for carbon regeneration mostly use oil or other fuel to indirectly heat the rotary kiln, and some plants use electric heating. Figure 4 is a new type of carbon regenerative kiln with a vibrating trough directly heated by the Mintek plant in South Africa. It has high thermal efficiency and can eliminate the adverse effects of moisture fluctuations in carbon, and is suitable for intermittent heating and The carbon is rapidly cooled, with low energy consumption, low investment and no moving parts.

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According to the practice of the Bessa plant in South Africa, 30 kg of charcoal is regenerated every hour in an electric rotary kiln. When the heating is 650 ° C, the power consumption is 5.5 kW. When the water content in the carbon reaches 60%, it consumes 34.2 kW. In order to reduce the amount of entrained water in the charcoal and improve the thermal efficiency, they dehydrate the wet charcoal through a screw loader and supply it to the kiln. Some manufacturers, such as the United States and Peru, also use it instead of rotary kiln. In addition, carbon regeneration is carried out using a multi-hearth furnace or the like.
Domestic carbon pulp mills use domestic horizontal rotary kiln. The rotary kiln is composed of a rotary cylinder body, a heat preservation furnace body, a feed screw, a head cover, a cooling discharge device, a furnace body inclination adjuster, and an electric control box. See Figure 5.

The head and the tail of the rotary cylinder are respectively inserted into the head cover and the cooling discharger, and the joint portion is sealed by the end face to ensure the airtightness of the cylinder. The blade of the feeding spiral is discontinuous to ensure that the spiral cylinder is filled with activated carbon, so that the inside of the rotating cylinder is not communicated with the outside, and the purpose is to ensure the airtightness of the cylinder.
The holding furnace body is composed of upper and lower furnace shells, furnace body cover, refractory bricks and electric heating body. The entire furnace body is divided into three heating zones. The heating temperature is automatically controlled to ensure that the temperatures in the three zones meet the process requirements.
The temperature inside the rotary kiln cylinder is not uniform, and the temperature from the feed end to the middle of the cylinder gradually increases, and the middle part is the regeneration activation zone, and the temperature is the highest. The temperature from the central portion to the discharge end is gradually lowered. However, the temperatures of the charcoal and gas at various points in the cylinder are very different. The temperature changes along the length of the rotating cylinder as shown in Fig. 6. [next]

The carbon adsorption rate and adsorption capacity of the carbon by acid regeneration and thermal regeneration reached the level of fresh activated carbon, thus indicating the importance of carbon regeneration. In the process of carbon regeneration, the loss rate of activated carbon is about 5%. The pure burning loss of activated carbon is 1% to 3%. The test also proves that the mechanical strength of the recycled carbon is similar to that of fresh carbon and can be recycled.

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