I. Introduction
In the chromium metal and chromium salt production process, a large amount of chromium slag. In view of the different grades of raw materials, different degrees of crushing, and different production equipment and processes, the amount of chromium slag generated also fluctuates. Generally, about 10 tons of chromium slag is discharged per ton of metal chromium produced, and 3 to 5 tons of chromium slag is discharged per lt of chromium salt produced. China's annual emission of chromium slag is about 200,000 tons, and the chromium slag that has been piled up to date has exceeded 3 million tons. The chemical composition of the chromium residue is shown in Table 1.
Table 1 Typical chemical composition of chromium slag
Cr 2 O 3 | Al 2 O 3 | Si O2 | CaO | MgO | K 2 O | Na 2 O | S | P | H 2 O | Fe 2 O 3 | Burning | |
Old slag /% | 4.66 | 5.74 | 10.17 | 30.02 | 22.33 | 0.042 | 2.18 | 0.008 | 0.08 | 14 | 9.44 | 19.28 |
New slag /% | 3.44 | 4.58 | 9.57 | 31.11 | 21.79 | 0.26 | 0.74 | 0.021 | 0.051 | twenty two | 8.13 | 19.65 |
It can be seen from Table 1 that the chromium residue is both a harmful waste residue and a secondary resource that can be utilized. On the one hand, the soluble Cr 6+ in chromium slag is highly toxic, which not only harms the ecological environment, but also affects the growth of animals and plants. It can enter the human body through the digestive tract and skin, distribute it in the liver and kidney, or accumulate in the lungs through the respiratory tract. Exposure to Cr 6+ in an environment of 100 μg/m 3 or more can cause diseases such as dermatitis, chrome sores, bronchitis, pneumonia, emphysema. At home and abroad, public hazard accidents caused by strong oxidizing, mutagenicity and carcinogenicity of Cr 6+ in chromium slag occur frequently; on the other hand, due to the lack of chromium resources in China, it is necessary to comprehensively use various forms of chromium in chromium slag. The abundant CaO, MgO, Fe 2 O 3 and other components can be used in industrial production instead of limestone , dolomite and other raw materials, which can save resources and reduce energy consumption.
The general trend of foreign chromium slag treatment is to decompose or fill the Cr 6+ after detoxification treatment. China since 1960's, it has chromium slag brick, calcium magnesium phosphate fertilizer production, dry (wet) reduction detoxification method, as the glass colorant, the color reducing chromium slag cement prepared using chromium slag and slag wool produced products and cast Methods such as stone products have been experimentally studied and progress has been made to varying degrees. In view of the ever-increasing chromium slag and its serious hazards, the development of its harmless treatment and comprehensive utilization technology is imminent.
Second, the harmless treatment of chromium slag
The phase composition of chromium slag is complex and it is difficult to control harmlessly. At present, the methods for treating chromium slag are basically divided into three categories: high temperature reduction method (dry method), wet method reduction method (wet method) and solidification method. The comparison of the three is shown in Table 2.
Table 2 Comparison of three methods for harmless treatment of chromium slag
method | principle | Application practice | Characteristics |
Dry method | The chromium slag having a particle size of less than 4 mm is mixed with the coal particles at a ratio of 100:15, and subjected to reduction calcination at a high temperature to reduce Cr 6+ to insoluble Cr 2 O 3 . | Sintered glass colorants, calcium magnesium phosphate fluxes, refining iron materials, cast stone and cement. | Valuable products can be obtained; however, the treatment cost is high, the amount of slag is small, and the detoxification of chromium slag is not thorough. |
Wet method | After the chromium slag having a particle size of less than 120 mesh is acidified or alkalized, a reducing agent such as Na 2 S or Fe S O 4 is added to the mixed solution to reduce Cr 6+ to Cr 3+ or Cr(OH) 3 . | Combined with the use of reducing papermaking waste liquid, monosodium glutamate wastewater, etc., it can achieve the purpose of waste treatment. | After treatment, Cr 6+ ≤ 2 × 10 -6 , but the treatment cost is high, it is not suitable to deal with bulk chromium slag. |
Curing method | After the chromium slag is pulverized, a certain amount of FeS0 4 , inorganic acid and cement are added, and water is stirred and solidified, so that the chromium slag is sealed in the cement and is not easily dissolved again. | Curing is mainly based on cement, and there are also a small amount of asphalt , lime, fly ash and chemical curing applications. | The method requires the addition of a considerable amount of curing agent, which is economically inefficient. |
(1) Reduction and detoxification treatment of chromium slag
In the reduction and detoxification treatment of chromium slag, dry and wet methods are most commonly used. Their basic principle is to reduce the highly toxic Cr 6+ to Cr 3+ with very little toxicity. The specific process is shown in Table 3.
Table 3 Reduction and detoxification process of chromium slag
Detoxification process classification | reducing agent | Method introduction | |
Wet method | Acid solution reduction | Sodium sulfite, ferrous sulfate, etc. | The process consumes a large amount of acid and is suitable for enterprises with waste acid emissions. The method is as follows: adjusting the alkaline chromium residue to acidity, and then adding a reducing agent such as Na 2 SO 3 or FeSO 4 to reduce Cr 6+ to Cr 3+ in a liquid-solid two-phase state (the mechanism is as follows: CrO 2 + 3 + 3 Fe 2 + +8H + →Cr 3+ +3Fe 3+ +4H 2 O). |
Alkaline solution reduction | Sodium sulfide, sodium hydrosulfide, etc. | The reduction reaction of Cr 6+ is carried out by directly adding sodium sulfide or sodium hydrogen sulfide to the alkaline chromium residue to form Cr(OH) 3 precipitate, and then the chromium sludge is collected by filtration. The detoxification mechanism of sodium sulfide is as follows: 8Na 2 CrO 4 +6Na 2 S+23H 2 O→8Cr(OH) 3 +3Na 2 S 2 O 3 +22NaOH | |
Soda solution reduction | Sodium carbonate, sodium sulfide | Chromium slag treatment after the wet grinding with sodium carbonate solution, wherein the acid-soluble calcium chromate so that chromium is converted to a water-soluble calcium aluminate is leached sodium chromate, sodium chromate recycled products. The remaining slag is treated with a sodium sulfide solution to reduce the remaining Cr 6+ to Cr 3+ , neutralized with sulfuric acid, and the excess S is fixed with ferrous sulfate. | |
Complex reduction | Lignosulfonate and ferrous sulfate | The lignosulfonate and ferrous sulfate in the papermaking waste liquid are used as reducing agents to reduce and complex the Cr 6+ in the chromium residue to form iron lignin sulfonate. The content of Cr 6+ after detoxification is lower than that. 1.8 mg/kg. The method not only reduces the environmental hazard of chromium slag, but also eliminates the environmental pollution of papermaking waste liquid. | |
Water vapor conversion | Reducing organic matter in wastewater | Using sugar or monosodium glutamate wastewater as a reducing agent, mixing with chromium slag to form a slurry, placing it in a pressure-sealed electric heating vessel, and electrically heating, so that the slurry in the vessel generates superheated steam above 300 ° C to promote the slag. The reduction reaction of Cr 6+ proceeds smoothly. The law also eliminates the contamination of sugar and monosodium glutamate wastewater. | |
Dry method | Carbon reduction | Toner, anthracite powder, etc. | The chromium slag and the carbon powder, anthracite powder, and the like are mixed in a certain ratio (about 100:15) in a reducing atmosphere and heated to about 800 ° C for a while until the Cr 6+ is converted into non-toxic Cr 3+ . |
Sinter | Silicon flux, auxiliary reducing agent | The iron concentrate and the chromium slag are mixed as raw materials to form a sintered ore, and the chromium residue is detoxified during the sintering process. This method produces 10t sinter ore to add 80% iron concentrate, and the amount of waste slag is small, so the cost is high, and the treatment problem of chromium slag cannot be fundamentally solved. | |
Seal roasting | Coal, rice husk or other organic matter | The chromium slag is mixed with an appropriate amount of coal or sawdust and rice husk, calcined at 540-600 ° C, the CO and H 2 produced by the process are used as a reducing agent, and water quenched under sealed conditions, and an excessive amount of ferrous sulfate is mixed with sulfuric acid. In order to consolidate the reduction effect, the Cr 6+ in the detoxification slag is extremely low and can be stored or utilized. | |
(2) Curing/stabilizing treatment of chromium slag
The curing/stabilization treatment of the chromium slag is to pulverize the chromium slag and then add a certain amount of inorganic acid or ferrous sulfate to reduce Cr 6+ to Cr 3+ , and then add a considerable amount of cement, stir with water, solidify, and In the hydration and condensation hardening process of cement, the chromium compound forms a stable crystal structure or chemical bond, and is enclosed in a solid substrate, which is difficult to be dissolved again, thereby achieving the purpose of stabilization and harmlessness.
In the solidification treatment of chromium slag, the blast furnace slag and fly ash are added to the cement substrate to cure/stabilize the chromium slag. The test shows that after the addition of the ultra-fine blast furnace slag, due to the reducing property of the slag, the solidified body The leaching toxicity of the strength and chromium slag has been greatly improved. The chromium slag can be added up to 40%, and the compressive strength of the cured body can reach 30 MPa or more, which can be used for building materials.
The research shows that the leaching toxicity of the chromium slag cured body obtained by pre-reduction of ferrous sulfate is more than 60% lower than that of the solidified body without pre-reduction treatment. Meanwhile, the feeding method of ferrous sulfate has a great influence on the treatment effect. A suitable feeding method is that the ferrous sulfate is firstly mixed into an aqueous solution and stirred with the chromium slag, which can increase the degree of the reduction reaction; the amount of ferrous sulfate added should be 1.25 times the theoretical calculation value.
Third, the comprehensive utilization of chromium slag
Chromium slag has the characteristics of high hardness and high melting point. Therefore, people often use chromium slag to make building materials such as cast stone and brick, or as an alternative raw material for certain products, and convert Cr 6+ into Cr 3+ or metal. Chromium, the dual purpose of detoxification and comprehensive utilization of resources. At present, the more mature methods for comprehensively utilizing chromium slag are:
(1) Building materials
1. Production of diabase cast stone
The diabase cast stone is an excellent acid and alkali resistant and wear resistant material. It is widely used in mining, metallurgy, electric power, chemical and other industrial sectors. It is necessary to use chromite as a nucleating agent when producing cast stone. Since the chromium residue contains residual chromium, it is a good crystal nucleating agent for producing cast stone. There is also a certain amount of silicon, calcium, aluminum, magnesium, iron, etc. in the chromium residue, which are all elements required for casting stone.
2, the production of chromium residue cotton
Slag wool is an excellent insulation and light construction material. The mass phase performance of the slag wool made of chromium slag is basically the same as that of the slag wool. Since it is detoxified at a high temperature of 1400 ° C, the detoxification is thorough. The results of the infiltration toxicity test showed that the water-soluble Cr 6+ content of the slag wool was 0.15 mg/kg, which was much lower than the relevant solid waste pollution control standards.
3, brick making
The technology of mixing chromium slag with clay and coal to burn red brick or green brick is simple, the investment and production cost are low, and the amount of slag is large. Studies have shown that because a large amount of clay in the raw material is acidic at high temperature, combined with the action of coal in the brick and its CO after gasification, it is beneficial to decompose Cr 6+ into Cr 3+ , which makes the Cr 6+ contained in the finished brick drop significantly. It is the CO formed by the kiln process of the blue brick, which not only reduces the reddish-brown iron oxide to the blue-gray Fe 3 O 4 , but also detoxifies the residual Cr 6+ , and the effect is better; when the amount of chromium slag is small, The compressive and flexural strength of the finished bricks have no significant effect. For example, the Guangzhou Chromium Salt Factory uses 40% chromium slag (pulvered to 100 mesh) and 60% clay. The analysis shows that Cr 3+ is about 0.5% to 3%, and the compressive strength of brick is 140kg/cm 3 or more. The flexural strength is 60 kg/cm 3 or more.
If the chromium slag is mixed with the base material prepared from the ceramic raw material in proportion, sprayed into the atomized water, mixed, granulated, formed by a press, dried and then burned, then glazed and then dried, and finally burned into the kiln. Made of glazed vitrified tiles. The brick has beautiful appearance, many decorative methods, and good market sales; and because of the dry material mixing method, the particle size is uniform, the reaction is complete, the glassing amount is large, the detoxification effect is good, and there is no secondary dyeing.
4, cement
The main mineral composition of chromium slag is dicalcium silicate, calcium aluminosilicate and periclase (the content of the three is 70%), which is similar to the mineral composition of cement clinker. There are three ways for chromium slag to be used in cement:
1 After the dry detoxification of chromium slag is used as a mixed material, cement is mixed with cement clinker and gypsum , and the amount of chromium slag is about 10% of the finished cement.
2 Chromium slag is used as one of the cement raw materials to burn cement clinker. The amount of chromium slag accounts for 5% to 10% of cement clinker.
3 Chromium slag replaces calcium fluoride as a mineralizer to burn cement clinker, and the amount of chromium slag accounts for 2% of cement clinker. The amount of chromium slag used in the three ways depends mainly on the magnesium content of the raw limestone.
Using fly ash (or coal gangue), limestone, chromium slag, slag, etc. as raw materials, calcined at 950 ~ 1100 ° C, can produce a chemical composition, mineral composition is different from ordinary Portland cement, but cement 28 days strength A new low temperature cement that can exceed 325 # cement markings.
(2) Used as a coloring agent for glass products
Glass is an amorphous solid that is randomly arranged by the cooling of the melt. When a chromium-containing compound is introduced during the glass melting process, the glass absorbs light of a certain wavelength and exhibits a color corresponding to light passing through a portion of the wavelength. When the glass frit melts at a high temperature, Cr 6+ is unstable and is converted into Cr 3+ , which makes the glass green. Previously, the main green glass colorants were chromite, sodium sulphate, chromium oxide and the like. Since the mid-1960s, Shenyang, Tianjin, and Qingdao began to use chromium slag instead of chrome ore and other chrome products as green glass colorants.
The method requires that the chromium slag particle size is about 0.2 mm and the water content should be less than 10%. Since the chemical composition of the raw materials used in each plant is not the same, the amount of chromium slag added varies. According to the experience of some manufacturers, the amount of chromium slag used as a glass colorant is 3% to 5%. The advantages of using chromium slag instead of other chrome-based materials as green colorants can be summarized as follows:
1 Cr 6+ detoxification is thorough, no secondary pollution, good stability and high degree of resource utilization. However, labor protection should be paid attention to during the process of crushing, transportation and loading and unloading.
2 The glass obtained by replacing the chrome ore powder with chromium slag is bright and the quality is improved.
3 Chromium slag is an active substance that is oxidized and burned at high temperature. It contains a certain amount of flux, which can reduce the melting temperature of the glass frit, shorten the melting time and save energy.
4 Chromium slag is cheap and easy to obtain. In addition to chromium ions, the glass can be colored. Among them, MgO, CaO, Al 2 O 3 and SiO 2 are also useful components of glass. Therefore, the use of chromium slag can reduce the amount of certain raw materials, thereby effectively reducing the production cost of the glass product.
(3) replacing lime for iron making
Iron making requires limestone and dolomite as flux. The chromium slag contains about 50% to 60% of MgO and CaO, and further contains 10% to 20% of Fe 2 O 3 , which are all components required for iron making. A small amount of chromium slag is used instead of slaked lime to mix with iron ore fines and pulverized coal in a sintering furnace, and then sent to a blast furnace for smelting. The high temperature and CO strong reducing atmosphere in the furnace reduces Cr 6+ in the slag to Cr 3+ or even metallic chromium. Into the molten iron, the other ingredients are melted into the slag, and the latter can be used as a cement mixture after water quenching. A small amount of chromium slag has no effect on the quality of sinter and blast furnace production, and the cost of iron making is slightly reduced.
(4) Production of calcium magnesium phosphate fertilizer instead of serpentine
The use of chromium slag instead of serpentine as a flux to produce calcium magnesium phosphate fertilizer, the quality of fertilizer is in line with the third grade of calcium magnesium phosphate fertilizer, through field trials, the fertilizer effect is the same as the calcium magnesium phosphate fertilizer made with serpentine. Since the use of calcium and magnesium in the chromium residue saves the serpentine, the cost of the calcium magnesium phosphate fertilizer is reduced by more than 10%, and the chromium magnesium phosphate fertilizer can treat about 400 kg of chromium residue per ton. In the case of sturdiness, coal or coke is used as a fuel and a reducing agent, so that Cr 6+ in the chromium slag can be reduced to Cr 3+ to achieve the purpose of harmlessness.
(5) Anti-rust pigments
The chromium slag is processed into a calcium iron powder by physical methods, has good anti-rust properties, and has stable quality. It has been applied to anti-rust paints of anti-rust paints such as phenolic, aldol and epoxy. The product has passed the inspection system and is non-toxic. , has been produced in two companies. The main point of the process is to use appropriate measures to accelerate the sedimentation rate of the particles, shorten the production cycle, and pay attention to the selection of packaging materials with good moisture resistance. The method has a large amount of chromium slag, and can consume 1.2 to 1.3 tons of chromium slag per 1 t of calcium iron powder.
(6) Preparation of other chromium products
The chromium slag can be made into Na 2 Cr 2 O 7 , Na 2 S and other products through reduction, separation, leaching, evaporation, acidification, etc.; the chromium slag is mixed with the waste hydrochloric acid, and the antidote and additives are added to prepare the chrome yellow. , gypsum and magnesium oxide.
The chromium slag is leached with water at 95 ° C to obtain a soluble chromium salt, then the pH value is adjusted to 13 with a 15% NaOH solution, and then Cr 3+ is oxidized to Cr 6+ with H 2 O 2 , and a PbAc 2 solution is added to precipitate. PbCrO 4 is formed , and after filtration and drying, the product lead chromate is obtained. The optimum ratio of raw materials in the test was chromium slag: H 2 O 2 (30%): PbAc 2 = 7: 3 :3.2, and lkg chromium slag can produce 0.457 kg of lead chromate.
IV. Conclusions and recommendations
(1) The chromium salt industry is an important basic raw material industry, involving more than 10% of the national economy, and possesses a prominent position in the national economy. The chromium slag is highly toxic, has a large discharge, and has a large stacking area, which seriously pollutes the surrounding environment and affects human health. However, in addition to chromium, the chromium slag contains useful components such as CaO, MgO, Fe 2 O 3 and the like. Utilization is necessary.
(2) The treatment of chromium slag should be based on the consideration of “reduction, harmlessness and resource utilizationâ€. In the production of chromium salt, the clean production process should be actively adopted first, and the negative treatment at the end should be used to minimize the amount of slag production; after the chromium slag is produced, appropriate technology should be adopted for harmless treatment, and the Cr 6+ should be exhausted. It may be reduced to Cr 3+ with very little toxicity, or it may be solidified in cement, fly ash, slag and other substrates so that it will not be dissolved. Finally, the chromium residue which has been harmlessly treated should be adapted to local conditions and comprehensively utilized. Make it a new resource.
(3) Practices at home and abroad have shown that chromium slag has great potential for utilization, and there are many technologies and products that can be developed. The existing level of harmlessness and comprehensive utilization still needs to be improved, and the scope of action must be expanded, especially for small and medium-sized enterprises. The chromium slag comprehensive utilization technology of chromium salt enterprises is still to be developed to completely solve this environmental hazard.
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