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1 Foreword <br> The chromium slag emitted by the chromium salt industry causes serious environmental pollution and waste of resources. At present, the country has accumulated about 2.5 million tons, and is increasing at a rate of more than 200,000 tons per year. It has been on groundwater, rivers and sea areas. It poses a serious threat to humans and various organisms. The treatment of chromium slag abroad is mostly after detoxification treatment of hexavalent chromium or landfill. The Japanese electrician company Tokushima Chemical Plant will treat toxic chromium slag and sulphite papermaking waste liquid. After being reduced and calcined in a rotary kiln, it is then stored or landfilled. The Deshan Chemical Plant in Japan mixes the chromium slag with a certain proportion of clay to make the building aggregate. The two large chromium salt plants in the United States basically detoxify the chromium residue. After the use of reclamation. In addition, there are a small number of ceramics, glass colorants, etc. and solidification treatment with cement [1] .
 "August" period, steel metallurgy portion Institute of sintered ore manufactured by chromium slag and pig iron chromium technology has been studied [1], with varying degrees of progress has been made, but there are some defects, such as large investment The cost is high, and enterprises are unable to withstand the enormous pressure of end-of-pipe governance. At present, there is no economic and reasonable plan for fundamental management at home and abroad. Many chromium salt plants are forced to stop production due to pollution problems. Chromium slag treatment has become a necessity for national economic and social development. Solving puzzles.
The new process for the clean production of chromium salts produced by continuous liquid phase chromite ore developed by the Institute of Chemical Metallurgy, Chinese Academy of Sciences, has successfully reduced the amount of chromium slag to 1/4 of the traditional process. New ecological, high specific surface area porous chromium residue The internal magnesium and iron have higher reactivity, which provides conditions for further resource utilization [2] . This study proposes a new preparation of ferrite by carbonation dissociation chromium slag-magnesium extraction-dry-wet method synergistic detoxification-rich iron slag The process of reducing chromium slag to zero emissions lays the foundation for the final goal of zero discharge and comprehensive utilization of resources in the whole process of chromium salt clean production. No new waste slag, waste water and waste gas are discharged into the environment. It is a typical green chemical process [3] .
2 Reaction mechanism  The chromium slag produced by the new process of liquid phase chromite ore clean production mainly contains crystalline magnesium hydroxide and amorphous iron oxide, which has the characteristics of small particle size, large specific surface area and high reactivity of magnesium hydroxide. The reaction of chromium slag carbonation to extract magnesium belongs to the gas-liquid-solid three-phase reaction process, and the reaction proceeds on the inner and outer surfaces of the chromium slag particles. During the carbonation process, the reaction mechanism of Mg(OH) 2 can be used. )~(4) means [4] : 
Since formulas (3) and (4) are rapid reactions, the macroscopic reaction is determined by the dissolution rate and hydration rate of CO 2 , ie, formulas (1) and (2), while being subject to the internal pores of the reactants and products in the porous particles. And the control of the diffusion process in the outer boundary layer. Increasing the partial pressure of CO 2 and the gas-liquid contact area, increasing the turbulent flow of the liquid phase, and reducing the particle size of the chromium slag particles will enhance the reaction rate. The product Mg under normal pressure (HCO 3 ) 2 decomposes according to formula (5) above 26 ° C, reducing the recovery of magnesium. Increasing the partial pressure of CO 2 will increase the concentration of HCO 3 and the equilibrium solubility of Mg(HCO 3 ) 2 , and Decomposition temperature of Mg(HCO 3 ) 2 . When pyrolysis of carbonation immersion zone liquid, Mg(HCO 3 ) 2 is decomposed according to formulas (6) and (7).
The unreaction shrinking core model (USCM) is applied to the process of carbon extraction from chromium slag. It is assumed that the reaction speed is very fast and the liquid film resistance is negligible, k s , k f D e , and the process is gray layer. In the case of diffusion control, the reaction time can be finally expressed as [5]
Where C A0 is the concentration of the fluid reactant, C B0 is the molar density of the solid reactant, D e is the effective diffusion coefficient in the gray layer, R is the diameter of the particle, t is the reaction time, ξc=r c /R For the dimensionless diameter, X b =1-(r c /R)3=1-ξ 3 c is the conversion rate of the solid reactant, t*=R 2 C B0 /(3D e C A0 ) is the complete conversion time. It is assumed that the concentration of the reactant HCO 3 - during the reaction is an equilibrium concentration, wherein Ka is the first dissociation constant of carbonic acid, P is the partial pressure of carbon dioxide, and H is the Henry's constant of CO 2 dissolved in water. Substituting formula (9) Equation (8) is available
 understood from the formula (10), when it is known C B0, D e, H, K a and when R, the reaction kinetics can be estimated by the reaction kinetics data is plotted (1-3ξ 2 c + 2ξ 3 c ). - The t-graph can estimate the effective diffusion coefficient in the particle and investigate the effect of the partial pressure of the reactant.
3 Experimental equipment and methods The experimental equipment for the atmospheric chrome slag carbonation leaching process consists of a bubble bed reactor with a glass sintered plate and a jacket (70 mm × 250 mm), a gas supply system and a temperature control system. The carbon dioxide gas and nitrogen gas are metered into the gas distribution chamber and the reactor through the gas rotameter, and the partial pressure is adjusted by controlling the flow rate of the CO 2 . The circulating water from the super constant temperature tank is returned through the jacket, and the temperature control precision is ± 0.5 ° C.
超声 A certain mass ratio of chromium slag and tap water are ultrasonically mixed and added to the reactor, and an appropriate amount of FeSO 4 reducing agent is added. During the reaction, the sample is intermittently sampled and the pH value is measured. After the reaction is completed, the suspension is filtered. After liquid pyrolysis, light magnesium carbonate can be prepared, and magnesium carbonate is calcined at a high temperature to obtain magnesium oxide. The iron-rich chromium residue after magnesium removal is dried and ground, and then magnetic ferrite is prepared by hydrogen reduction. The liquid phase is determined by atomic absorption. Magnesium in the solid phase and iron in the solid phase.
4 Experimental results and discussion 4.1 Main components of chromium slag
X The typical composition of chromium slag is 14.2% Mg, 40% Fe, 0.59% Cr, 0.35% Si, 0.60% Al and 1.41%. The X-ray diffraction pattern of ΣNa. chromium slag shows that the magnesium in the chromium slag is mainly crystalline. the Mg (OH) 2 is present, aluminum oxide and silicon oxide mainly crystalline aluminosilicate (1.08Na 2 O · Al 2 O 3 · 1.68SiO 2 · 1.81H 2 O) is present, in an amorphous iron The presence of iron oxide.
Due to the large specific surface area of ​​the chromium slag, magnesium hydroxide has high reactivity and is easy to react in a weakly acidic medium during carbonation, while iron oxide in the slag does not react, and hexavalent chromium is reduced to three by ferrous sulfate. The valence chromium enters the chromium slag in the form of Cr(OH) 3 , which not only ensures the high purity of the product magnesium carbonate, but also increases the iron content in the slag phase. During the carbonation process, the free NaOH in the chromium slag reacts into sodium carbonate. After entering the liquid phase, the filtration performance of the treated chromium slag is obviously improved. The specific surface area of ​​the chromium slag before the carbonation leaching is 33.9 m 2 /g and the surface immersion is 69.4 m 2 /g.
4.2 Analysis of the results of magnesium carbonate extraction experiments
实际 The main source of CO 2 in actual industrial production is flue gas. The volume concentration of CO 2 is 13%-18%. To obtain higher purity CO 2 gas, complex carbon dioxide refining equipment must be added. Therefore, to save The CO 2 refining process is intended to directly use the flue gas as a reaction gas, simplifying the process flow and reducing equipment investment. Now the atmospheric pressure operation is used to simulate the pressure operation process by changing the partial pressure of CO 2 in the reaction gas. The bubbling bed reactor is used to strengthen the gas-liquid-solid three-phase reaction process.
4.2.1 Effect of reaction temperature on magnesium leaching rate Figure 1 shows the relationship between magnesium leaching rate and reaction temperature. It can be seen that low reaction temperature contributes to higher leaching rate. Carbonate leaching from chromium slag The mechanism shows that the reaction temperature has an effect on the equilibrium solubility of CO 2 in water, the effective diffusion coefficient of the reactants in the particles, the surface reaction rate and the decomposition rate of magnesium bicarbonate, but the decomposition rate of magnesium carbonate is more sensitive to temperature. The low reaction temperature is more suitable for atmospheric pressure operation.
4.2.2 Effect of partial pressure of carbon dioxide on magnesium leaching rate  The partial pressure of CO 2 has a great influence on the reaction rate. The high partial pressure is fast and the equilibrium concentration is high. The experimental data are processed by formula (10). See Figure 2. For a pure CO 2 reaction gas with P = 0.1 MPa, the linearity of the data correlation is very good. However, the CO 2 partial pressure is 0.05 MPa and 0.013 MPa of reaction gas, and the associated line has a distinct inflection point, indicating that in the initial stage of the reaction The concentration of the reaction gas has little effect on the reaction rate. When the concentration of the product Mg(HCO 3 ) 2 is high, the partial pressure of CO 2 is lowered, which not only reduces the equilibrium concentration of the product, but also the effective diffusion coefficient and liquid in the particle. The membrane mass transfer coefficient has a certain influence.
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| Figure 1 Effect of reaction temperature on magnesium leaching rate Fig.1Effect of reaction temperature on the yield of magnesium | Figure 2 Calculation results of the nuclear contraction model Fig.2 Calculated results of USCM model |
4.2.3 Effect of liquid-solid ratio on magnesium leaching rate 液 The liquid-solid ratio of water and chromium slag is an important factor affecting the leaching rate of magnesium in chromium slag, and it also determines the energy consumption index of the whole process. The main pressure leaching experiment is mainly The liquid-solid ratio was selected as 10:1 and 20:1, and the effect on the leaching rate of magnesium was studied. The results are shown in Fig. 3. As can be seen from Fig. 3, the leaching rate of magnesium is faster when L: S=20:1. The highest concentration of phase Cmax=3.81 g/L. L:S=10:1, Cmax=5.60 g/L.
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| Figure 3 Effect of liquid-solid ratio on magnesium leaching rate Fig.3 Effect of liquid to residue ratio on the yield of magnesium | Figure 4 Effect of additives and ultrasonic fields on magnesium leaching rate Fig.4 Effect of addition and ultraso nic field on the yield of Sichuan |
4.2.4 Effect of ammonium bicarbonate on magnesium leaching rate  It can be seen from Fig. 4 that the addition of ammonium bicarbonate to the solution is not conducive to the leaching of magnesium in the chromium slag, which not only increases the pH of the liquid phase, but also the leaching rate of magnesium. The pH value of the leachate is the key factor determining the magnesium leaching rate. After adding 0.5 mol/L ammonium bicarbonate to the liquid phase, the average pH of the liquid phase increases from 7.56 to 7.94, and the ionic strength increases. , causing the leaching rate of magnesium to decrease. Without ammonium bicarbonate, the reaction gas with a partial pressure of CO  2 of 0.05 MPa, the concentration of liquid magnesium in the reaction end point is 4.48 g / L, the time is 3.5 h, the experimental results are very good .
4.2.5 Effect of ultrasonic wave on magnesium leaching rate SEM analysis and BET measurement results of chromium slag particles show that the chromium slag particles have a large specific surface area, and there are a large number of pores inside and on the surface, which determines the magnesium in the chromium slag. The leaching rate is controlled by the mass transfer diffusion in the particle, which increases the turbulence in the reactor, which will strengthen the mass transfer process and increase the macroscopic reaction rate. We use ultrasonic technology to strengthen the reaction and mass transfer process, and get better experimental results. In the ultrasonic field, the reaction time required to reach the highest magnesium concentration is 1/6 of the original, and the maximum concentration of liquid magnesium is also increased from 4.48 g/L to 4.65 g/L. It also accelerates the decomposition of the product magnesium hydrogencarbonate. The reaction time is short (Figure 4).
4.3 Characterization of magnesium and iron products SEM image of light magnesium carbonate (Fig. 5) shows that the crystal has no fixed shape, its particle size is about 10 μm, and the purity is over 99%. X-ray diffraction pattern of magnesium oxide (Fig. 6) The middle curve a) shows that the crystal has a good crystal structure and its purity is more than 99.5%. Experiments on the extraction of magnesium from chromium slag show that for the chromium slag of low magnesium and high iron, the grade of iron after magnesium extraction is as high as 54%. Further reduction roasting can not only reduce Cr 6+ to Cr 3+ and seal it in the spinel structure of ferrite, but also prepare ferromagnetic magnetic ferrite, and then magnetically select it as hard magnetic material. Such as magnetic flaw detection powder, magnetic iron core, etc., and then convert the toxic chromium residue into a product with higher added value. In this paper, only preliminary research work has been done in this place, in the reducing atmosphere of hydrogen and water vapor, 350 ° C After 1 h of reaction, a magnetic ferrite with higher magnetic properties can be obtained. The XRD pattern of the synthesized product is shown in curve b of Figure 6, and the main component is triiron tetroxide.
After the hydrazine sulphate wet reduction and hydrogen dry reduction treatment, the toxic chromium in the chromium slag is sealed in the lattice of the ferrite to the maximum extent, and the magnetic ferrite obtained after the reduction roasting is standard. Acid-base leaching experiments showed that no soluble chromium leaching showed good detoxification after reduction roasting.
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| Figure 5 SEM image of light magnesium carbonate Fig.5 SEM image of carbo nate magnesium ligh | Figure 6 X-ray diffraction pattern of magnesium oxide and ferrite Fig.6 XRD image of magnesium oxide and magnetic ferrite |
5 Conclusions <br> Reaction kinetics of atmospheric pressure carbonation leaching magnesium show that increasing the partial pressure and liquid-solid ratio of CO 2 and lowering the reaction temperature will increase the leaching rate of magnesium; the addition of ultrasonic field greatly enhances the leaching rate and accelerates The decomposition rate of magnesium carbonate is suitable for short-time operation; the addition of ammonium bicarbonate in aqueous solution reduces the leaching rate of magnesium. The purification experiment of magnesium shows that high-purity light magnesium carbonate and magnesium oxide products can be prepared; After the magnesium chromium residue is reduced and calcined and magnetically selected, magnetic ferrite can be prepared as a hard magnetic material.
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