Method for preparing sodium chromate by high-temperature separation and recovery of chromium slag

Abstract

The application discloses a method for preparing sodium chromate by high-temperature separation and recovery of chromium slag, and belongs to the technical field of sodium chromate separation and preparation. The process comprises the following steps: adding sodium carbonate with a molar ratio close to that of chromium into the chromium slag and stirring uniformly, then, passing in air and roasting to 1200 DEG C-1400 DEG C and keeping the temperature until the weight is no longer reduced, when the temperature reaches 1200 DEG C, collecting the condensed material volatilized, namely, sodium chromate. Since the volatilization temperature range of sodium chromate is relatively wide, and the sodiumized roasting high-temperature volatilization temperature section 1200 DEG C-1400 DEG C of the chromium-iron slag has only sodium chromate, the process heating parameters can be easily adjusted and industrial control can be realized. Therefore, the method for preparing sodium chromate by high-temperature separation of chromium based on the volatilization phenomenon of sodium chromate combines the advantages of the previous processes, and has the advantages of simple process, environmental protection and remarkable economic benefits.

Description

Technical Field

[0001] This invention relates to the field of sodium chromate preparation technology, and more specifically to a method for preparing sodium chromate by high-temperature separation and recovery of chromium slag. Background Technology

[0002] Currently, most chromium-containing slag is initially stored in an open environment. When chromium-containing slag is landfilled, it needs to be stabilized to prevent chromium contamination of groundwater and soil. However, only a small portion of chromium-containing slag is directly recycled and used as cementitious materials or roadbed materials. The low reactivity and high chromium content of chromium-containing slag limit its utilization. On the other hand, chromium-containing slag has become a potential resource for chromium recovery.

[0003] Currently, methods for recovering chromium from chromium-containing slag generally include physical separation processes, high-temperature reduction processes, plasma smelting, alkaline roasting-water leaching processes, and bioleaching processes. Physical separation methods, including magnetic separation and gravity separation, utilize the different magnetic properties or densities of the chromium-containing phase compared to other non-chromium phases. While a certain amount of chromium can be recovered from chromium-containing slag, the metal recovery rate is low, as small metal droplets and lattice-substituted chromium are difficult to recover through physical separation. Smelting reduction processes are considered a promising steelmaking method. This process is based on the reduction of selected metal oxides using a reducing agent at high temperatures. In this process, valuable metals can be recovered by forming alloys with molten iron. Through the reduction reaction of the chromium-containing phase with the reducing agent (carbon) at high temperatures (1600℃), the metals can be easily separated from the slag, with a metal recovery rate of 98% for various metals and no secondary pollution. However, because carbothermic reduction is an endothermic reaction and the chromium-containing phase has a high melting point, this process has the disadvantages of high energy consumption and large consumption of refractory materials. In plasma melting, a large number of high-energy particles are generated in the plasma reactor. These particles are violently bombarded by high-energy electrons, becoming ionized or excited, leading to a series of complex physical and chemical reactions. Due to these reactions, the metal components melt to form slag and metallic slag. Plasma melting uses a plasma arc to generate heat. The temperature of the plasma arc can reach up to 3000℃. At such high temperatures, metals can melt rapidly, which is beneficial for separation and purification. However, the energy consumption and refractory material consumption of thermal plasma are higher than those of carbothermal reduction processes, making commercial application difficult. The alkaline roasting and water leaching process involves alkaline roasting to around 800℃ to form water-insoluble waste residue and chromates. The water-insoluble waste residue and chromates-containing water are then separated by water leaching. Finally, the chromates are purified by barium salts or evaporation. This method is currently the main way to generate chromates, but it consumes a large amount of water, and there is a risk of heavy metals such as manganese and chromium diffusing into the water during the leaching process, requiring subsequent treatment of large amounts of wastewater. Bioleaching is a hydrometallurgical technique that dissolves metals primarily through inorganic or organic acids produced by microorganisms. Bioleaching offers advantages such as low energy consumption and low investment costs, and is currently widely used for processing pyrite-rich and low-grade copper sulfide ores. However, bioleaching efficiency is very low, making it unsuitable for processing large quantities of chromium-containing waste in a short period, and therefore it currently lacks industrial application value.

[0004] Therefore, how to provide a method for high-temperature separation and recovery of chromium slag to prepare sodium chromate is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present invention has discovered through experiments that sodium carbonate will gradually react with chromium ore to form sodium chromate under high temperature air conditions exceeding 600°C. Based on this, the present invention provides a method for preparing sodium chromate by high-temperature recovery of chromium slag.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing sodium chromate by high-temperature separation and recovery of chromium slag includes the following steps: adding sodium carbonate to chromium slag and stirring evenly; then, calcining with air to 1200℃-1400℃ and holding at that temperature until the weight no longer decreases; when the temperature reaches 1200℃, collecting the volatilized condensate; repeating the above process multiple times to obtain sodium chromate; wherein the ratio of chromium slag to sodium carbonate is configured according to a molar ratio of chromium in chromium slag to sodium in sodium carbonate of 1:2.

[0008] This method allows chromium in chromium slag to be completely converted into sodium chromate. Sodium chromate is formed during the first roasting, but because both chromite and sodium carbonate are powdered solids, the reaction is incomplete. Therefore, multiple mixing and roasting cycles are required to ensure a complete reaction. This method raises the heating temperature to 1200℃-1400℃. The higher temperature improves the fluidity and viscosity of the liquid slag, resulting in good temperature reaction kinetics. Furthermore, TGDSC analysis shows that sodium chromate has begun to evaporate from the liquid phase into the gas phase.

[0009] Sodium chromate has a melting point of 800℃. Existing technologies use a roasting temperature of 1020-1040℃ to produce liquid sodium chromate. After roasting and condensation, water leaches out the water-soluble sodium chromate. Multiple roasting processes ensure a more complete reaction between chromium oxide and sodium carbonate, improving reaction efficiency. This invention, by increasing the roasting temperature, enhances reaction kinetics. As sodium chromate vaporizes and volatilizes at high temperatures, it shifts from the roasting system to the volatilization system, shifting the chemical equilibrium in the roasting system towards sodium chromate, resulting in even higher reaction efficiency. Traditional methods use low roasting temperatures and require multiple roasting cycles after leachation to achieve a complete reaction, while this high-temperature roasting separation method only requires one roasting. Traditional separation methods are solid-solid separations requiring water as a solvent, while this method is a gas-solid high-temperature separation that does not require water. Therefore, this method eliminates the need to consider water pollution from sodium chromate, resulting in lower production costs and greater environmental protection.

[0010] Preferably, in the above technical solution, an electrostatic precipitator is used to collect the volatilized condensate.

[0011] Preferably, in the above technical solution, when air is introduced, the ratio of chromium slag to sodium carbonate is 166g:5L.

[0012] As can be seen from the above technical solution, compared with the prior art, the present invention provides a method for preparing sodium chromate by high-temperature recovery of chromium slag, achieving the following technical effects:

[0013] 1) This method not only separated chromium from chromium-containing iron slag, but also obtained sodium chromate as a reaction byproduct, thus producing a high-value-added product during the pollution control process;

[0014] 2) This method only requires energy costs for heating slag. Since chromium separation can be achieved by heating at 1400℃ and holding for a certain time, it is not necessary to reach the carbothermic reaction temperature of 1600℃. Therefore, the energy consumption and refractory material costs are lower than those of the carbothermic reaction.

[0015] 3) Unlike the traditional process of "roasting water leaching + wet separation of chromium in water" which consumes a lot of acid, alkali and water resources and has subsequent wastewater and waste acid and alkali treatment costs, this method does not consume chemical reagents and acid and alkali or water from heating to product separation and treatment, and will not cause secondary pollution to the environment.

[0016] 4) Since sodium chromate has a wide volatilization temperature range, and the only escaping phase in the high-temperature volatilization temperature range of 1200℃-1400℃ during sodium roasting of ferrochrome slag is sodium chromate, it is easy to adjust the process heating parameters and achieve industrial control. Therefore, the high-temperature separation of chromium to prepare sodium chromate based on the phenomenon of sodium chromate volatilization combines the advantages of previous processes and has the advantages of simple process, environmental protection and significant economic benefits. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0018] Figure 1 The attached figure shows the XRD results of chromite + sodium carbonate roasted at 1200 degrees Celsius in air according to the present invention.

[0019] Figure 2 The attached figure shows the TGDSC curve of chromite + sodium carbonate under high temperature roasting in air.

[0020] Figure 3 The attached figure shows the SEM-EDS results of the condensate;

[0021] Figure 4 The attached figure shows a comparison of the Raman spectra of the volatiles collected at 1400℃ and the sodium chromate standard.

[0022] Figure 5 The attached diagram shows the process flow chart for chromium slag treatment. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1

[0025] A mixed sample of 200 mg was weighed and heated from room temperature to 1200 °C at a rate of 20 °C per minute in air under a simultaneous thermal analyzer. After cooling, the sample was collected and subjected to XRD analysis. XRD results revealed that under high-temperature air conditions exceeding 600 °C, sodium carbonate gradually reacts with chromite to form sodium chromate. Figure 1 As shown, the main phase is sodium chromate.

[0026] Based on XRD results, sodium chromate was separated. Sodium chromate has a melting point of 792℃. When the temperature exceeds 1200℃, the vapor pressure of sodium chromate changes, and sodium chromate escapes as a gas. Figure 2 The green curve represents the thermogravimetric change curve. It can be seen that after 1200℃, the curve declines, indicating a decrease in weight on the thermogravimetric balance. The weight loss in this experiment is approximately 8%. Figure 2 As shown, the process is as follows: Origin: 20mg of sample is weighed in a synchronous thermal analyzer and heated to 1400℃ in air atmosphere at a rate of 20℃ per minute. The real-time weight change and heat change of the sample during the heating process are tested.

[0027] First, sodium carbonate with a similar molar ratio to chromium was added to the chromium slag and stirred evenly. The ratio of chromium slag to sodium carbonate was configured according to a molar ratio of chromium in the chromium slag to sodium in the sodium carbonate of 1:2. The sample composition was tested by XRF as shown in Table 1. Then, air was introduced and the mixture was calcined to 1200℃-1400℃ and held until the weight no longer decreased. When air was introduced, the ratio of chromium slag to sodium carbonate was 166g:5L. When the temperature reached 1200℃, an electrostatic precipitator was turned on to collect the volatilized condensate. Finally, the precipitator collected nano-sized sodium chromate, and the slag was chromium-free iron slag, realizing the separation and purification of chromium from chromium-iron slag.

[0028] Table 1. Composition of chromite and sodium chromite (Wt%)

[0029]

[0030] By placing a silicon wafer at the exhaust port of the simultaneous thermal analysis, volatiles emitted during the thermal analysis process are deposited on the wafer. The wafer is then placed in a SEM (Scanning Electron Microscopy) system to analyze the microstructure and energy dispersive spectroscopy (EDS) spectra of the volatiles. Since the emitted gas condenses at the exhaust port, forming condensate, SEM-EDS analysis of the condensate revealed that the main phase is nano-sized sodium chromate, such as... Figure 3 As shown; SEM-EDS analysis of the condensate revealed that the main phase was nano-sized sodium chromate, and the only volatile condensate elements were sodium, chromium, and oxygen. Raman spectroscopy analysis showed that its structure was consistent with that of standard sodium chromate. The sample preparation method for Raman spectroscopy was the same as that for SEM. Figure 4 As shown in the diagram, sodium chromate will continue to escape from the slag as the temperature continues to rise and the holding time extends. The mechanism of the entire reaction process is illustrated in the diagram below. Figure 5 As shown.

[0031] The collected volatile samples were tested using ICPOES, and the purity of sodium chromate was 98.5%. The purification efficiency at high temperature was 100%, indicating that high temperature can completely convert chromium oxide into sodium chromate.

[0032] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0033] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing sodium chromate by high-temperature separation and recovery of chromium slag, characterized in that, The process includes: adding sodium carbonate to chromium slag and stirring evenly; then, calcining with air to 1200℃-1400℃ and holding at that temperature until the weight no longer decreases; when the temperature reaches 1200℃, collecting the volatilized condensate; repeating the above process multiple times to obtain sodium chromate; wherein, the ratio of chromium slag to sodium carbonate is configured according to a molar ratio of chromium in chromium slag to sodium in sodium carbonate of 1:

2.

2. The method for preparing sodium chromate by high-temperature separation and recovery of chromium slag according to claim 1, characterized in that, Electrostatic precipitators are used to collect the condensate that has evaporated.

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