RU2261844C2 - Method for preparing light grades of pure magnesia from serpentinite - Google Patents

Abstract

FIELD: chemical industry, inorganic chemistry, chemical technology.

SUBSTANCE: invention relates to methods for preparing magnesia light grades from serpentinite. Method for preparing pure magnesia light grades from serpentinite involves its dissolving in sulfuric acid, cooling the prepared mixture up to crystallization temperature, washing out the crystallized mixture with water, separation of undissolved residue from prepared sulfate solution followed by successive treatment of prepared filtrate from metal impurities by their precipitation as hydroxides, separation of precipitate, treatment of prepared solution with alkaline reagent, separation of magnesium-containing precipitate, washing out precipitate with water and thermal treatment. In precipitation of magnesium-containing compound from sulfate solution previously purified from metal impurities method involves using 7-15% caustic soda solution and magnesium-containing compound precipitate is steamed out in boiling solution for 15-30 min up to the complete decomposition of soluble magnesium hydrocarbonates. Invention provides solving the problem for preparing light grades of pure magnesia from raw containing silicon oxides by economy and ecologically pure method.

EFFECT: improved preparing method.

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Description

The invention relates to the chemical industry, and in particular to methods for producing light grades of pure magnesia with a bulk density of 0.8-1.8 g / cm ³ from natural raw materials, in particular from serpentinite.

A known method of producing magnesium oxide from serpentinite and serpentinite dump waste in chrysotile asbestos deposits [1], including the decomposition of serpentinite with sulfuric acid with a concentration in the range of 10-50%, followed by removal of hydroxide impurities at a solution pH of 6-8.5, carbonization of pure magnesium sulfate solution at a solution pH of 10-12 and thermal decomposition of the formed magnesium carbonate to its oxide. In order to intensify the process, decomposition of serpentinite with sulfuric acid is carried out at 90-100 ° C. The thermal decomposition of the formed magnesium carbonate is carried out at a temperature of 900 ° C. This method is environmentally friendly and waste-free, since the process of processing serpentinite is carried out in a closed cycle. However, according to this method, it is difficult to obtain a product with a high degree of purity. When implementing this method, an increased amount of electric energy is spent on filtering a magnesium-containing compound formed at the stage of carbonization of magnesium sulfate, which consists largely of hard-to-filter magnesium hydroxide. When carbonizing a magnesium sulfate solution at a solution pH of 10-12 in the absence of boiling with steaming the precipitate, along with magnesium carbonate and magnesium hydroxide, easily soluble magnesium bicarbonate is formed, which reduces the yield of the final product due to the loss of magnesium with the solution during filtering of the precipitate. The magnesium-containing compound formed at the stage of carbonization of magnesium sulfate, consisting of magnesium carbonate and, to a large extent, magnesium hydroxide, cannot be completely washed with water from impurities of an alkaline agent formed at the stage of carbonization of a solution of magnesium sulfate, since magnesium hydroxide has significant sorption properties. Therefore, it is not possible to obtain the final product - magnesium oxide with a high degree of purity.

A known method of producing pure magnesium oxide from natural serpentinite [2], the closest in technical essence to the claimed method and selected by the authors for the prototype.

The method includes grinding natural serpentinite to a particle size of not more than 0.5 mm, dissolving in sulfuric acid with a concentration of at least 20%, separating the insoluble residue from the solution by cooling the mixture formed at the stage of dissolving the raw material to a crystallization state, followed by washing the crystallized mixture with water in such amount, so that the concentration of magnesium sulfate salt in the solution was 290-330 g / l, cleaning the solution from impurity metals by precipitating them in the form of hydroxides with a 40-50% sodium hydroxide solution at pH p solution 5-7 and separating the precipitate from the solution, precipitation of magnesium hydroxide with 40-50% sodium hydroxide solution at a solution pH of 10-12 when the solution is heated to 30-40 ° C, washing the target product in two stages to the concentration of sodium sulfate in wash water 0.5-4 g / l, heat treatment of the dried material at 850 ° C for 40 minutes, the final washing of the product with water and filtering the precipitate.

However, to obtain the final product - magnesium oxide with a basic substance content of at least 95% according to this technology, after heat treatment of magnesium oxide at a temperature of 850 ° C, an additional washing operation is required from the residual sodium sulfate content, which is caused by the fact that when washing magnesium hydroxide from the residual content sodium sulfate in the previous stage of the process cannot completely remove the impurity of sodium sulfate due to the fact that magnesium hydroxide has sorbing properties and sorb well Fat-ions. Magnesium hydroxide is a hard-to-filter precipitate, therefore, the heat treatment of the precipitate of magnesium hydroxide after filtration comes with significant humidity, which requires additional energy for its heat treatment. After washing the magnesium oxide from the residual content of sodium sulfate, the final product is subjected to additional drying. An additional stage of washing and drying the final product requires additional energy costs, which makes the process energy intensive and increases the cost of the final product. Due to the fact that the magnesium hydroxide precipitate adsorbs and retains sulfate ions well, and the purity of the final product, magnesium oxide, largely depends on the efficiency of washing magnesium hydroxide from sulfate ions at the previous stage of the process, an increased amount of water is required to obtain pure magnesium oxide washing, which increases the cost of the final product and increases the amount of wastewater.

The proposed method solves the problem of creating an economical and environmentally friendly method for producing light grades of pure magnesia with a basic substance content of 97-99% from natural serpentinite. The essence of the method lies in the fact that all the previous stages of the process - grinding of natural serpentinite to a particle size of not more than 0.5 mm, dissolution in sulfuric acid with a concentration of at least 20%, separation of the insoluble residue by cooling the mixture formed at the stage of dissolution of the raw material to a crystallization state, followed by washing the crystallized mixture with water in such an amount that the magnesium sulfate salt content in the solution is 290-330 g / l; purification of the solution from impurity metals by precipitation in the form of hydroxide rows 40-50% sodium hydroxide solution at pH = 7 and 5 separating the precipitate from the solution is carried out as described in the prototype [2], and precipitation of the magnesium compound solution is performed 7-15% soda ash solution at pH 10-12. The suspension of precipitated precipitate is heated with boiling steam to a boil and the precipitate is evaporated for 15-30 minutes to destroy soluble magnesium bicarbonates formed during precipitation of the magnesium-containing precipitate. The composition of the steamed precipitate of a magnesium-containing compound corresponds to the chemical formula 3MgCO ₃ × Mg (OH) ₂ × 3H ₂ O. The precipitate is filtered off, washed and dried at a temperature of 200-300 ° C to a moisture content of not more than 0.5% to obtain a light grade of pure magnesia with bulk weighing 160-180 kg / m ³ , the chemical composition corresponding to the above chemical formula. When calcining dried magnesia at a temperature of 500-600 ° C, light pure magnesium oxide with a bulk density of 80-100 kg / m ³ with the highest activity of 100 m ² / g is obtained.

New in the method is that the precipitation of a magnesium-containing compound from a solution of magnesium sulfate is carried out by soda ash with a concentration of the main substance in the solution of 7-15%, and the suspension of the precipitated precipitate is heated to boiling, for example, with hot steam, and the precipitate is evaporated for 15-30 minutes until the complete destruction of soluble magnesium bicarbonates formed during the deposition of the magnesium-containing precipitate. Fine crystalline magnesia formed after steaming with boiling, consisting mainly of magnesium carbonate, is easily filtered, practically does not have sorption properties, it is easily washed from the residual sodium sulfate content with a minimum amount of water. Due to the fact that fine crystalline magnesia does not have sorption properties and does not retain free moisture in the volume, the magnesia filtration process, which takes place with low energy costs, is carried out to a final moisture content of 10-15% and the subsequent drying process is not energy intensive. When boiling a suspension of the precipitated precipitate of a magnesium-containing compound and steaming the precipitate, for example, with hot steam, the decomposition of soluble magnesium bicarbonates does not completely decompose for less than 15 minutes and the yield of the final product — light magnesia or, during its heat treatment, magnesium oxide decreases, since soluble magnesium bicarbonates may be lost with filtrate when filtering a precipitate of magnesia. When boiling a suspension of precipitated precipitate of a magnesium-containing compound and steaming the precipitate, for example, with hot steam, for more than 30 minutes, with the complete decomposition of magnesium bicarbonates, an additional amount of electricity is wasted. When precipitating magnesium sulfate from a solution with a concentration of 290-330 g / l of a magnesium-containing compound at a solution pH of 10-12 with a solution of soda ash with a concentration of less than 7%, the process is not economical due to the formation of an increased amount of solution at the stage of magnesium deposition, for example, 18, 6 l per 1 kg of the initial serpentinite containing sodium sulfate salt with a concentration of 70 g / l. After filtering the precipitate, steamed, for example, with sharp steam, the solution containing the dissolved sodium sulfate salt must be disposed of. The most economical method of disposal, allowing to obtain an additional product, a dry salt of sodium sulfate, is to dry a solution of sodium sulfate salt in a spray dryer or to evaporate the solution. The dry salt of sodium sulfate is a valuable raw material and is used in the glass industry and in the chemical industry. The process of drying a sodium sulfate solution in a spray dryer or evaporating a sodium sulfate solution proceeds economically when the concentration of sodium sulfate in the solution is more than 70 g / l, while the costs of the drying or evaporation process do not significantly increase the cost of the by-product - dry sodium sulfate salt. The upper limit of the concentration of soda ash solution is 15% due to the fact that with a higher concentration of soda ash solution, for example 20%, additional energy costs for the process are required. When the concentration of the soda ash solution is more than 15%, for example, 20%, the total volume of the working solution during the deposition of the magnesium-containing compound from the magnesium sulfate solution at a concentration of the latter in the solution of 290-330 g / l will be 5.76 l per 1 kg of the initial serpentinite, while the concentration of the dry salt of sodium sulfate in the solution will be 138 g / l. Due to the fact that the solubility limit of the dry sodium sulfate salt in the solution at 15 ° C is 117 g / l, at the temperature of the working solution after precipitation of the magnesium-containing compound with a soda solution with a concentration of more than 15% from a magnesium sulfate solution of 15 ° C-18 ° C, the salt concentration sodium sulfate in the solution will be greater than the solubility limit, while crystallization of the sodium sulfate salt in the solution is possible. Precipitated crystals of sodium sulfate salt will not allow to qualitatively separate by filtration a solution of a magnesium sulfate salt from a precipitate of a magnesium-containing compound, which will lead to contamination of the final product.

Examples of the method.

Example 1

As a raw material for obtaining light grades of pure magnesia, an aqueous suspension of natural serpentinite was used with a content, wt.%: Silicon dioxide - 36, magnesium oxide - 34, iron oxides of divalent and trivalent and other metals: manganese, aluminum, titanium, nickel, chromium - 6, water - 13, impurities removed by calcination - 11.

1 kg of raw material was dissolved in technical sulfuric acid at a concentration of 40%. The process was accompanied by heat generation with heating of the mixture to 80 ° C-90 ° C. Dissolution was carried out within 2 hours. After one hour from the start of the dissolution process, an oxidizing agent was added to the solution — 15 ml of a 36% hydrogen peroxide solution. The addition of an oxidizing agent was carried out with the aim of converting Fe ^{+ 2} ferric iron present in the feed to ferric Fe ⁺³ , capable of forming a precipitated compound at the stage of purification from metal impurities of a magnesium-containing compound. The addition of this oxidizing agent also provides a change in the valency of some other impurity metals to form precipitated compounds, for example, the conversion of divalent manganese Mn ⁺² to tetravalent Mn ⁺⁴ . The introduction of hydrogen peroxide earlier than one hour from the beginning of the dissolution of the feedstock in acid is undesirable, as this can disrupt the leaching process.

The mixture obtained in the dissolution step was cooled to a crystallization temperature of 20 ° C, after which the crystallized mixture was washed with water. Water for washing the crystallized mixture was taken in an amount of 3.5 l to obtain a concentration of magnesium sulfate in an aqueous solution of 290 g / l. The insoluble residue was separated by filtration. The separated residue was a by-product resulting from the implementation of the method is silica gel. The separated filtrate was a solution of magnesium sulfate and impurity metal sulfates. 300 ml of sodium hydroxide solution with a concentration of 40% was added to the filtrate to a solution pH of 5-7, while impurity metal compounds precipitated as their hydroxides. The precipitate formed was filtered, after which 13.25 ml of soda ash solution with a dry salt concentration of 7% to a solution pH of 10-12 was added to the filtrate, and a magnesium-containing compound consisting of magnesium carbonate and magnesium hydroxide precipitated. The precipitated solution was heated to boiling and boiled for 15 minutes with steaming the precipitate. The precipitate was filtered. The filtrate containing sodium sulfate with a concentration of 70 g / l was sent to a spray dryer to obtain an additional product - dry sodium sulfate with a basic substance content of 97%. The filtered precipitate was washed with water to a concentration of sodium sulfate in wastewater of not more than 4 g / l. Thermal drying of the precipitate was carried out at a temperature of 200 ° C. The sediment was dried to a moisture content of not more than 0.5%. Got light magnesia with a bulk density of 180 kg / m ³ . After that, heat treatment of dried magnesia was carried out at 500 ° C for 40 minutes. The indicated time is sufficient for the process of complete thermal decomposition of magnesium carbonate and magnesium hydroxide, which are part of light magnesia. As a result, the target product was obtained with a content, wt.%: Magnesium oxide — not less than 97.5, Fe — 0.254, MnO — 0.085 (total oxides of iron and other metals except magnesium oxide — 0.339). The bulk density of magnesium oxide was 80 kg / m ³ .

Example 2

Light grades of pure magnesia were prepared as described in Example 1. The raw materials were dissolved in sulfuric acid at a concentration of 50%. The mixture obtained in the dissolution step was cooled to its crystallization temperature of 20 ° C, after which the crystallized mixture was washed with water. Water for washing was taken in an amount of 3.1 l to obtain a concentration of magnesium sulfate in an aqueous solution of 330 g / l. After separation of the insoluble precipitate, 200 ml of sodium hydroxide solution with a concentration of 50% was added to the filtrate to a solution pH of 5-7, and impurity metal compounds precipitated in the form of their hydroxides. The precipitate formed was filtered, after which 4.6 l of soda ash solution with a concentration of 15% was added to the filtrate to a solution pH of 10-12, and a magnesium-containing compound consisting of magnesium carbonate and magnesium hydroxide precipitated. The precipitated solution was heated to boiling and boiled for 30 minutes with steaming the precipitate. The precipitate was filtered. The filtrate containing sodium sulfate with a concentration of 110 g / l was sent to a spray dryer to obtain a dry salt of sodium sulfate. The precipitate after separation of the filtrate was washed with water. Thermal drying of the precipitate was carried out at a temperature of 350 ° C. Got light magnesia with a bulk density of 160 kg / m ³ . After that, heat treatment of dried magnesia was carried out at a temperature of 600 ° C for 40 minutes. As a result, the target product was obtained with a content, wt.%: Magnesium oxide — not less than 99%, Fe — 0.115, MnO — 0.059 (total oxides of iron and other metals except magnesium oxide — 0.174). The bulk density of magnesium oxide was 60 kg / m ³ .

Example 3

Light grades of pure magnesia were prepared as described in Examples 1, 2. The raw materials were dissolved in sulfuric acid at a concentration of 20%. The process was accompanied by the release of heat with heating of the mixture to 60 ° C. During dissolution, the pulp was additionally heated to 90 ° C. The mixture obtained in the dissolution step was cooled to a crystallization temperature of 20 ° C, after which the crystallized mixture was washed with water. Water for washing the crystallized mixture was taken in an amount of 3.25 L to obtain a concentration of magnesium sulfate salt in an aqueous solution of 310 g / L. After separation of the insoluble precipitate, 266 ml of sodium hydroxide solution with a concentration of 45% was added to the filtrate to a solution pH of 5-7, while impurity metal compounds precipitated in the form of their hydroxides. The precipitate formed was filtered, after which 6.6 L of soda ash solution with a concentration of 19% was added to the filtrate to a solution pH of 10-12, and a magnesium-containing compound consisting of magnesium carbonate and magnesium hydroxide precipitated. The precipitated solution was heated to boiling for 25 minutes with steaming the precipitate. The precipitate was filtered. The filtrate containing 105 g / L sodium sulfate was sent to a spray dryer to obtain dry anhydrous sodium sulfate. The precipitate was washed with water. Thermal drying of the precipitate was carried out at a temperature of 300 ° C. Got light magnesia with a bulk density of 170 kg / m ³ . After that, heat treatment of dried magnesia was carried out at 550 ° C for 40 minutes.

As a result, we obtained the target product with a content, wt.%: Magnesium oxide — not less than 98%, Fe — 0.1, MnO — 0.14 (total oxides of iron and other metals except magnesium oxide — 0.24). The bulk density of magnesium oxide was 70 kg / m ³ .

Information sources.

1. RF patent No. 2038301, C 01 F 5/06, publ. 1995 year

2. RF patent No. 2209780, C 02 F 5/02, publ. 2003 year

Claims (1)

A method of obtaining light grades of pure magnesia from serpentinite, including dissolving it in sulfuric acid, cooling the resulting mixture to crystallization temperature, washing the crystallized mixture with water, separating the insoluble residue from the obtained sulfate solution, sequential purification of the obtained filtrate from impurity metals by precipitation in the form of hydroxides, separating the precipitate treatment of the resulting solution with an alkaline reagent, separation of the precipitate of a magnesium-containing compound, washing the precipitate with water, thermal treatment otku, characterized in that when precipitating a magnesium-containing compound from a sulfate solution previously purified from impurity metals, a 7-15% solution of caustic soda is used, and the precipitate of the magnesium-containing compound is steamed in a boiling solution for 15-30 minutes until the complete decomposition of soluble magnesium bicarbonates.