Processing of serpentenites of the Arvaten deposit of Uzbekistan with the use of ammonium sulphate

. The article presents the results of a study of the processes of obtaining magnesium oxide by fusing serpentinites of the Arvaten deposit in the presence of ammonium sulfate, followed by grinding and treatment with water to separate silica from the suspension, neutralizing the filtrate with ammonia to pH 8.5 for precipitation and separation of impurity metal ions, precipitation and separation of ions magnesium from solution using ammonia and ammonium carbonate, thermal decomposition of magnesium corbanate to obtain magnesium oxide. It has been established that the use of ammonium sulfate for fusion of serpentite at a ratio of serpentenite: (NH4)2SO4 = 100: 201.77 and further processing of the resulting solutions allows you to obtain magnesium oxide with a composition (wt.%): MgO - 97.55; Fe2O3 - 0.86; A12O3 - 0.12; CaO - 0.18; SO3 - 0.48.


Introduction
Magnesium compounds are used in the production of magnesite, spinel, forsterite, dolomite, talc, talc-magnesite and chromo-magnesian refractory materials, in the manufacture of magnesia cements, for clarification of petroleum products, as an active filler in the rubber industry. Highly active magnesium oxide and compositions based on it are used as a heat-resistant electrical insulating material for transformer steel and electrical insulators. Magnesium oxide is abrasive and is therefore often used in the electronics industry. Agriculture is considered a promising segment of magnesium oxide consumption. With its help, they prevent caking of fertilizers and are used as an independent additive for soil enrichment. The lack of magnesium on farmland affects the production, therefore the use of such fertilizer is very important for the harvest. Magnesium compounds are also added to medicines, used in the independent or complex treatment of people [1][2][3][4][5][6].
In Uzbekistan, there is no production of magnesium oxide and its other compounds used in various industries based on local raw materials. It should be noted that only in the production of ammonium nitrate, to eliminate its caking and improve other commercial properties, a magnesian additive is used about 6,000 tons per year. The raw material for its preparation is caustic magnesite supplied from Russia. Serpentenites of the Arvetensky deposit can serve as one of the raw materials for obtaining magnesium compounds in Uzbekistan. The Arveten serpentenite deposits are located in the Jizzakh region, 9 km northwest of the Jizzakh lime plant, 1.5 km south of the village of Kiyabash. It is confined to the northeastern slopes of the Northern Nurata mountains. The Arvaten deposit is an array of serpentenites up to 1300-1400 m and a thickness of 90 to 125 m. The stock of serpentenite deposits was calculated by the method of vertical parallel sections to a horizon of 500 m in the amount of 1363.7 thousand m 3 [7][8][9].
At the present time, various methods of processing magnesium containing raw materials and waste are known. And also there are various ways of processing intermediates formed in the process of separating magnesium compounds from the feedstock [10]. In work [10] the processing of serpentenite is given by the acid-base combined method. It is shown that when treating serpentinite with solutions of strong mineral acids, the degree of magnesium leaching into solution can be increased to 95 wt. %. After cleaning the resulting solution from metal impurities by neutralization, it is used to obtain a magnesium compound according to existing technologies, and when processing the solid phase with caustic alkali, about ~ 90 wt. % SiO2 contained in it goes into solution. The resulting solutions and the solid phase are processed according to existing technologies, and the corresponding chemical compounds are obtained.
In [11], in order to obtain purer concentrates of metal hydroxides (Fe 3+ , Al 3+ , Cr 3+ , Ni 2+ , Mn 2+ , etc.) with a smaller amount of magnesium oxide, the effect of the concentration of a solution of magnesium nitrate on the solubility of magnesium oxide was studied. It was found that with an increase in the concentration of magnesium nitrate solution, the solubility of magnesium oxide increases. It is shown that ultrasonic treatment of the precipitant suspension increases its reactivity. The precipitator was tested in the separation of metal hydroxides (Fe 3+ , Al 3+ , Cr 3+ , Ni 2+ , Mn 2+ , etc.) from acidic nitrate solutions obtained by leaching serpentinite and oxidized nickel ore with nitric acid.
The work [1] presents the results of a study of the processes of thermal decomposition of magnesium nitrate hexahydrate, obtained by nitric acid processing of oxidized nickel ores or serpentinite. Thermographic analysis established the main stages of this process and the minimum temperature required for the complete decomposition of Mg(NO3)2·6H2O into magnesium oxide and nitrous gases. Intermediate solid products formed at different temperatures were determined by IR spectroscopy. It has been shown that in the temperature range 80-165 °C, 4 molecules of H2O are removed and Mg(NO3)22H2O is formed, up to 300 °C the substance is completely dehydrated. At temperatures above 400 °C, Mg(NO3)2 is completely decomposed to MgO, the purity and individuality of which is confirmed by X-ray and chemical analysis. X-ray diffraction patterns confirmed the purity and individuality of the starting material (Mg(NO3)2·6H2O) and the final product (MgO).
Other methods of processing serpentinites are also known, which do not differ significantly from each other, however, the direct use of existing methods for processing serpentenites from other deposits is impossible, since serpentenites are polygenic minerals formed in different geological conditions. Literature data show that various polymorphic modifications of serpentenite are stable at different temperatures and pressures, and the characteristics of minerals change depending on the phase composition of serpentenite and on its origin. There are three groups of serpentinite with oriented and non-oriented structure, formed by different protoliths: mantle, lithosphere and crust, and these serpentenites also differ in composition and properties [12,13].
In work [4], in order to clarify the information on the mineralogical composition and structure of the initial serpentinite of the Arvetensky deposit, we studied the composition and properties, and also studied the processes of obtaining magnesium oxide by decomposition of serpentinites of the Arvaten deposit with sulfuric acid and with the addition of nitric acid to the sulfuric acid solution by subsequent separation silica from suspension, neutralization of the filtrate with ammonia to pH 8.5 to precipitate and separate ions of impurity metals, precipitation and separation of magnesium ions from solution using ammonia and ammonium carbonate, thermal decomposition of magnesium corbanate to obtain magnesium oxide.

Main part
The purpose of this work is to study the processes of processing serpentenites of the Arvetensky field using ammonium sulfate, which is produced in Uzbekistan by two large industrial enterprises: Chirchik JSC "Maksam Chirchik" and JSC "Navoiazot". It is mainly used as a fertilizer. For laboratory experiments, we took serpentenite from the Arvetinsky deposit, the chemical composition of which is shown in Table 1.  Table 2 shows the results of mass spectrometric (ICP -MS) analysis of serpentenite from the table it follows that serpentenite contains in its composition a number of elements necessary for various sectors of the national economy. As reagents for processing serentenite were taken (wt.%): Ammonium sulfate -Ntot. -21.2; 25% ammonia and crystalline ammonium carbonate of the "Pure" brand. The norm of ammonium sulfate was taken in the amount of 90,100, 100, 120% of the stoichiometry for MgO, CaO, Fe2O3 and AL2O3 containing in the serentenite of the Arvetensky deposit according to the following reactions (reactions 1-4).
At a rate of ammonium sulfate of 100% of stoichiometry, 176.55 g of crystalline ammonium sulfate is required for the decomposition of 100 g of serpentenite. The process of decomposition of serpentenite in the presence of ammonium sulfate was carried out as follows. First, serpentenite and ammonium sulfate were crushed to a fraction size of no more than 0.1 mm separately. Then they were thoroughly mixed in various weight ratios corresponding to the standards in the range of 90-120% of stoichiometry for MgO, CaO, Fe2O3 and AL2O3 containing in serepentenite. The resulting charge was fused at a temperature of 350°C for 3 hours. The resulting alloy containing iron, aluminum, chromium, nickel and other metals was crushed and treated with water. At the end of the dissolution process, the suspension was separated into liquid and solid phases by vacuum filtration. The solid phase was washed with a small portion of disvoda twice (10 ml per 100 ml suspension) and dried at 105°C to constant weight (1-precipitate). The liquid phase and all the washing solutions were collected together, then it was heated to 45-50 °C and, using an ammonia solution, was ammonized to a pH of 8.5, precipitating hydroxides of iron, aluminum, chromium, nickel and other metals, and the hydroxides of metals were filtered. The precipitates remaining on the filter were dried in a place with filter paper in a drying oven to constant weight at 105 °C (2-precipitate). And the filtrate containing magnesium sulfate was neutralized using ammonium carbonate to pH 11-11.5. Magnesium carbonate was filtered off, washed with water, dried and then thermally decomposed to magnesium oxide at a temperature of 900 °C (3-precipitate). The final filtrate consists mainly of ammonium sulfate. The ammonium sulfate solution obtained at the stage of precipitation of magnesium hydroxide was evaporated and used for sintering serpentinite, and the ammonia released at this stage was captured and used to precipitate Mg(OH)2. Chemical analysis of the initial serpentenite and sediments was carried out by known methods [13].
The determination of the content of CaO and MgO was carried out by the volumetric complexometric method: titration with 0.05 N solution of Trilon B in the presence of fluorexon and dark blue chromium indicators. The content of Al2O3 and Fe2O3 was determined by the complexometric method [14]. The pH value of the suspension of the test samples was measured using an I-130M laboratory ionometer with an electrode system of ESL 63-07, EVL-1M3.1, and TKA-7 electrodes with an accuracy of 0.05 pH units. The research results are given in Tables 3 and 4. Table 3 shows the composition of the dried sediments obtained by sintering the original serpentenite in the presence of ammonium sulfate. The table shows that as the amount of ammonium sulfate in the charge in the first sediment increases, the SiO2 content increases from 76.84% (the ratio of serpentenite: (NH4)2SO4 = 100: 168.145) to 78.23% (the ratio of serpentenite: (NH4)2SO4 = 100: 201.77), and the content of Al2O3, Fe2O3, CaO and MgO decreases from 0.58 to 0.51; 1.52 to 1.20; from 0.82 to 0.7; from 3.25 to 1.49%, respectively. From these data it can be seen that the first precipitate is mainly composed of silicon dioxide. In the second draft, i.e. obtained after ammonization of the filtrate to a pH of 8.5 with an increase in the amount of ammonium sulfate, the content of SiO2 decreases from 9.57% to 7.11%, the content of Al2O3, Fe2O3, CaO and MgO increases from 12.29 to 13.30; from 38.70 to 41.76, and the content of CaO and MgO decreases from 0.58 to 0.39; from 3.61 to 2.30%, respectively. The composition of the third precipitate obtained after neutralization of the filtrate using ammonium carbonate to a pH value of 11.5 drying and firing at 900 °C consists mainly of 97.35-97.55% of magnesium oxide.   Table 4 shows the degree of distribution of components by precipitation, depending on the ratio of serpentenite and ammonium sulfate, from it, as the amount of ammonium sulfate increases, the degree of transfer of MgO increases from 88.55% to 97.37%. From these data it can be seen that the use of ammonium sulfate for the processing of serpentenite allows the most complete extraction of magnesium oxide and other metals. For the processing of serpentenite, the ratio of serpentenite: (NH4)2SO4 = 100: 201.77 can be considered optimal. The choice of this ratio is due to the fact that with its decrease, the degree of transition to the solution of magnesium ions decreases. Depending on the ratio of the starting materials, the loss of MgO with precipitation is 11.45-2.63%. The composition of the main sediment obtained under optimal conditions has the following composition (wt.%): MgO -97.55; Fe2O3 -0.86; A12O3 0.12; CaO -0.18; SO3 -0.48, p.p. -0.81 (Table  4).
The ammonium sulfate solution obtained at the precipitation stage is evaporated, granulated and used as a fertilizer, which is in high demand in agriculture, or it can be returned to the sintering stage, and the released ammonia is captured and used to precipitate metals, the rest of the precipitates are also in high demand for obtaining highly dispersed silica and a number of valuable metals.
Thus, the fusion of serpentenites from the Arvaten deposit in the presence of ammonium sulfate makes it possible to more completely extract magnesium from raw materials. It has been established that the most complete leaching of magnesium ions from serpentenite occurs after fusion of the initial substances at a ratio of serpentenite: (NH4)2SO4 = 100: 201.77 at a temperature of 350°C and within 2 hours.