RU2051101C1 - Method of producing chemically precipitated chalk - Google Patents

Abstract

FIELD: production of inorganic materials. SUBSTANCE: calcium substance is treated with soda solution containing 30-60 g/l Na₂O at 80-100 C in the presence of carbon dioxide. After chalk suspension separation precipitate is washed with water quantity of which is equal to the moisture content in precipitate. EFFECT: sewage elimination, enhanced carbonization of lime suspension with gaseous mixtures. 1 dwg, 1 tbl

Description

 The invention relates to the production of inorganic materials, in particular to methods for the manufacture of chemically precipitated chalk, intended for food, perfume, cosmetics, medical, chemical and other industries.

A known method of producing chemically precipitated chalk by preparing and purifying lime milk, carbonizing this milk with calcining gases of calcareous furnaces at elevated pressure (0.35 MPa), purifying and filtering chalk milk to obtain a precipitate with a moisture content of 55-63% and drying the separated precipitate [1]
The disadvantages of this method are the long duration of the carbonization process (10-12 hours), the need to use calcining gases with a high content of carbon dioxide, high pressure and complexity of the equipment.

A method of obtaining a chemically precipitated chalk by causticizing a soda solution (10-15% Na ₂ CO ₃ or 70-100 g / l Na ₂ O) with lime or lime milk at 80 ° ^C in the absence of carbon dioxide, filtering the resulting slurry, and wash sludge drying [2]
The disadvantages of this method are the relatively low yield of the finished product, as a result of which it contains a significant amount (more than 4 wt.) Of free calcium hydroxide, the inability to accelerate the process by raising the temperature due to an even greater decrease in the yield of the product, the need for multi-stage washing of the precipitate from alkali, which leads to to the formation of a significant amount of alkaline wastewater, and the need for scarce raw materials of soda ash.

There is also known a method for producing chemically precipitated chalk [3] by which sodium-calcium hydroxylate is treated at elevated temperature with a soda solution containing 150-220 g / l of Na ₂ O carbon dioxide in the presence of carbon dioxide. As a result of this treatment, a pulp is obtained containing lichit Na ₂ CO ₃ ˙CaCO ₃ ˙ 2H ₂ O and a silicic acid gel SiO ₂ ˙nH ₂ O. The pulp is separated into pomit and silicic acid gel in a settling apparatus. The resulting personit is treated with water at 95-100 ^about With obtaining a solution of Na ₂ CO ₃ and a precipitate of calcium carbonate CaCO ₃ , which is separated from the solution, washed and dried. This method is characterized by complexity and multistage, pollution of the resulting chalk with silica, the need for additional energy for decomposition of persona and a significant consumption of water for washing with the formation of wastewater.

There is also known a method of producing chemically precipitated chalk [4] in which milk of lime is prepared by slaking lime with a soda solution containing 1-1.5 wt. soda (6-9 g / l Na ₂ O carbon dioxide). Then the pulp is carbonized by calcining gases of lime kilns. The presence of soda in the liquid phase of the pulp accelerates the carbonation process by 10-12 times. The precipitate of precipitated chalk is separated from the solution, washed and dried.

This method has the following disadvantages: a sharp deceleration carbonization process while reducing the concentration of carbon dioxide in the gas mixture, the bubble pulp, which does not allow to use the flue gases boilers, since they are not CO ₂ content exceeds 10 wt. and in the gases of lime kilns contains 16-30 wt. carbon dioxide; low carbon dioxide use due to the low absorption capacity of the soda solution containing 6-9 g / l Na ₂ O.

 The aim of the invention is to increase the intensity of carbonization of the lime slurry with gas mixtures with a low content of carbon dioxide, increase the degree of use of carbon dioxide and the exclusion of wastewater.

The object is achieved in that the treatment of the calcium compound produce soda solution containing 30-60 g / l Na ₂ O of carbon, the resulting suspension was purified and carbonized at 80-100 ^{° C} with a gas mixture, allowing a reduced content of CO _2, and the precipitate was separated from the solution and washed water, the amount of which is taken equal to the amount of moisture in the precipitate.

 The essence of the proposed method is illustrated by the circuit shown in figure 1.

Реакция 1 при обычной температуре практически необратима и протекает с достаточно высокой скоростью. Реакция 2 обратимая, благодаря чему при определенных условиях (температуре и концентрации) наступает состояние равновесия. Направление реакции зависит от растворимости Са(ОН)₂ и СаСО₃ в щелочном растворе при данных условиях. Для повышения скорости взаимодействия между реагирующими веществами и получения легко отмываемого от щелочи осадка процесс проводят при 80^оС, не превышая ее, поскольку при повышении температуры из-за уменьшения растворимости гидроксида кальция уменьшается значение константы равновесия реакции 2. По мере протекания реакции 2 содержание NaOH в растворе увеличивается и вследствие повышения концентрации гидроксильных ионов в растворе понижается растворимость Са(ОН)₂, что приводит к прекращению образования карбоната кальция. В реакционной смеси остается значительное количество гидроксида кальция. Для нарушения установившегося равновесия и перевода всего гидроксида кальция в карбонат с высокой скоростью следует понизить концентрацию гидроксильных ионов в растворе. Этот эффект можно достичь, связав NaOH в Са₂СО₃ по реакции 3 путем барботажа реакционной смеси углекислым газом, чем обратимый процесс каустификации соды превращается в практически необратимый процесс карбонизации гидроксида кальция, что позволяет повысить температуру проведения процесса до максимально возможной при атмосферном давлении, одновременно обеспечивая высокую скорость карбонизации и высокую степень использования СО2, полное превращение гидроксидов кальция и натрия в соответствующие карбонаты и получение легко отделяющегося от раствора и хорошо отмываемого от воды осадка. Очевидно, что скорость реакций 2 и 3 и степень использования СО₂будут тем выше, чем выше концентрация Na₂O в растворе. За счет тепла, выделяемого в процессе реакций 1 и 2, обработку известкового молока возможно организовать автотермично.Treatment of a calcium compound, for example calcium oxide, with a soda solution in the presence of carbon dioxide under conditions excluding the formation of persona, allows the conversion of calcium oxide into its carbonate. This process can be described by the following equations:

Reaction 1 at normal temperature is practically irreversible and proceeds at a fairly high speed. Reaction 2 is reversible, due to which, under certain conditions (temperature and concentration), a state of equilibrium sets in. The direction of the reaction depends on the solubility of Ca (OH) ₂ and CaCO ₃ in an alkaline solution under these conditions. To increase the rate of reaction between the reactants and to obtain easily washed from alkali sludge process is carried out at 80 ° ^C, without exceeding it, because when the temperature rises due to decrease of solubility of calcium hydroxide decreases the value of the equilibrium constant of the reaction 2. As the reaction proceeds 2 NaOH content in the solution increases and due to an increase in the concentration of hydroxyl ions in the solution, the solubility of Ca (OH) ₂ decreases, which leads to the cessation of the formation of calcium carbonate. A significant amount of calcium hydroxide remains in the reaction mixture. To disturb the established equilibrium and transfer all calcium hydroxide to carbonate at a high speed, the concentration of hydroxyl ions in the solution should be lowered. This effect can be achieved by combining NaOH in Ca ₂ CO ₃ by reaction 3 by sparging the reaction mixture with carbon dioxide, which transforms the reversible soda causticization process into an almost irreversible process of carbonization of calcium hydroxide, which makes it possible to increase the temperature of the process to the maximum possible at atmospheric pressure, simultaneously providing a high carbonization rate and a high degree of CO2 utilization, the complete conversion of calcium and sodium hydroxides to the corresponding carbonates and obtaining easily separating from a solution and a sediment well washed from water. Obviously, the rate of reactions 2 and 3 and the degree of use of CO ₂ will be the higher, the higher the concentration of Na ₂ O in solution. Due to the heat generated during reactions 1 and 2, the processing of milk of lime can be organized autothermally.

Нижняя величина концентрированного предела содового раствора, равная 30 г/л Na₂O углекислого, объясняется тем, что при содержании в растворе менее 30 г/л Na₂O углекислого продолжительность карбонизации известковой суспензии газовой смесью, содержащей ниже 16 мас. СО₂, резко возрастает с одновременным понижением степени использования СО₂.The speed of the carbonization process and the degree of use of carbon dioxide decreases with a decrease in the concentration of CO ₂ in the gas mixture sparging the lime suspension, and increases with increasing concentration of the soda solution. Therefore, for the case of carbonation of a lime suspension with gas mixtures with a low content of CO ₂ , solutions with the highest possible concentration of soda should be used, however, a conditioned product without the formation of wastewater can be obtained only at a concentration not exceeding 60 g / l Na ₂ O carbon dioxide. This is due to the following. Since the content of free alkali in chemically precipitated chalk is limited to 0.03 wt. CaO or 0.033 wt. Na ₂ O, then after separation of the precipitate, it must be washed with water, the amount of which depends on the alkali content in the non-washed precipitate and the degree of saturation of the washed precipitate with the initial solution. If this amount exceeds the amount of water removed from the technological cycle with the finished product, it accumulates and the formation of wastewater. Therefore, in order to avoid the formation of wastewater, it is necessary that the alkali content in the non-washed sediment be no more than it could be washed with the amount of water corresponding to the humidity of the sediment. In this case, the degree of saturation of the washed precipitate with the initial solution is reduced to 10 wt. The dependence of the content of Na ₂ O carbon dioxide in the product K on its concentration in the initial solution Co, g / l, the moisture content of the precipitate W, and the degree of saturation of the washed precipitate with the initial solution m, are determined by the ratio
K

The lower value of the concentrated limit of soda solution, equal to 30 g / l Na ₂ O carbon dioxide, is explained by the fact that when the content in the solution is less than 30 g / l Na ₂ O carbon dioxide, the duration of carbonization of the lime suspension with a gas mixture containing below 16 wt. CO ₂ increases sharply while reducing the degree of use of CO ₂ .

62,7 г/л
обеспечивает получение качественного продукта с достижением остальных целей решения при влажности осадка химически осажденного мела до 5% что без затруднения осуществляется на современных промышленных фильтрующих центрифугах.In FIG. 2 is a graph illustrating the proposed method. The value of the upper limit of concentration of 60 g / l Na ₂ O carbon dioxide
C _o =

62.7 g / l
provides a high-quality product with the achievement of other solution goals when the sediment moisture content of chemically precipitated chalk is up to 5%, which is easily carried out on modern industrial filter centrifuges.

PRI me R. To 60 g quicklime, 660 g hot soda solution was added. Under such conditions, the slaking of lime took place quickly and a slowly exfoliating lime suspension was obtained. The resulting suspension was stirred for 5-10 minutes and passed through a sieve to separate unquenched particles. The purified suspension was transferred to a thermostatically controlled glass packet 60 mm in diameter. Carbonation of lime slurry was carried out at 80-100 ^{° C,} dispersing therein at a rate of 4-5 l / min gas mixture containing about 9 wt. carbon dioxide. Exhaust gases from the patch were removed through a reflux condenser. Proceedings carbonization process was monitored by sampling the liquid phase and determining therein the content of Na ₂ O caustic Na ₂ O and carbon successive sample titration by hydrochloric acid with two indicators (phenolphthalein and methyl orange). The carbonization process was completed only in the absence of caustic Na ₂ O in the solution. Then, the resulting Cretaceous suspension was separated (in a laboratory filter centrifuge, separation factor 216) to a predetermined sediment moisture. The end of centrifugation was determined by the weight of the obtained centrate. The thickness of the sediment layer reached 5 mm. The precipitate, directly in the centrifuge rotor, was washed with hot water, the amount of which was determined as the difference between the mass of the initial soda solution and the mass of the resulting centrate. The degree of saturation of the washed precipitate with the initial solution was maintained equal to 10%. The content of unwashed alkali in the washed dry precipitate was determined by the method described in GOST 8253-79.

 The experimental procedure is the same for all experiments, and the results are shown in the table.

The proposed method for the preparation of chemically precipitated chalk in comparison with known methods has the following advantages: increased intensity of the process of carbonization of the lime suspension in gas mixtures containing CO ₂ less than 16 wt. and an increased degree of use of carbon dioxide, since the process proceeds at elevated temperature using soda solutions containing 30-60 g / l of Na ₂ O carbon dioxide; chemically precipitated chalk is washed from soda without the formation of wastewater, since the precipitate is washed from soda with hot water, the volume of which corresponds to the moisture content of the precipitate.

Claims (1)

METHOD FOR PRODUCING CHEMICALLY DEPOSITED CHALK, including treating the calcium compound with carbon dioxide in the presence of a soda solution at 80-100 ^o C, separating the chalk suspension and washing the precipitate, characterized in that they use a soda solution with a concentration of 30-60 g / l Na ₂ O, and the amount of water for washing is taken equal to the moisture content in the precipitate.

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