CS272041B1 - Method of magnesium sulphite and calcium carbonate mixture production - Google Patents

Abstract

For an aqueous suspension of half-burned dolomitic material containing 10 to 90 wt. % of magnesium oxide and 10 to 90 wt. % calcium carbonate is treated gases containing sulfur dioxide; \ t The pH of the reaction mixture is maintained at 3 to 9, preferably 5.6 to 7, or stoichiometric magnesium oxide to oxide ratio sulfur in the feedstock in the range of i: 0.7 to 0.7: 1, and is preferred 1 5 1.

Description

The invention relates to a process for the production of a mixture of magnesium sulphite and calcium carbonate and enables the capture of sulfur dioxide from waste gases. The output emem can be used, for example, as a fertilizer.

Recently, ecology has enjoyed great interest in science and practice, with the capture of sulfur dioxide from waste gases (desulphurisation) in the foreground »gentle fertilization of soils and protection of groundwater as problems that need to be addressed urgently. A number of methods have been proposed for the desulfurization of waste gases, especially flue gases from power plants and heating plants. Methods that use calcium and magnesium compounds in their oxide or carbonate form have been most widely used. This is because they are found in nature in large quantities and in a concentrated state, and when used they are the product of desulphurisation of substances which can be further recovered. Approximately 90% of the implemented desulphurisation plants use one of the variants of non-regenerative limestone technology (so-called • wet or wet-dry), which is based on the reaction according to the equation:

CaCOg + S0 ₂ + 1/2 0 ₂ + 2H ₂ 0 »caso ₄ , 2H ₂ O + C0 ₂ (1),

The product here is gypsum, which is used in construction or adhered to. In order for the gypsum to be usable and of high quality, it is necessary to use the purest possible input material (limestone), which, however, is becoming a shortage in nature and the effort is to preserve it more for the construction and chemical industries. Furthermore, the so-called lime technologies were designed, where the active ingredient is calcium oxide or hydroxide, produced by thermal decomposition of limestone. These technologies are used only to a small extent because very fine-grained calcium sulphate is formed as a desulphurisation product, which is difficult to separate from the desulphurisation slurry. Although lime has a higher reactivity to sulfur dioxide than limestone, its use is still very uneconomical. The so-called Magnesite flue gas desulphurisation technology has so far only been implemented in a few cases, because the world's magnesite reserves are much smaller than limestone, and compared to it, magnesite has a lower affinity for sulfur dioxide. It must therefore be based on magnesium oxide or hydroxide, similar to the limestone and magnesite desulphurisation process, the dolomite process could be used, which, using unfired dolomite, would proceed according to the equation:

MgCO ₃ .CaCO ₃ + so ₂ + 1/2 0 ₂ »MgCO ₃ + CaSO ₄ + co ₂ , (2) when using calcined dolomite or hydrated calcined dolomite, respectively, according to the equation;

MgO.Cao + 2SO ₂ + 1/2 0 ₂ MgSO ₃ + CaSO ₄ (3)

Mg / 0H / ₂ .Ca / 0H / ₂ + 2SO ₂ + 1/2 0 ₂ »MgSOg + CaSO ₄ + 2H ₂ 0 (4).

In this sense, a number of theoretical and experimental works focused on thermodynamic and kinetic aspects of reactions (2), (3) and (4) have been performed with dolomite, but the fact remains that dolomite is not yet used in gas desulphurisation for economic and environmental reasons. using unfired dolomite according to equation (2), magnesium carbonate would come out of the process as an inert in the form of a filter cake, which would make it impossible to use the product for gypsum production. This difficult waste would be generated significantly more than with limestone technology. When using calcined or hydrated calcined dolomite according to equations (3) and (4), magnesium would also be used for desulphurisation, but the resulting mixture of magnesium sulphite and calcium sulphate would be non-negotiable from an ecological point of view, because spontaneous oxidation formed magnesium sulphate is highly soluble and poses a serious threat to groundwater in large quantities. Reducing the acid response and replenishing magnesium reserves in soils is a permanent part of soil care. Acid reCS 272041 B1 ₀ action is reduced by so-called liming, when limestone, lime, dolomite or magnesite is applied to the field. The disadvantage of using these materials is the need to grind them very finely in order to be effective. Due to the low solubility of magnesium carbonate in water, the fertilizing effect on soils is insufficient and it is still necessary to apply water-soluble magnesium fertilizers. Kieserite is traditionally used for this purpose, but it is acidic in nature and too soluble in water (35% by weight). Recently, magnesium sulfite has been introduced as a magnesium fertilizer, which has a neutral reaction, optimal solubility in water (0.7% by weight) and, in addition, does not sinter, does not moisten, does not wash, is easily granulated and has high mechanical strength. In combination with calcium carbonate, stabilization of magnesium sulphite in soils and good granulation properties of the mixture are achieved. A process for the production of a mixture of magnesium sulphite and calcium carbonate has not yet been proposed. Merely mixing the two components would not be sufficient due to the need for intimate contact and homogeneity of the mixture. Due to the different physical properties of the two materials, it would be necessary to first grind them finely, and then homogenize them and possibly granulate them, which would be economically disadvantageous.

The above-mentioned drawbacks are eliminated by the process for the preparation of a mixture of magnesium sulfite and calcium carbonate according to the invention, the essence of which consists in the fact that an aqueous suspension of semi-fired dolomitic material containing 10 to 90 wt. % of magnesium oxide and 10 to 90 wt. % of calcium carbonate is treated with sulfur dioxide-containing gases, the reaction proceeding according to the equation;

MgO + CaCO ₃ + SO2 + xH ₂ 0 ® MgSO ₃ .xH ₂ O + CaCO ₃ (5) The pH of the reaction mixture is maintained in the range of 3 to 9, or the stoichiometric ratio of magnesium oxide to sulfur dioxide in the feedstock is maintained in the range of 1 : 0.7 to 0.7: 1. The dolomite material may be dolomite, which contains magnesium carbonate and calcium carbonate in a molar ratio not very different from one, it may be dolomagnesite with a predominance of magnesium carbonate, or dolomitic limestone with a predominance of calcium carbonate. . The input material can also be a mixture of the above raw materials, ením by firing them at a temperature of about 700 ° C (so-called half-firing), the magnesium component is thermally decomposed according to the equation;

MgCO ₃ .CaCO ₃ MgO + caco ₃ + co ₂ (6)

As regards the accuracy of the half-firing of the starting dolomitic raw material, the process according to the invention allows a certain non-firing of magnesium carbonate as well as a certain firing of calcium carbonate to calcium oxide in accordance with technical practice where the half-fired dolomite contains about 2 wt. % CaO and about 2 wt. % MgCO ₃ . The natural impurities accompanying the starting raw material (Al ₂ 0 ₃ , ^Fe ₂ ° 3 'S10 ₂ ), which exceed 10 wt. %. In order for the reaction according to equation (5) to proceed without undesired decomposition of the calcium carbonate according to equation (1), it can be provided in two ways, as has been experimentally demonstrated. In particular, by maintaining the pH of the reaction mixture at 3 to 9, which can be achieved by controlling the amount of input semi-fired dolomitic material introduced into the production process. Alternatively, it is possible to control the amount of semi-calcined dolomitic feedstock and sulfur dioxide-containing feed gases introduced so that the stoichiometry according to equation (5) is met. To meet this requirement, it is sufficient for the molar ratio of sulfur dioxide to magnesium oxide at the inlet to be in the range i; 0.7 to 0.7: 1. The mixture of magnesium sulphite and calcium carbonate formed by the process according to the invention can be further processed as required by known methods, e.g. filtration, sedimentation, drying, granulation, with a magnesium sulphate-containing liquid separated at dewatering the mixture, returning it to the production process, to the reaction mixture intended to capture the sulfur dioxide. If the production of a solid product is desired, it is advantageous to use the heat of the desulfurized flue gases to evaporate the water during dewatering. The resulting mixture

CS 272041 B1 according to the invention can be used as a fertilizer, it is also possible to apply it directly to the field as a suspension fertilizer. In the production of the fertilizer mixture, it is necessary to process semi-fired dolomitic material containing 10 to 90 wt. % of magnesium oxide and 10 to 90 wt. % of calcium carbonate. When using semi-fired material with MgO content below 10 wt. This is because a product with too low a content of magnesium sulphite is formed, which has an insufficient content of water-soluble magnesium and is poorly granulated, using a semi-calcined raw material with a MgO content of more than 90 wt. % there is a low content of calcium carbonate in the manure produced, which already poorly stabilizes magnesium sulphite.

The main advantage of the process according to the invention is the use of available feedstocks, which are very large in the world, are easy to extract and are usually substantially lower in impurities and magnesite or limestone. The process according to the invention makes it possible to use gases with a low content of sulfur dioxide, it is simple, waste-free and easy to implement on equipment used hitherto in lime, limestone or magnesite flue gas desulphurisation technology, both wet and wet-dry. The invention is gentle and homogeneous.

Flue gases with a temperature of 180 ° C in an amount of 20,000 Nm ³ / hour were introduced into a continuously operating absorber. The absorber was sprinkled with a circulating suspension at an intensity of 50 m ³ / hour. The pH and bulk density of the circulating suspension were measured. The pH was maintained between 6.2 and 6.9 by the addition of semi-fired dolomite (automatically controlled). The bulk density of the slurry was maintained at 1.2 to 1.25 t / m ³ by discharging a portion of the slurry from the process and replacing it with feed water. This produced a suspension in an amount of 1 m ³ / h, which was freed of the mother liquor by filtration and the filter cake was freed of residual free water in a fluid bed granulator. The filtrate was returned to desulfurization. 300 kg of granulated fertilizer with a composition (% by weight) was produced per hour; 55% MgSO ₃ .3H ₂ 0.35% CaCO _{3 <} 5% MgSO 4 and 5% impurities. The semi-fired dolomite used contained 70 wt. % CaCO ₃ and 25 wt. % MgO.

Claims (3)

OBJECT OF THE INVENTION A process for the preparation of a mixture of magnesium sulphite and calcium carbonate, in particular by capturing sulfur dioxide from gases, characterized in that an aqueous suspension of semi-calcined dolomitic material containing 10 to 90 wt. % of magnesium oxide and 10 to 90 wt. % of calcium carbonate is treated with sulfur dioxide-containing gases, the pH of the reaction mixture being maintained in the range of 3 to 9, preferably 5.5 to 7, or the stoichiometric ratio of magnesium oxide to sulfur dioxide in the feedstocks in the range of 1: 0.7 to 0.7: 1, preferably 1: 1. 2. The process according to item 1, characterized in that the resulting mixture is dewatered by drying, using the heat of the desulfurized flue gases. 3. The process according to item 1, characterized in that the liquid separated during dewatering of the resulting mixture is returned to the production process, namely to the reaction mixture intended for the capture of sulfur dioxide.