CS241848B1 - Process for the production of pure magnesium sulphite - Google Patents

Abstract

The aim of the solution is to produce pure magnesium sulfite by treating contaminated magnesium raw material with sulfur dioxide. The method addresses the use of a product from flue gas desulfurization. An aqueous suspension prepared from the above magnesium raw material and containing optionally dissolved sulfate and magnesium sulfite is treated with a gas containing sulfur dioxide so that the final reaction mixture has a temperature above 40 °C, a pH above 3.5, preferably 5.5 to 8.5, and a magnesium sulfite content above 0.7% by weight, preferably 4 to 8% by weight. The liquid phase is then separated from the suspended impurities from the reaction mixture thus obtained no later than three hours later, and this supersaturated magnesium sulfite solution is processed into a magnesium compound, e.g. by cooling, crystalline magnesium sulfite is separated. The liquid phase obtained after separation of the magnesium sulfide is used to prepare an aqueous suspension from the starting contaminated magnesium raw material.

Description

The task of the solution is the production of pure magnesium sulphite, in which sulfur dioxide is treated on contaminated magnesium raw material. The method solves utilization of the product from flue gas desulfurization. The aqueous suspension prepared from said magnesium-containing feedstock and optionally containing dissolved magnesium sulphate and sulphite is treated with a gas containing sulfur dioxide such that the final reaction mixture has a temperature above 40 ° C, pH, above 3.5, preferably 5.5 and a magnesium sulphite content above 0.7% by weight, preferably 4 to 8% by weight. Thereafter, from the reaction mixture so obtained, the liquid phase is separated from the suspended impurities no later than three hours and this supersaturated magnesium sulphate solution is treated to a magnesium compound, e.g. by cooling, crystalline magnesium sulphite is separated. The liquid phase obtained after separation of the magnesium sulphate is used to prepare an aqueous suspension from the starting contaminated magnesium feedstock.

The invention also relates to the production of pure magnesium sulphide, which is contaminated with magnesium raw materials.

Magnesium sulphite is a pig that has been receiving nAw & amine in recent years, mainly due to the magnesite flue gas desulfurization method where it is produced as an intermediate. Its advantageous chemical properties are known which make it a raw material for the production of other magnesium compounds. For example, it is relatively easily decomposed by heat to form MgO and concentrated SO2, can be decomposed with mineral acids to yield other magnesium salts, and can also serve as a raw material in the production of cellulose by the magnesium bisulfite process. The use of magnesium sulphate in practice is hampered by the fact that it is very impure in industrial scale production. In fact, it contains impurities originating from the raw and magnesium-based raw material, i.e. magnesite, silica, alumina, iron and calcium, and, in the case of flue-gas desulfurization, also contains fly ash. Several methods of purification of crude magnesium sulfite are known, but all of them ^! MgO _(s) + SO _{2 (g)} -I-6H ₂ O ₍₁₎ = MgSO 3. 6H ₂ O ₍₈₎ ns basically use two principles. Older (processes convert very small crystalline magnesium sulphite MgppJ. XH ₂ O 1 (x = 7 3 by introducing sulfur dioxide to magnesium sulfide) insoluble magnesium hydrogen sulphite and insoluble impurities are separated. However, it is not possible to separate the iron, which is the most serious impurity, as ferrous hydrogen sulphite is also very soluble.An additional disadvantage is (laborious and costly, because the preparation of Mg (HSO3) ₂ requires a high SO ₂ gas and A large amount of pure magnesium oxide is consumed to recover MgSO ₃ · xH ₂ O. The newer process proposed in recent years uses the finding that one of the MgSO 3 hydrates, MgSO 3 · 6H 2 O, is able to provide a relatively concentrated metastabile solution (about 8% by weight) of MgSO 4. a suspension of crude crystals of MgSO ₃ .8H ₂ O by the action of sulfur dioxide on the magnesium feedstock according to one or both of the following equations:

'(1)

Mg (OH) _{2 (s)} + SO _{2 (g} > + 5H ₂ O ₍ i, = MgSO 3. &Amp; H ₂ O ( _S) '(2)' The crude crystals of MgSO 3 .6H ₂ O are then mixed with water and this The suspension is rapidly heated by direct mixing with water vapor to a temperature near boiling point (about 100 DEG C.) The resulting MgSO4 solution is rapidly separated from insoluble impurities, e.g., by filtration, and the filtrate is very pure and recovered by crystallization according to temperature conditions.

. 6H ₂ O or MgSO 4. 3H ₂ O.

Its purity is up to 99.9 percent. The advantage of this method is the unusually high cleaning efficiency and apparatus simplicity. The disadvantage is high heat consumption, resp. water vapor to heat the suspension and dissolve the crystals. Also unpleasant is the fact that magnesium sulfide hexahydrate must be started, since the trihydrate does not form a supersaturated solution. The recovery of the hexahydrate intermediate is bound to the low temperatures at its formation since it is stable up to 42 ° C so that the reaction mixture must be cooled during decomposition according to equations (1) and (2) and then reheated by direct steam to dissolve the magnesium sulfite hexahydrate to supersaturated solution.

These disadvantages are overcome by a process for the production of pure magnesium sulfite in which sulfur dioxide is treated on the magnesium raw material. According to the invention, the aqueous suspension prepared from the contaminated magnesium raw material, optionally containing dissolved magnesium sulphate and magnesium sulphite, is treated with a sulfur dioxide-containing gas so that the final reaction mixture has a temperature above 40 ° C, pH above 3.5, preferably 5.5 to 8.5 and a magnesium sulphite content above 0.7% by weight, preferably 4 to 8% by weight, after which the liquid phase is separated from the reaction mixture so obtained no later than three hours from suspended impurities and the 'supersaturated magnesium sulphite solution' is processed to a magnesium compound, e.g. upon cooling, crystalline magnesium sulphite is separated. Also according to the invention, the liquid phase obtained after the separation of magnesium sulphite is used to prepare an aqueous suspension from the starting contaminated magnesium feedstock.

The process for producing pure magnesium sulfite according to the invention is much simpler than the prior art processes and reduces the steam consumption for direct heating by achieving a supersaturated magnesium sulfite solution by a mere chemical reaction between the sulfur dioxide and the magnesium starting material. In another embodiment of the invention, the heat of the sulfur dioxide-containing waste gases can be used to heat the reaction mixture to a desired temperature above 40 ° C. E.g. in the desulfurization of flue gas from thermal power stations having a temperature of 170 to 200 degrees Celsius, their residual heat is sufficient to heat the reaction mixture to a temperature of about 70 ° C. The process according to the invention also opens up possibilities for the use of the desulfurization product in the chemical industry and is a source of very pure, easily reworkable magnesium compounds. It has advantageous use in the production of cellulose by magnesium-magnesium sulphite technology and in the desulphurization of flue gas by the magnesite process. Prerequisites are also created for the mutually beneficial link between these two remote activities. The sulphurous magnesium salt of the flue gas desulfurization process is purified to the extent that it is useful for the preparation of boiling acid. The cellulose replaces the previously used pure MgO and the consumption of elemental sulfur is considerably reduced.

Said pH of the reaction mixture above 3.5 is required in order to limit, preferably eliminate the existence of a bisullfit ion, which, on the one hand, increases the solubility of impurities, in particular iron, thereby deteriorating the cleaning effect according to the invention and degree of waste gas desulphurisation. At pH above 5.5, the bisulfite ion is no longer present in the reaction mixture. However, to lead the process to a pH above 8.5 is no longer useful in terms of SO2 utilization and, moreover, the utilization of the second reactant, i.e. the magnesium feedstock, is sharply decreasing. The desired supersaturated magnesium sulphite solution in the final reaction mixture is formed by mixing the streams containing the main reaction components, i.e., the gases containing SO2 and the aqueous suspension containing the magnesium feedstock, at approximately the steehiometric ratio given by equations (3j and (4)). ).

iMgO (s, -j-SO 2 (g) = MgSO 3 (e) (3j

Mg (OH) 2 (s) -j-SO 2 (g) = MgSO 3 (e) 4 H 2 O is (4)

The mixing is preferably carried out continuously. The conversion of the main reaction components, when the magnesium raw material is finely ground and sufficiently reactive, proceeds very rapidly and with a high yield of about 90% according to equations (3) and (4). The concentration of magnesium, magnesium, in the final reaction mixture required according to the invention above 0.7% by weight, is readily adjusted by the amount of water used to prepare the suspension of the magnesium feedstock and optionally the amount of hot water, preferably steam, used to heat the reaction mixture. Working with concentrations below 0.7% by weight of magnesium sulphite in the final reaction mixture is ineffective, since then large volumes of reaction components are circulated and the yield in the production of pure crystalline magnesium sulphite is low. In these aspects, an optimal concentration of 4-8% by weight of magnesium sulfite is optimal. An important condition of the process according to the invention is the rapid separation of the metastable solution of magnesium sulfite formed by the chemical reaction from the water-insoluble impurities. This time is determined by the concentration of magnesium sulfite in the final reaction mixture. At 0.7% by weight of magnesium sulfite, this time is up to 3 hours, at 5% by weight it is about 311 minutes, and above 8% by weight, the existence of a supersaturated solution is so short that separation of impurities is difficult or even impossible.

When the supersaturated magnesium sulphite solution is converted into crystalline magnesium sulphite or another magnesium compound of the process of the invention, a dilute magnesium sulphite solution is formed as a by-product. it is preferred to return it to the process for preparing the starting aqueous suspension of the magnesium feedstock. The issue of wastewater will be solved and product yield will increase.

Example 1

100 L of a 20% magnesium sulfate solution at 40 ° C was charged to the stirred reactor and finely ground burnt maignesite having a grain size of 100% under 0.2 min was added in an amount of 2 kg of MgO 72.5%, GaO 2 7%, SiO 2 1.4%, FeO 6 6.5%, loss on ignition 15. The suspension had a pH of 9.5. Sulfur dioxide was introduced from the cylinder with stirring for 30 min, when the pH dropped to 6. A total of · 1.8 kg of SO 2 was consumed and the reaction mixture reached 63 ° C. Then the reaction mixture was filtered to ice over 10 min and the filtrate was cooled to 20 ° C. The crystallized magnesium sulfite hexahydrate was separated from the mother liquor on a centrifuge and washed with cold water. 3.7 kg of wet pure hexahydrate were obtained, containing 99.5% by weight of MgSO4. 6H2O. Example 2

50 L of water at 50 ° C was charged to the reactor and 2 kg of fired magnesite ground below 0.2 mm were added. Sulfur dioxide was introduced from the cylinder under stirring for 25 min, when the pH dropped to 6.4. A total of 1.7 kg of SO2 was consumed. The reaction mixture was then filtered on suction to separate impurities and the filtrate, a supersaturated magnesium sulfite solution, was cooled to 25 ° C. The crystallized magnesium sulfite hexahydrate was separated from the mother liquor on a centrifuge. 5 kg of wet pure hexahydrate were obtained, containing 99.5% by weight of MgSO4. 6Η2Ό,

Claims (2)

1. A process for the production of pure magnesium sulphite by treating sulfurous material with sulfur dioxide, characterized in that an aqueous suspension prepared from contaminated magnesium raw material containing optionally dissolved magnesium sulphate and sulphite is treated with a gas containing sulfur dioxide, such that the final reaction mixture has a temperature above 40 ° C, a pH above 3.5, preferably 5.5 to 8.5, and a magnesium sulfite content above 0.7% by weight, preferably 4 to 8% by weight, after which thus obtained, of ^L exempted EZÚ ^ i · kanié'reakční mixture collected at the latest before the 'expiry of three hours the liquid. phase = from the suspended impurities and this supersaturated "magnesium sulphite" solution is worked up into a magnesium compound, e.g. by cooling, crystalline magnesium sulphite is separated. 2. The method of claim 1, wherein the liquid phase obtained after the separation of magnesium sulfite is used to prepare an aqueous suspension from the starting contaminated magnesium feedstock. Severography, n. P., Plant 7, Most Price 2,40 Kčs