RU2617569C2 - Method for acid-base treatment of black liquor from sulphate cellulose production - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to sulphate cellulose production. Disclosed is an acid-base method of recovering chemicals, which includes separating organic compounds from black liquor solutions from cellulose production, treating the solution with sulphuric acid to pH>1-5 to obtain a suspension of organic compounds (lignin etc.), filtering and washing the solid phase from the solution, followed by processing into a commercial product. The obtained acidic solution is treated with alkali metal salts, e.g. KOH, to pH>7 and evaporated with crystallisation of the salts to 100-500 g water per 100 g dry salt. The mother salt solution after separating the solid and liquid phases and correcting the solution is fed for cooking pulp chips, and the solid product after separation and drying thereof is removed from the process as a commercial product.

EFFECT: invention improves production efficiency based on reducing power consumption when producing cellulose, providing environmental safety of the alkali recovery process by excluding high energy consuming and environmentally hazardous processes from the process chain: burning black liquor in a soda recovery boiler, decarbonisation of limestone, causticisation of soda liquor; reduces material consumption and improves diversification of production.

1 dwg

Description

The invention relates to the field of sulfate production of cellulose and can be used for the regeneration of sodium salts from a solution of black liquor.

A known method of regeneration of alkalis from black liquor in a soda recovery boiler (IBS) - see Nepenin Yu.N. “Production of sulfate cellulose”, t.2. - M .: Forest industry, 1990, p. 372-397. According to this method, black liquor is concentrated to 70-80% of absolutely dry substances (a.s.w.). Absolutely dry substances mean a mixture of organic and mineral compounds (65-70% of organic substances and 30-35% of minerals). The fortified solution is burned in the IBS to produce sodium sulfides and sodium carbonates in the remainder of the melt.

The disadvantages of the described method include:

1. The high energy intensity and environmental hazard of processes in the IBS, the decarbonization of limestone and the evaporation of solutions of black liquor.

2. A large number of technological stages of the process of processing black liquor: evaporation, burning of a solution of black liquor in the SRK, decarbonization of limestone, caustization of soda solution, characterized by high capital and operating costs.

The high energy intensity of processes in the IBS is determined by the following factors:

- high fuel consumption (organic compounds mainly in the form of lignin and its other forms) when moisture is removed from black liquor in IBS;

- excessive consumption of fuel in the IBS, due to the need to restore sodium sulfates to sulfides. At the same time, only 30% of sodium sulfide to that recovered in IBS is used in the process of cooking technological chips - see Novikova A.I. “Production of fibrous semi-finished products”: Textbook. - 2nd ed. / SPbGTURP. SPb., 2006, p. 32. The remainder is oxidized to thiosulfates by air, introducing a special technological redistribution;

- high (250-280 ° C) temperature of the flue gases discharged into the environment due to the high temperature of the dew point.

The energy intensity of the process of decarbonization of limestone is determined by the following factors:

- fuel consumption for evaporation of water during drying and decarbonization of lime sludge in drum kilns;

- low heat recovery of flue gases and calcined lime.

The environmental hazard of the process in IBS is due to the following main reasons:

- flue gases contain a large amount of water vapor and sulfur-containing compounds, which upon cooling produce sulfuric acid;

- large ablation of process dust;

- when cooling the melt of mineral salts with a solution of white or green liquor, explosions can form.

The causes of the environmental hazard of limestone decarbonization are:

- large ablation of technological dust in the amount of 5-15% to the environment - see Milovidova A.A., Sevastyanova Yu.V., Komarova G.V. et al. “Chemical regeneration in the production of sulphate pulp (causticization and lime regeneration)”. Tutorial. Arkhangelsk, 2010, p. 75, 134 .;

- the removal of flue high-temperature (200 ° С) gases with high moisture content and calcined dusty lime (150 ° С) is discharged into the environment - see Milovidova A.A., Sevastyanova Yu.V., Komarova G.V. et al. “Chemical regeneration in the production of sulphate pulp (causticization and lime regeneration)”. Tutorial. Arkhangelsk, 2010, p. 75, 134.

Known hydrochemical method of regeneration of sodium alkali - see patent No. 2415984 dated April 10, 2011, according to which a solution of black liquor is subjected to carbonization by treatment with flue gases of decarbonization furnaces to obtain a lignin suspension. After separation of the suspension, lignin is sent for processing to obtain a marketable product, and the mother liquor of lignin is sent for evaporation with crystallization of sodium salts. After evaporation, the resulting suspension is separated into a soda-sulfate mixture and a mother liquor of a soda-sulfate mixture. The solid product is sent for partial reduction of sulfate to sodium sulfide, after which it is mixed with the mother liquor of the soda-sulfate mixture and sent for caustization. The solution after caustization is adjusted for alkalis. The resulting working solution is used for cooking chips.

In this method, it is possible to deduce from the technological circuit of the IBS devices. However, the remaining disadvantages inherent in the classical method of processing black liquor remain: the process of calcining limestone, causticization, high capital and operating costs.

As a prototype, a method was selected for isolating lignin from a portion (10%) of the total black liquor stream by treating it with sulfuric acid to form a lignin suspension - see - V.A. Demin. "Chemistry and technology of sulfate liquors." Textbook, Syktyvkar, SLI, 2013, p. 96. After separation of the suspension, lignin is processed into a commercial product, and the mother liquor of lignin is mixed with the main stream of black liquor, which is processed according to traditional technology.

The aim of the invention is to remedy these disadvantages in the process of regeneration of black liquor chemicals.

The technical result of the invention is the elimination of energy and operating costs in the processes of burning black liquor in the IBS, calcining limestone, causticizing the soda solution, increasing the operational reliability of the equipment and eliminating the environmental hazards of these processes by removing them from the technological circuit of the apparatus.

The technical result is achieved by a method comprising treating black liquor with sulfuric acid to a pH> 1-5, obtaining a suspension of organic compounds, separating the solid and liquid phases with processing the solid phase into a commercial product, characterized in that the black liquor after isolation of organic compounds is treated with alkali metal salts to pH> 7 and evaporated to 100-500 g of H ₂ O / 100 g of dry salts with crystallization of these salts from solution; while the resulting suspension is divided: the solid product is sent for processing into a marketable product, and the circulating solution after adjustment is sent to the cooking chips.

The drawing shows a simplified schematic diagram of the proposed method and an example of its implementation.

The solution of black liquor after cooking technological chips enters the mixer (1), where it is treated with a solution of sulfuric acid to pH = 3. At this stage, the reactions of the decomposition of organic compounds associated with alkali, and their precipitation. In addition, chemical reactions of the interaction of caustic alkali and sulfuric acid with the formation of sulfates take place. In the resulting suspension contains up to 90% of the total mass of lignin contained in the solution. The suspension is filtered on the filter (2). The solid phase of lignin is sent for processing into a marketable product. The mother liquor of lignin enters the reactor (3). In the reactor, the solution is neutralized with potassium hydroxide (KOH) to a pH> 7, which ensures effective subsequent evaporation of the solution. The resulting solution was evaporated to a concentration of 150 g of H ₂ O / 100 g of dry salts in a multi-stage evaporator (4). This ratio is determined by the technological capabilities of the evaporation of the solution in the presence of a large amount of solid phase in the production housing of the evaporator. The resulting suspension is separated on the filter (5): the solid potassium sulfate product is sent for further processing into a commercial product, and the mother liquor of potassium sulfate is adjusted with alkali metals - Na ₂ O, Na ₂ S, Na ₂ SO ₄ (if necessary) and diluted with wash water . In the adjusted solution, the concentration of active alkali in units Na ₂ O is 100 g / l with a sulfide content of 30%. Obtained in the mixer to adjust the solution (6), the circulating solution is sent to the cooking process chips.

According to the proposed method, it is possible to carry out a radical modernization of pulp production on the basis of energy-saving technology for the regeneration of chemicals at low investment.

The effectiveness of the proposed method is determined by the reduction of energy, capital and operating costs in the processing of black liquor. At the same time, environmental safety of the processes is ensured on the basis of withdrawal from the technological circuit of SRK apparatuses, limestone decarbonization furnaces and caustization of soda solutions.

The implementation of the proposed method allows you to:

1. To simplify the hardware-technological scheme and, accordingly, to increase the operational reliability of the installations of the alkali recovery process from black liquor.

2. Eliminate fuel consumption on the IBS and limestone kilns.

3. Reduce the consumption of circulating water and, accordingly, reduce the steam consumption for the evaporation of black liquor.

4. Increase energy efficiency and reduce capital costs for the process of evaporation of a solution of potassium sulfate.

5. Improve the environmental safety of the process of processing black liquor by eliminating the processes of burning black liquor in the SRK, the process of calcining limestone, causticizing soda solutions, and eliminating the introduction of lime into the process.

6. To exclude the release of high temperature greenhouse gases into the environment from the production process.

7. To carry out the diversification of production based on the expansion of products (commercial products based on lignin and soda products).

Claims (1)

An acid-base method for the recovery of chemicals, including the separation of organic compounds from solutions of black liquor for cellulose production, processing the solution with sulfuric acid to pH> 1-5 to obtain a suspension of organic compounds (lignin, etc.), filtering and washing the solid phase from the solution with subsequent processing for a commercial product, characterized in that in order to reduce energy and capital costs, diversify production and improve environmental safety of technology, the acid solution is treated with salt and alkali metals, for example KOH, to pH> 7 and evaporate with crystallization of salts to 100-500 g of water per 100 g of dry salts, while the mother liquor of the salts after separation of the solid and liquid phases and adjustment of the solution is sent to the cooking chips, and solid the product after its separation and drying is removed from the process as a marketable product.