JP2012091970A - Method for manufacturing calcium carbonate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing calcium carbonate with a low insoluble sodium content in an alkali recovery step during pulp manufacturing.SOLUTION: In a causticizing step during the alkali recovery step, the insoluble sodium content in the resulting calcium carbonate can be reduced by adjusting the amount of quicklime or slaked lime added to a green liquor relative to the sodium carbonate concentration in the green liquor.

Description

  The present invention relates to a method for producing calcium carbonate in an alkali recovery step in kraft pulp production.

  The cooking chemical used in the kraft pulp manufacturing process is an alkaline aqueous solution containing sodium hydroxide and sodium sulfide, and is generally called white liquor. Using this white liquor, wood chips are cooked under conditions such as white liquor addition amount, reaction temperature, pressure, time, sodium sulfide content in white liquor, etc., which are usually determined by kraft cooking. The black liquor generated by separating from pulp after kraft cooking is composed of organic components such as lignin generated during cooking and inorganic components such as sodium salts. The black liquor is concentrated and then used as fuel for the chemical recovery boiler. The inorganic component remaining after combustion in the boiler is called smelt and is dissolved in a weak liquid which is a dilute alkaline aqueous solution to form a green liquid. This green liquor is converted into a white liquor in the alkali recovery step and used again as a cooking chemical.

  In the alkali recovery process, the green liquor is directly mixed with calcium oxide or calcium hydroxide, or the mixed solution with the calcium oxide added to water or alkaline solution is mixed with the green liquor, and the sodium carbonate in the green liquor is derived from calcium oxide. A white liquor is prepared by generating sodium hydroxide by causing a causticizing reaction with calcium hydroxide. During this causticization reaction, calcium carbonate that is hardly soluble in water is produced simultaneously with sodium hydroxide, but this calcium carbonate is separated from the white liquor and then calcined in a kiln to form calcium oxide, which again becomes a caustic reaction. used.

  When this calcium carbonate is baked in the kiln, a scale-like deposit called a dam ring mainly composed of calcium is formed on the inner wall of the kiln, which may inhibit the baking of the calcium carbonate. Since the operation trouble of the kiln due to the dam ring formation causes great damage to the alkali recovery process that is continuously operated, it is an important issue to suppress the formation of the dam ring.

  Several factors are considered as the cause of dam ring formation in the kiln. One of them is related to a sodium component contained in calcium carbonate baked in a kiln. It has been suggested that the sodium component brought into the kiln together with calcium carbonate promotes the generation of dam rings by melting at a high temperature. Therefore, in order to suppress the formation of the dam ring, it is necessary to reduce the sodium component present in the calcium carbonate as much as possible (see, for example, Non-Patent Document 1). Particularly emphasized in this non-patent document is the presence of an insoluble sodium component contained in calcium carbonate. This insoluble sodium is present in a form that does not dissolve in water when separating the calcium carbonate from the white liquor. Therefore, it is difficult to remove the calcium carbonate by washing with water.

  Moreover, in order to control the property of calcium carbonate, the technique which controls conditions, such as causticization reaction, is disclosed (for example, refer patent documents 1-5). However, in any document, adjusting the sodium content in calcium carbonate is not considered, and a technique for reducing the content of the insoluble sodium component has not been disclosed so far.

JP 2010-132522 A JP 2002-234725 A JP 2001-199721 A JP 2001-199720 A JP 2000-264629 A

JOURNAL OF PULP AND PAPER SCIENCE, 2003, 29, 6, 185-189

  This invention is in view of the said situation, Comprising: It is providing the manufacturing method of calcium carbonate with little content of an insoluble sodium component by controlling the conditions of causticization reaction.

  As a result of studying to solve the above problems, it is considered that the insoluble sodium component in calcium carbonate is generated in the causticization reaction. The amount of calcium oxide and calcium hydroxide added to the green liquor can be changed, It was found that the content of the insoluble sodium component in the calcium carbonate to be produced can be adjusted by changing the alkali concentration.

  That is, in a green liquor having an alkali concentration of 90 g / L or more, insolubility in calcium carbonate produced when a calcium oxide or calcium hydroxide having a molar amount 1.0 times the molar amount of sodium carbonate in the green liquor is added. It has been found that the content of the insoluble sodium component decreases as the content of the sodium component is maximized and the amount added is larger than that.

  According to the present invention, in the alkali recovery step, it is possible to produce calcium carbonate with a reduced content of insoluble sodium components, and it is possible to reduce the risk of occurrence of dam rings that cause trouble when calcining calcium carbonate with a kiln. .

  The alkali recovery step in the present invention is an alkali for cooking called white liquor mainly composed of sodium hydroxide or sodium sulfide from an alkaline slurry called black liquor separated from pulp after cooking of chips in kraft pulp production. In the present invention, particularly effective in the causticizing step of converting sodium carbonate in inorganic residue called smelt after burning black liquor as fuel in a boiler to sodium hydroxide. It is.

  The green liquor in the present invention is obtained by burning the black liquor with a boiler and dissolving the remaining smelt in water or a dilute alkaline aqueous solution, and includes sodium carbonate, sodium hydroxide, sodium sulfide, and the like. . This green liquor is converted into white liquor in the causticizing step and used again as a cooking chemical.

  The alkali concentration of 90 g / L or more in the present invention means that the total concentration of sodium carbonate, sodium hydroxide, and sodium sulfide measured by the US Tapi Standard Methods T624 method is 90 g / L in terms of the sodium oxide concentration. It is shown above. The alkali concentration in the green liquor can be adjusted by changing the amount of water or dilute aqueous alkali solution when adjusting the green liquor by dissolving smelt with water or dilute alkaline solution. However, if the alkali concentration is too high, the causticization reaction may not proceed sufficiently, the washing efficiency of calcium carbonate may decrease, and the amount of sodium hydroxide accompanying it may increase, so an alkali concentration of 90 g / L or more and 150 g / L or less, more preferably an alkali concentration of 90 g / L or more and 130 g / L or less is optimal.

  The reaction between the green liquor and calcium oxide or calcium hydroxide in the present invention is carried out in a slaker or caustic tank used in a normal causticizing process, and calcium oxide or calcium hydroxide is added to the green liquor in the slaker. In addition, calcium carbonate can be recovered by solid-liquid separation from a reaction slurry that has passed a sufficient reaction time in residence in a slaker and a causticizing tank. The recovered calcium carbonate is baked in a kiln through a washing step and a concentration step, converted into calcium oxide, and used again for a causticizing reaction.

  In the present invention, the concentration of the insoluble sodium component in calcium carbonate is reduced even when calcium oxide or calcium hydroxide having a molar amount of 1.0 or less of the molar amount of sodium carbonate in the green liquor is added to the green liquor. However, inadequate addition of calcium oxide or calcium hydroxide reduces the efficiency of the causticizing reaction, and the white hydroxide sodium hydroxide concentration used for cooking the chips lowers the pulp production efficiency. End up. Further, if the amount of calcium oxide or calcium hydroxide added is too large, the efficiency of the causticizing reaction is lowered and the treatment load of calcium carbonate in the subsequent process is increased, which is not desirable. Therefore, the amount of calcium oxide or calcium hydroxide added is more than 1.0 times and less than 1.5 times, more preferably more than 1.0 times and less than 1.3 times the molar amount of sodium carbonate in the green liquor. Is optimal.

  Examples of the present invention will be described.

  The alkali concentration in the green liquor collected at Mitsubishi Paper Industries' Hachinohe Factory was measured based on the US Tapi Standard Methods T624 method. Distilled water was appropriately added to this green liquor to prepare green liquors A, B, and C having alkali concentrations of 130, 90, and 70 g / L, respectively. The concentration ratios of sodium carbonate, sodium hydroxide, and sodium sulfide in green liquors A, B, and C were the same for all green liquors.

  Each 500 mL of green liquor A, B, C is heated and boiled, and the molar ratio of calcium hydroxide to sodium carbonate contained in each green liquor is 0.5, 0.7, 1.0, 1.3, 1 Calcium hydroxide (special grade reagent, manufactured by Kanto Chemical Co., Inc.) was added so as to have a.

  The causticization reaction solution was filtered with a filter paper, and the collected filtration residue was washed with water to obtain a calcium carbonate slurry. The filtration residue may contain unreacted calcium hydroxide and other solids present in the green liquor, but in this example, all of them were used as calcium carbonate.

  This calcium carbonate slurry was dried and a part of it was precisely weighed out and 10N hydrochloric acid was added until the solid content was completely dissolved, and then distilled water was added until a predetermined volume was reached. The sodium concentration in the solution was quantified by atomic absorption, and the total sodium concentration contained in calcium carbonate was calculated. Separately weigh the above-mentioned dried calcium carbonate and add distilled water to a predetermined volume. The sodium concentration in the filtrate filtered through filter paper is quantified by atomic absorption spectrometry, and water-soluble sodium contained in calcium carbonate. Concentration was calculated. A value obtained by subtracting the water-soluble sodium concentration from the total sodium concentration was calculated as the insoluble sodium concentration in calcium carbonate. The measurement results are shown in Table 1.

  On the other hand, for the green liquors A and B, the filtrate obtained by filtering the causticization reaction solution with a filter paper was measured for the concentrations of sodium carbonate and sodium hydroxide in the same manner as the green liquor. The ratio of the sodium hydroxide concentration increase after the reaction to the sum of the concentration increase and the sodium carbonate concentration after the reaction was calculated as the causticizing rate. The results are shown in Table 2.

  As a result, in Table 1, in green liquors A and B, when the added molar amount of calcium hydroxide is 1.0 times the molar amount of sodium carbonate in the green liquor, the concentration of insoluble sodium in the generated calcium carbonate is maximized. The insoluble sodium concentration could be decreased as the amount added was larger. In the green liquor C, when the molar amount of calcium hydroxide added is 1.3 times the molar amount of sodium carbonate in the green liquor, the concentration of insoluble sodium in the generated calcium carbonate is maximized. The addition of calcium oxide or calcium hydroxide exceeding the molar amount of 1.0 times the molar amount of sodium carbonate does not necessarily reduce the concentration of insoluble sodium in the generated calcium carbonate.

  In Tables 1 and 2, in green liquors A and B, the concentration of insoluble sodium decreased when the molar amount of calcium hydroxide added was 0.7 or 0.5 times the molar amount of sodium carbonate in the green liquor. However, at the same time, the causticizing rate has also been greatly reduced, so adding a molar amount of calcium oxide or calcium hydroxide that is less than 1.0 times the molar amount of sodium carbonate also increases the efficiency of the causticizing reaction. It is greatly lowered and is not desirable.

Claims (1)

  In the alkali recovery process in kraft pulp production, a molar amount of calcium oxide or calcium hydroxide exceeding 1.0 times the molar amount of sodium carbonate in the green liquor is added to the green liquor having an alkali concentration of 90 g / L or more. A method for producing calcium carbonate, characterized by reacting.