CN1114011C

CN1114011C - Alkaline pulping process of wheel straw without drainage of black liquid

Info

Publication number: CN1114011C
Application number: CN 01115000
Authority: CN
Inventors: 焦为成; 林军; 彭学伟; 冯一民
Original assignee: Weifang University
Current assignee: Weifang University
Priority date: 2001-05-30
Filing date: 2001-05-30
Publication date: 2003-07-09
Anticipated expiration: 2021-05-30
Also published as: CN1317610A

Abstract

The present invention relates to a treatment method of papermaking waste liquid, particularly to a comprehensive treatment method of black liquid generated by making pulp with wheat straw by an alkaline method, which comprises the steps: firstly, polyacrylamide as a flocculating agent is used for separating lignin by an acid eduction method; then, light yellow water is solidified by sodium silicate, the obtained solidified substance is used for producing water glass which can be used as a solidifying agent for solidifying the light yellow water, the rest of water glass can be sold for gaining profits, tail gas generated in the process of producing the water glass can be led into a black liquid absorbing tower, and the black liquid absorbs harmful ingredients, such as carbon dioxide, sulfur dioxide, etc., in the black liquid absorbing tower. The present invention has the advantages of little investment, low running cost and no secondary pollution.

Description

Method for zero discharge of black liquor in wheat straw alkaline pulping

The invention relates to a method for treating paper-making waste liquor, in particular to a method for comprehensively treating black liquor generated by wheat straw alkaline pulping.

At present, straw pulp papermaking occupies a very important position in China, waste water (black liquor) generated in the papermaking process is always an important pollution source of environmental pollution, and two methods for treating the black liquor are generally adopted: one is to add acid into black liquor to carry out neutralization reaction, then precipitate, filter and recover lignin, and discharge sewage after biological fermentation, and the defects are that alkali in the sewage cannot be recovered, the sewage still contains more pollutants, and the pollution to the environment is great. The other method is to concentrate the black liquor, then to spray-burn, then to hydrolyze, then to filter, causticize into liquid alkali to recover. The method burns out 70% of organic matter in black liquor, and has large investment, low benefit and white mud pollution. The two methods can not realize the zero emission of harmful substances and are not easy to popularize and utilize.

The invention aims to provide a method for zero discharge of wheat straw alkaline pulping black liquor, which has the advantages of high extraction rate of lignin, no secondary pollution, no black liquor discharge and low operation cost.

The purpose of the invention is realized as follows:

firstly, sending the squeezed black liquor into an absorption tower, then sending into a dissolving tank for dissolving a flocculating agent polyacrylamide, extracting lignin, and then carrying out the following steps: A) curing light yellow water, namely mixing the light yellow water after lignin separation with sodium silicate according to a certain proportion, standing to cure the light yellow water, and drying a cured substance to be used as a raw material for producing sodium silicate in the next step; B) regenerating the curing agent, drying the cured product, measuring the content of sodium sulfate and silicon dioxide in the cured product, adding corresponding components, and placing the cured product in a reflecting furnace for producing sodium silicate.

As a further improvement of the invention, before extracting lignin in black liquor by polyacrylamide and sulfuric acid, waste gas containing sulfur dioxide and carbon dioxide generated in the production process of sodium silicate is led into a black liquor absorption tower, the sulfur dioxide and the carbon dioxide react with residual alkali in the black liquor, the pH value of the black liquor after absorbing the waste gas is reduced to 6-8, and then the lignin is extracted.

As a further improvement of the invention, sodium silicate with the modulus of more than 2.5 is mixed with the light yellow water for solidification in the solidification process of the light yellow water, and the mass ratio of the sodium silicate to the light yellow water is (0.37-0.03): 1.

As a further improvement of the invention, in the regeneration process of the curing agent, the mass ratio of the sodium sulfate, the carbon and the silicon dioxide in the cured material is 1 to (0.02-0.04) to (0.4-1.7) by adding corresponding components.

The method for treating the papermaking black liquor mainly has the following advantages: 1) the method firstly treats the papermaking black liquor with acid to extract lignin, then completely solidifies the light yellow water after the lignin is extracted, the dried condensate is used for producing liquid sodium silicate, and the waste gas containing sulfur dioxide and carbon dioxide generated in the production process is absorbed by the thick black liquor, so that the whole production process has no discharge of any black liquor, no discharge of waste gas such as sulfur dioxide and the like, and no secondary pollution thoroughly. 2) The product after the condensate is dried is added with a proper amount of quartz sand and coal powder for producing the sodium silicate, the price is low, the production technology is mature, and compared with the current common alkali recovery technology, the method has the advantages of low equipment investment, low operation cost and no secondary pollution. Because the use of the sodium silicate is wide, the produced sodium silicate can be sold for profit except for being used as a curing agent for curing light yellow water. 3) The black liquor is used for absorbing the waste gas generated in the process of producing the sodium silicate, so that the waste gas can be prevented from polluting the environment, the pH value of the black liquor can be reduced, the hydrolysis of polyacrylamide and the use amount of sulfuric acid in the subsequent process can be reduced, and the operation cost is further reduced.

The invention is further illustrated by the following detailed description in conjunction with the drawings in which:

the figure is a process flow diagram of oneembodiment of the present invention.

Example (b):

the method comprises the steps of producing 300 tons of black liquor with COD of more than 8 ten thousand per day, absorbing tail gas generated by reaction in a reverberatory furnace when the black liquor passes through an absorption tower, reducing the pH value of the black liquor to about 8, adding 24.75 kilograms of flocculating agent polyacrylamide into one fourth of the black liquor, slowly stirring for 4 hours in a dissolving tank to completely dissolve polyacrylamide in the black liquor, and feeding the black liquor dissolved with the polyacrylamide into a mixer. In the section A of the mixer, the black liquor dissolved with polyacrylamide and the residual black liquor after tail gas absorption are fully mixed in the section A according to the volume ratio of 1: 4 and then enter the section B of the mixer. And adding 93% sulfuric acid into the section B, uniformly mixing to make the pH value of the black liquor be 1, and conveying the black liquor into an inclined-net type lignin separator through a pipeline. 127.5 tons of wet lignin containing 80 percent of water and 172.5 tons of light yellow water can be obtained, and the extraction rate of lignin is as high as 90 percent. 172.5 tons of light yellow water with the pH value of 1 and the sodium silicate with the modulus of 3 sequentially enter a static mixer according to the mass ratio of 1: 0.125, are fully mixed in the mixer, and generate chemical reaction, and the chemical reaction equation is as follows:

since silica sol is extremely unstable in an acidic medium, it rapidly coagulates to form silica gel. Therefore, after the sodium silicate is mixed with the light yellow water, the mixture can flow into a curing pool forcuring within 30 seconds, the curing time is about 5 minutes, the cured substance is dried to obtain 21 tons of solid slag containing 12 tons of sodium sulfate and 7.3 tons of silicon dioxide, 7.9 tons of quartz sand and 0.5 ton of coal powder, the raw materials are continuously placed in a reverberatory furnace after being uniformly mixed, the reaction temperature is 1500 ℃, the crushed material is discharged after 5 hours of reaction and enters a material melting device for melting, and 23 tons of sodium silicate with the modulus of 3.0-3.2 are produced. A part of the water is used as a curing agent for curing light yellow water, and the rest can be sold for profit. The tail gas generated in the reverberatory furnace in the process of producing the sodium silicate can be guided into the absorption tower, so that the black liquor absorbs the tail gas, and the pH value of the black liquor is reduced.

In the process of producing the sodium silicate, the sodium sulfate, the silicon dioxide and the carbon are adjusted in proportion and the production process can produce the sodium silicate with the modulus of 1-4, and the sodium silicate can be used for curing light yellow water, but the curing time and effect of the sodium silicate with different moduli are slightly different, and the sodium silicate is not detailed here.

Claims

1. A method for zero discharge of black liquor in wheat straw alkaline pulping comprises a process of extracting lignin by using sulfuric acid and polyacrylamide, and is characterized in that: the method also comprises the following processes of A) curing the light yellow water, mixing the light yellow water after lignin separation and liquid sodium silicate according to a certain proportion, standing to cure the light yellow water, and drying cured substances to be used as raw materials for producing the sodium silicate in the next step; B) regenerating the curing agent, drying the cured product, measuring the content of sodium sulfate and silicon dioxide in the cured product, adding corresponding components, and placing the cured product in a reflecting furnace for producing sodium silicate.

2. The method for zero discharge of black liquor in alkaline pulping of wheat straw according to claim 1, characterized in that: before extracting lignin by polyacrylamide and sulfuric acid, waste gas containing sulfur dioxide and carbon dioxide generated in the production process of soda foam is introduced into a black liquor absorption tower, so that the sulfur dioxide and the carbon dioxide react with residual alkali in the black liquor, the pH value of the black liquor after absorbing the waste gas is reduced to 6-8, and then the lignin is extracted.

3. The method for zero discharge of black liquor in wheat straw alkaline pulping according to claim 1 or 2, characterized in that: in the process of curing the light yellow water, sodium silicate with the modulus of more than 2.5 and the light yellow water are mixed and cured, and the mass ratio of the sodium silicate to the light yellow water is (0.37-0.03): 1.

4. The method for zero discharge of black liquor in alkaline pulping of wheat straw according to claim 1, characterized in that: in the regeneration process of the curing agent, the mass ratio of the sodium sulfate, the carbon and the silicon dioxide in the cured material is 1 to (0.02-0.04) to (0.4-1.7) by adding corresponding components.