CN1084413C

CN1084413C - Process for causticizing green solution of alkali recovered from black liquid of paper-making

Info

Publication number: CN1084413C
Application number: CN98109098A
Authority: CN
Inventors: 董学芝; 艾天召; 赵海兰
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-06-14
Filing date: 1998-06-14
Publication date: 2002-05-08
Anticipated expiration: 2018-06-14
Also published as: CN1239166A

Abstract

The present invention relates to a novel technology for causticizing green liquor of alkali recovered from paper-making black liquor, which mainly comprises the following steps: green liquor and quicklime are respectively pretreated before a causticization reaction, and the impurities are removed; then successive twice causticizations, filtering, washing, etc. are carried out so as to directly produce two products of caustic soda(white liquor) and light calcium carbonate without generation of waste slag-white slime. The technology of the present invention avoids the secondary pollution, eliminates the treatment process of the whites lime, reduces the investment for equipment and the energy consumption of the recovery of alkali from paper-making black liquor, and obviously improves the benefit.

Description

Causticizing process for recovering green liquor from black paper-making liquor alkali

The invention relates to a process for recovering alkali from papermaking black liquor, in particular to a novel causticizing process for recovering green liquor from the alkali from the papermaking black liquor. It is used for treating the pollution of black liquor in paper making.

At present, the pollution of the papermaking black liquor to the environment is a problem which draws much attention. The basic approach to remediating such pollution is usually to perform caustic recovery. More mature alkali recovery technology is described in detail in the pulping and papermaking process (Jiang Xiongxiang Shu, published by the light industry Press of China in 2 months 1992). The alkali recovery process comprises the following steps: the black liquor is first evaporated, burnt, dissolved and converted into green liquor, clarified, and then causticized with excessive lime through the causticization reaction:

make Na in green liquor₂CO₃Conversion to HaOH (white liquor). The latter step of alkali recovery is referred to as green liquor charging. The alkali recovery process can eliminate the pollution of black liquor to environment and recover caustic soda (white liquor). However, the following problems still remain: firstly, while recovering caustic soda, a large amount of waste residue, namely white mud, which seriously pollutes the environment is generated, for example, a medium-scale pulping and paper-making factory generates tens of tons to hundreds of tons of white mud every day, and serious secondary pollution is caused. At present, the main methods adopted internationally for the treatment of the white mud are as follows: the white mud is burnt into lime for recycling. The treatment method has the advantages of large equipment investment, high energy consumption, but low economic value, and is only suitable for wood pulp factories. For a straw pulp plant, due to the fact that the silicon content of white mud is high, if the white mud is burnt into lime for recycling, silicon accumulation can be caused, and vicious circle of silicon interference in alkali recovery is formed. In a word, no suitable technology exists at home and abroad for the treatment of white mud recovered from straw pulp black liquor alkali and secondary pollution thereof. At present, most of the grass pulp factories in China discharge white mud obtained by alkali recovery of black liquor of grass pulp into rivers and lakes or fill ditches. The method is one of important factors for restricting the alkali recovery development of the papermaking black liquor; secondly, the white liquor after causticization is not easy to clarify, the white mud is not easy to settle, filter and wash, the residual alkali content in the white mud is high, and the alkali loss is large, which is mainly caused by that colloidal substances in the marginal liquor are brought into a causticization reaction system.

The present invention aims at providing a new causticizing process for recovering green liquor from alkali in paper-making black liquor, which can avoid the generation of waste residue (white mud) in the process of recovering alkali from paper-making black liquor, and can directly produce another chemical product, namely light calcium carbonate, while producing caustic soda (white liquor). Thus, it not only can eliminate the pollution of black liquor, but also can completely convert the black liquor and lime into two important chemical raw materials necessary for paper-making.

In the process of recovering alkali from papermaking black liquor, Na is removed from the cloudy green liquor from the combustion section₂CO₃In addition to the useful alkali, silt and incompletely burnt carbon particles, also contains metal hydroxides, such as Fe (OH)₃Al(OH)₃、Mg(OH)₂Sulfides, e.g. Fe₂S₃And colloidal substances such as FeS and silicon-containing compounds. If the impurities enter the next causticization reaction system, not only the white liquor (caustic soda) is not easy to clarify and the white mud is not easy to precipitate, filter and wash, thereby influencing the normal operation of the causticization process, but also the white precipitate (CaCO) generated in the causticization reaction is necessarily mixed₃) Once mixed, it is difficult to remove. Therefore, it is necessary to completely remove the green liquor before the green liquor enters the causticization reaction so that the impurities do not enter the causticization reaction system at all.

The solution is as follows:

1. in the process of recovering alkali from black liquor in paper making, before causticization reaction, the turbid green liquor from combustion section is pretreated to remove impurities. The green liquor pretreatment process comprises the following steps: (1) and placing the turbid green liquor in a green liquor preprocessor, heating to 80-100 ℃, adding lime milk while stirring, fully stirring, and transferring into a green liquor clarifier for clarification. However, the lime milk and the green liquor should meet the following specified amounts: i.e. every 1m³Na₂CO₃Lime milk with CaO of 1-5Kg is added into green liquor with the content of 100 g/L. Thus, before the green liquor is clarified, the green liquor is heated, a small amount of lime milk is added, and under the condition of fully stirring, colloid, carbon particles and other impurities in the green liquor are adsorbed, co-precipitated and broken down through the strong surface adsorption effect, so that the sedimentation speed of the impurities in the clarifier is accelerated. (2) Filtering the clarified green liquor through a fine filter to completely remove the clarificationThe obtained clear and transparent green liquor is placed in a storage tank for causticization.

2. Lime is digested into lime milk in advance before the causticization reaction, coarse lime ash is removed, and then the lime milk is sieved by a sieve with 120-180 meshes to further remove impurities in the lime. Fine lime milk after sieving is prepared for causticization.

3. Two causticizations are carried out successively to produce two products of caustic soda (white liquor) and light calcium carbonate. The process comprises the following steps:

(1) will contain Na₂CO₃106 parts (by weight) of the pretreated refined green liquor and 56-67.2 parts (by weight) of fine lime milk containing CaO are placed in a causticizer, and the causticization is carried out for one time at the temperature of 70-100 ℃, wherein the causticization reaction is as follows: the caustic soda solution (white liquor) separated by filtering after causticization is sent to the cooking section of papermaking for use.

In the process, the excessive lime milk can make Na in green liquor₂CO₃Conversion to NaOH to Na as complete as possible₂CO₃The conversion rate of (2) is high.And after the CaO in the calcium carbonate is completely reacted, filtering, returning filtrate for causticization, and washing, drying, crushing and sieving the filtered solid to obtain the finished product of the light calcium carbonate. However, the term "essential green liquor" means Na contained in the essential green liquor₂CO₃The weight ratio of CaO which is contained in the solid matter filtered after the first causticization and does not participate in the reaction is 2.5-20.

The solids filtered out after thefirst causticization are removed of CaCO produced by the causticization₃In addition, excessive CaO which does not participate in the reaction is also contained, when excessive refined green liquor is added into the system for secondary causticization, the Na in the system is₂CO₃Excess amount of CaO in the solid can completely participate in the reaction and conversion into CaCO₃. Filtering, washing to remove NaOH and Na entrained in solid₂CO₃The soluble substances are mixed to obtain pure water-containing light CaCO₃Then drying and the likeLight CaCO can be obtained₃And (5) finishing. The filtrate after the second causticization contains excessive Na besides NaOH₂CO₃Therefore, it cannot be sent to the cooking section as white liquor, but should be returned for primary causticization.

The present invention uses green liquor and quicklime as raw materials in the alkali recovery process of paper-making black liquor, and before causticization reaction, the green liquor and quicklime are respectively pretreated to remove impurities, then the causticization, filtration, washing, drying, pulverizing and sieving are successively carried out twice, so that two products of caustic soda (white liquor) and light calcium carbonate can be directly produced. Compared with the existing alkali recovery process, the process has the advantages of no waste residue (white mud) production, no secondary pollution, no white mud recovery treatment process, no raw material and no cost increase, and can produce another important chemical product, namely light calcium carbonate, besides the same amount of caustic soda as the original process. The invention can effectively reduce the equipment investment and energy consumption of the alkali recovery of the papermaking black liquor, and obviously improve the benefit.

The attached figure is a flow chart of a new causticizing process for recovering green liquor from papermaking black liquor alkali.

The process of the present invention, an industrial exchange, is further illustrated below with reference to the following examples.

Example 1:

collecting turbid green liquor from wood pulp and paper mill, collecting Na-containing solution₂CO₃212 parts (by weight, the same below) of turbid green liquor is placed in a preprocessor, heated to 90 ℃, stirred and added with 56 parts of lime milk containing CaO, the mixture is transferred into a green liquor clarifier to be clarified after being fully stirred, and the clarified green liquor is filtered to obtain clear and transparent refined green liquor for causticization. And taking lime containing 112 parts of effective CaO, digesting, removing coarse lime ash, sieving (140 meshes), and preparing fine lime milk after sieving for causticization.

Collecting Na₂CO₃106 parts of the pretreated refined green liquor are added with 67.2 parts of CaO-containing fine lime milk and put into a causticizerCausticizing at 90 deg.C, filtering to obtain white liquor, and steaming. Adding 106 parts of Na into the filtered solid₂CO₃The pretreated refined green liquor is subjected to secondary causticization. Filtering after the CaO in the solid matters completely reacts, sending the filtrate to the next round of causticization, and washing, drying, crushing and sieving (120 meshes) the filtered solid matters to obtain the finished product of the light calcium carbonate. Will be describedThe finished light calcium carbonate product is analyzed and measured according to the method specified in GB4794-84, and the results are as follows:

CaCO₃(dry basis): 97.82 percent; moisture content: 0.26 percent; screen residue: 125 μm, O; 45 μm, 0.48%; sedimentation volume: 3.1 ml/g; hydrochloric acid insoluble substance: 0.089%; free base (calculated as CaO): 0.025 percent; iron: 0.051%; manganese: 0.0032%.

It is clear from the results of the analytical measurements that the light calcium carbonate (CaCo) produced by the process₃) And meets the standard specified in GB 4794-84.

Example 2:

the turbid green liquor sent from the burning section is collected from a straw pulp paper mill, and the light calcium carbonate finished product obtained in the same way as the example 1 is analyzed and determined to have the following results:

CaCO₃(dry basis): 97.15 percent; moisture content: 0.3 percent; screen residue: 125 μm; o; 45 μm, 0.46%; sedimentation volume: 3.0 ml/g; hydrochloric acid insoluble substance: 0.56 percent; free base (calculated as CaO): 0.058%; iron: 0.066%; manganese: 0.0045 percent.

The analysis and measurement result shows that: light calcium carbonate (CaCO) produced by adopting the process₃) Except that the content of hydrochloric acid insoluble substances is slightly higher than the standard, the other substances all meet the standard specified in GB 4794-84. The reason why the hydrochloric acid insoluble substance is slightly higher is because the silicon content in the black liquor of straw pulp papermaking is high, and the quality of the product can completely meet the requirements of industrial use of papermaking, rubber, plastics and the like.

Claims

1. A causticizing process for recovering green liquor from black paper-making liquor alkali includes such steps as evaporating black liquor, burning, dissolving molten substance to obtain green liquor, causticizing the green liquor with lime to obtain white liquor and waste sludge dregs,

1) pretreating the turbid green liquor from a combustion section to remove impurities in the turbid green liquor before carrying out the causticization reaction, wherein the pretreatment process comprises the following steps,

(1) placing the turbid green liquor in a preprocessor, heating to 80-100 ℃, adding lime milk while stirring, transferring into a green liquor clarifier for clarification after fully stirring,

(2) filtering the clarified green liquor with fine filter to completely remove solid substances remained in the clarified green liquor, placing the obtained clear and transparent refined green liquor in a storage tank for causticization,

2) lime is digested into lime milk in advance before the causticization reaction is carried out, coarse lime ash is removed, then the lime milk is sieved, the used sieve is 120-180 meshes to further remove impurities in the lime, the sieved fine lime milk is prepared for causticization,

3) two causticizations are carried out successively to respectively produce two products of caustic soda solution (white liquor) and light calcium carbonate, the process comprises the following steps,

(1) will contain Na₂CO₃106 parts by weight of the pretreated refined green liquor and 56-67.2 parts by weight of fine lime milk containing CaO are placed in a causticizer for causticization at the temperature of 70-100 ℃, caustic soda solution (white liquor) is separated by filtration and sent to a cooking section for papermaking,

(2) adding refined green liquor into the solid matters filtered after the primary causticization for secondary causticization untilCaO in the solid matters is completely reacted, filtering, returning the filtrate for primary causticization, and washing, drying, crushing and sieving the filtered solid matters to obtain the finished product of the light calcium carbonate.

2. The process for causticizing green liquor from alkali recovery of paper-making black liquor as claimed in claim 1, wherein the amount of turbid green liquor and lime milk used in pretreatment of green liquor is 1m³Na₂CO₃Lime milk containing 1-5kg CaO is added into the turbid green liquor with the content of about 100 g/L.

3. The causticizing process of green liquor from alkali recovery of black liquor in paper making according to claim 1, wherein the refined green liquor added during the second causticizing is Na contained in the refined green liquor₂CO₃The weight ratio of CaO which is contained in the solid matter filtered after the first causticization and does not participate in the reaction is 2.5-20.