CN112030591A - Novel method for recycling alkali from straw pulp black liquor by acid-alkali circulation - Google Patents

Abstract

Aiming at the problem that the black liquor is difficult to evaporate and burn due to the high content of silicon cells and the existence of sodium silicate in the conventional straw pulp black liquor, the invention provides a novel method for recycling acid and alkali from the straw pulp black liquor. The method adopts phosphoric acid as acid precipitation liquid, and lignin is precipitated by adjusting the pH value of the black liquid, so that the lignin is extracted from the black liquid. Phosphoric acid is used as a medium strong acid, the pH required for completely depositing lignin can be reached, sodium dihydrogen phosphate generated in a filter liquor is subjected to causticization treatment by calcium hydroxide, the products are calcium phosphate and sodium hydroxide, wherein the sodium hydroxide can be recycled as a cooking white liquor, the phosphoric acid and the calcium hydroxide can be obtained by a phosphogypsum recycling way, substances in the whole process are completely recycled, no harmful wastewater is generated, and clean production of non-wood fiber pulping and papermaking can be really realized.

Description

Novel method for recycling alkali from straw pulp black liquor by acid-alkali circulation

Technical Field

The present invention belongs to a new method for recovering acid-alkali regeneration circulation alkali of paper-making black liquor, which relates to the pollution treatment of alkaline pulping straw pulp black liquor, and is especially suitable for the technological transformation of black liquor acid-out method.

Background

The silicon cells of the non-wood raw material contain a large amount of silicon elements, and in the traditional alkaline pulping and cooking process, the silicon cells are dissolved by alkali liquor, and the silicon elements are dissolved out in the form of sodium silicate and exist in black liquor, so that the viscosity of the black liquor is increased, the extraction rate of the black liquor is low, and the subsequent alkali recovery and reutilization of the black liquor are difficult, namely the problem of silicon interference is solved. While the lignin in the black liquor can make degradable and renewable polymers, the hemicellulose can be used as raw material for producing chewing gum, etc. If the useful components can be separated from the black liquor, the waste is changed into valuable, and remarkable environmental benefit, economic benefit and social benefit can be obtained.

In recent years, a plurality of novel black liquor treatment methods have appeared, such as a method of treating lignin components by combining ionic liquid catalytic conversion, separating phenolic lignin from black liquor by virtue of the super-strong solubility of ionic liquid, and then performing catalytic hydrolysis on the phenolic lignin to obtain petroleum products. The supercritical fluid method is a novel method for recovering hydrogen in black liquor, separated gas products and natural gas are supplied to heat energy produced by pulping, and nitrogen oxides are generated in the process of gas discharge by using air as an oxidant, so that secondary pollution is caused to the environment. These methods have complicated requirements for equipment, instruments and experimental conditions, and thus are always in the laboratory stage, which makes large-scale industrialization difficult. The traditional black liquor evaporation combustion technology is mature, is always a common black liquor treatment method in factories, and most researches are made on a method for generating lignin by acid deposition in black liquor, wherein the key point is the problem of acid selection, the required pH value can be easily reached by using strong acid such as sulfuric acid, hydrochloric acid and the like, but the generation of sodium sulfate and sodium chloride in filtrate is difficult to recycle due to too low concentration, and secondary pollution is generated. Carbon dioxide is introduced into the black liquor to reduce the pH, but carbonic acid is weak acid and can not reach the pH required for completely depositing the lignin, so that the search for a safe and effective acid treatment method capable of recycling is very important.

The process adopts an acid-base circulation concept, and two acid-base treatment processes of phosphoric acid acidification and calcium hydroxide causticization are adopted, so that not only can organic matters in the black liquor be effectively extracted, but also the water circulation and the alkali circulation of the black liquor can be realized. Macromolecular substances are effectively combined with micromolecules such as sugar and phenolic acid in the acid and alkali treatment processes, so that COD (chemical oxygen demand) of the circulating alkali liquor is in a stable state, and a feasible theoretical basis is provided for the cooking circulation of the alkali liquor.

Disclosure of Invention

The invention aims to mainly solve the problems of difficult treatment, serious pollution and the like of non-wood pulping and papermaking black liquor, designs a novel straw pulp black liquor alkali recovery process technology, and aims to realize the clean production of straw pulp by solving the key problem of black liquor treatment. On one hand, the problem of shortage of papermaking raw materials in China is solved, the imported waste paper is not relied on, and the native straw resources are fully utilized; on the other hand, based on the concept of biological refining, the components of straw resources are separated, the separated cellulose can be used for pulping and papermaking and also can be used for producing other cellulose products, and lignin extracted from black liquor can be used for producing fertilizers such as humic acid and the like. The straw fiber resource is fully utilized, so that economic benefits can be brought, energy crisis can be relieved, and sustainable development is realized.

1. A new method for recycling alkali from straw pulp black liquor by acid-alkali circulation is characterized by comprising the following steps:

(1) preparing black liquor: cutting straws into 3cm-10cm, adding the cut straws into a digester, wherein the solid-liquid ratio is 1: 3-1: 10 (mass ratio), and the cooking conditions are as follows: the alkali for cooking is NaOH, the alkali amount accounts for 15-25% of the mass of the absolutely dry straw raw material, the cooking temperature is 150-170 ℃, and the cooking time is 1-3 h. After cooking, filtering the obtained slurry, fully washing the solid material until the pH value of the solid material is 6-9, and collecting all filtrate and water washing liquid to obtain required black liquid;

(2) black liquor phosphoric acid acidification treatment: adding NH into the black liquor obtained in the step (1)₄H₂PO₄Adjusting pH to 5-6, precipitating to obtain part of lignin, centrifuging to remove precipitate, adding phosphoric acid into the supernatant to adjust pH to less than 2, and centrifuging to obtain extracted lignin and NaH₂PO₄Red liquor;

(3)NaH₂PO₄causticizing reaction of red liquid: the NaH obtained in the step (2) is used₂PO₄Adding calcium hydroxide into red liquid at normal temperature for 30-60 min, stirring to make reaction uniform, and performing solid-liquid separation to obtain final productCalcium phosphate solids and NaOH lye;

(4) circulation of cooking liquor: after the concentration and the volume of the NaOH alkali liquor obtained in the step (3) are measured, a certain amount of solid NaOH and the amount of water for cooking are added until the amount of the alkali for cooking in the step (1), the added cooking alkali liquor is dissolved and uniformly mixed, and the operation of the step (1) is repeated, so that the circulation of the cooking alkali liquor is realized;

(5) regeneration of NaOH and H from calcium phosphate₃PO₄And (3) circulation: the calcium phosphate solid generated in the step (3) is used for circularly regenerating NaOH and H₃PO₄Mixing solid calcium phosphate with NH₄H₂PO₄Adding the mixture into a 250 ℃ high-pressure autoclave according to the molar ratio of more than 1:1.5 for reacting for 2 to 3 hours to obtain CaHPO₄Adding CaHPO₄Solid and NH₄HCO₃Adding the solid into a reactor at 600 ℃ according to a molar ratio of more than 3:1 to obtain NH₄H₂PO₄Solution and CaCO₃Solid, NH₄H₂PO₄Introducing equal mass of CO into the solution at 250 DEG C₂Obtaining H required by the circulation process₃PO₄，CaCO₃Calcining the solid at high temperature to obtain CaO, and further obtaining Ca (OH) from the CaO₂For supplementing causticizing process Ca (OH)₂Consumption of (d);

the reaction principle is as follows:

Detailed Description

The invention is further illustrated by the following examples.

Example 1

(1) Preparing black liquor: cutting straws into 3cm, adding the straws into a cooking pot, wherein the solid-liquid ratio is 1:5 (mass ratio), and the cooking conditions are as follows: the alkali for cooking is NaOH, the amount of the alkali accounts for 15% of the mass of the absolutely dry straw raw material, the cooking temperature is 150 ℃, and the cooking time is 1.5 h. After cooking, filtering the obtained slurry, fully washing the solid material until the pH value of the solid material is 6, and collecting all filtrate and water washing liquid to obtain required black liquid;

(2) black liquor phosphoric acid acidification treatment: adding NH into the black liquor obtained in the step (1)₄H₂PO₄Adjusting pH to 5, precipitating to obtain part of lignin, centrifuging to remove precipitate, adding phosphoric acid into the supernatant to adjust pH to less than 2, and centrifuging to obtain extracted lignin and NaH₂PO₄Red liquor;

(3)NaH₂PO₄causticizing reaction of red liquid: the NaH obtained in the step (2) is used₂PO₄Adding calcium hydroxide into the red liquid at normal temperature for reaction for 30min, continuously stirring to ensure that the reaction is uniform, and carrying out solid-liquid separation after the reaction is finished to obtain calcium phosphate solid and NaOH alkali liquor;

(4) circulation of cooking liquor: after the concentration and the volume of the NaOH alkali liquor obtained in the step (3) are measured, 50% of solid NaOH and 20% of cooking water consumption are added until the cooking alkali consumption in the step (1) is reached, the supplemented cooking alkali liquor is dissolved and uniformly mixed, and the operation in the step (1) is repeated, so that the circulation of the cooking alkali liquor is realized;

(5) regeneration of NaOH and H from calcium phosphate₃PO₄And (3) circulation: the calcium phosphate generated in the step (3) is used for circularly regenerating NaOH and H₃PO₄Mixing solid calcium phosphate with NH₄H₂PO₄Adding the mixture into a 250 ℃ high-pressure autoclave according to the proportion of 1:1.5 for reaction for 2 hours to obtain CaHPO₄Adding CaHPO₄Solid and NH₄HCO₃Adding the solid into a reactor at 600 ℃ according to the ratio of 3:1 to obtain NH₄H₂PO₄Solution and CaCO₃Solid, NH₄H₂PO₄Introducing equal mass of CO into the solution at 250 DEG C₂Obtaining H required by the circulation process₃PO₄，CaCO₃Calcining the solid at high temperature to obtain CaO, and further obtaining Ca (OH) from the CaO₂For supplementing causticizing process Ca (OH)₂Consumption of (d);

example 2

(1) Preparing black liquor: cutting straws into 5cm, adding the cut straws into a cooking pot, wherein the solid-liquid ratio is 1:6 (mass ratio), and the cooking conditions are as follows: the alkali for cooking is NaOH, the alkali amount accounts for 20% of the mass of the absolutely dry straw raw material, the cooking temperature is 160 ℃, and the cooking time is 2 hours. After cooking, filtering the obtained slurry, fully washing the solid material until the pH value of the solid material is 7, and collecting all filtrate and water washing liquid to obtain required black liquid;

(2) black liquor phosphoric acid acidification treatment: adding NH into the black liquor obtained in the step (1)₄H₂PO₄Adjusting pH to 5, precipitating to obtain part of lignin, centrifuging to remove precipitate, adding phosphoric acid into the supernatant to adjust pH to less than 2, and centrifuging to obtain extracted lignin and NaH₂PO₄Red liquor;

(3)NaH₂PO₄causticizing reaction of red liquid: the NaH obtained in the step (2) is used₂PO₄Adding calcium hydroxide into the red liquid at normal temperature for 45min, continuously stirring to ensure that the reaction is uniform, and carrying out solid-liquid separation after the reaction is finished to obtain calcium phosphate solid and NaOH alkali liquor;

(4) circulation of cooking liquor: after the concentration and the volume of the NaOH alkali liquor obtained in the step (3) are measured, 60% of solid NaOH and 30% of cooking water consumption are added until the cooking alkali consumption in the step (1) is reached, the supplemented cooking alkali liquor is dissolved and uniformly mixed, and the operation in the step (1) is repeated, so that the circulation of the cooking alkali liquor is realized;

(5) regeneration of NaOH and H from calcium phosphate₃PO₄And (3) circulation: the calcium phosphate generated in the step (3) is used for circularly regenerating NaOH and H₃PO₄Mixing solid calcium phosphate with NH₄H₂PO₄Adding the mixture into a 250 ℃ high-pressure autoclave according to the ratio of 1:2 for reaction for 2.5h to obtain CaHPO₄Adding CaHPO₄Solid and NH₄HCO₃Adding the solid into a reactor at 600 ℃ according to the ratio of 4:1 to obtain NH₄H₂PO₄Solution and CaCO₃Solid, NH₄H₂PO₄Introducing equal mass of CO into the solution at 250 DEG C₂Obtaining H required by the circulation process₃PO₄，CaCO₃Calcining the solid at high temperature to obtain CaO, and further obtaining Ca (OH) from the CaO₂For supplementing causticizing process Ca (OH)₂Consumption of (d);

example 3

(1) Preparing black liquor: cutting straws into 10cm, adding the cut straws into a cooking pot, wherein the solid-liquid ratio is 1:7 (mass ratio), and the cooking conditions are as follows: the alkali for cooking is NaOH, the alkali amount accounts for 25% of the mass of the absolutely dry straw raw material, the cooking temperature is 170 ℃, and the cooking time is 3 hours. After cooking, filtering the obtained slurry, fully washing the solid material until the pH value of the solid material is 8, and collecting all filtrate and water washing liquid to obtain required black liquid;

(2) black liquor phosphoric acid acidification treatment: adding NH into the black liquor obtained in the step (1)₄H₂PO₄Adjusting pH to 6, precipitating to obtain part of lignin, centrifuging to remove precipitate, adding phosphoric acid into the supernatant to adjust pH to less than 2, and centrifuging to obtain extracted lignin and NaH₂PO₄Red liquor;

(3)NaH₂PO₄causticizing reaction of red liquid: the NaH obtained in the step (2) is used₂PO₄Adding calcium hydroxide into the red liquid at normal temperature for 60min, continuously stirring to ensure that the reaction is uniform, and carrying out solid-liquid separation after the reaction is finished to obtain calcium phosphate solid and NaOH alkali liquor;

(4) circulation of cooking liquor: after the concentration and the volume of the NaOH alkali liquor obtained in the step (3) are measured, 80% of solid NaOH and 50% of cooking water consumption are added until the cooking alkali consumption in the step (1) is reached, the supplemented cooking alkali liquor is dissolved and uniformly mixed, and the operation in the step (1) is repeated, so that the circulation of the cooking alkali liquor is realized;

(5) regeneration of NaOH and H from calcium phosphate₃PO₄And (3) circulation: the calcium phosphate generated in the step (3) is used for circularly regenerating NaOH and H₃PO₄Mixing solid calcium phosphate with NH₄H₂PO₄Adding the mixture into a 250 ℃ high-pressure kettle according to the proportion of 1:3 to react for 3 hours to obtain CaHPO₄Adding CaHPO₄Solid and NH₄HCO₃Adding the solid into a reactor at 600 ℃ according to the proportion of 5:1 to obtain NH₄H₂PO₄Solution and CaCO₃Solid, NH₄H₂PO₄Introducing equal mass of CO into the solution at 250 DEG C₂Obtaining H required by the circulation process₃PO₄，CaCO₃Calcining the solid at high temperature to obtain CaO, and further obtaining Ca (OH) from the CaO₂For supplementing causticizing process Ca (OH)₂The consumption of (c).

Claims (1)

1. A new method for recycling alkali from straw pulp black liquor by acid-alkali circulation is characterized by comprising the following steps:

(1) preparing black liquor: cutting straws into 3cm-10cm, adding the cut straws into a digester, wherein the solid-liquid ratio is 1: 3-1: 10 (mass ratio), and the cooking conditions are as follows: the alkali for cooking is NaOH, the alkali amount accounts for 15-25 percent (mass ratio) of the absolutely dry straw raw material, the cooking temperature is 150-170 ℃, and the cooking time is 1-3 h. After cooking, filtering the obtained slurry, fully washing the solid material until the pH value of the solid material is 6-9, and collecting all filtrate and water washing liquid to obtain required black liquid;

(2) black liquor phosphoric acid acidification treatment: adding NH into the black liquor obtained in the step (1)₄H₂PO₄Adjusting pH to 5-6, precipitating to obtain part of lignin, centrifuging to remove precipitate, adding phosphoric acid into the supernatant to adjust pH to less than 2, and centrifuging to obtain extracted lignin and NaH₂PO₄Red liquor;

(3)NaH₂PO₄causticizing reaction of red liquid: the NaH obtained in the step (2) is used₂PO₄Adding calcium hydroxide into the red liquid at normal temperature, reacting for 30-60 min, continuously stirring to make the reaction uniform, and performing solid-liquid separation after the reaction is finished to obtain calcium phosphate solid and NaOH alkali liquor;

(4) circulation of cooking liquor: after the concentration and the volume of the NaOH alkali liquor obtained in the step (3) are measured, a certain amount of solid NaOH and the amount of water for cooking are added until the amount of the alkali for cooking in the step (1), the added cooking alkali liquor is dissolved and uniformly mixed, and the operation of the step (1) is repeated, so that the cyclic utilization of the cooking alkali liquor is realized;

(5) regeneration of NaOH and H from calcium phosphate₃PO₄And (3) circulation: the calcium phosphate solid generated in the step (3) is used for circularly regenerating NaOH and H₃PO₄Mixing solid calcium phosphate with NH₄H₂PO₄Adding the mixture into a 250 ℃ high-pressure autoclave according to the molar ratio of more than 1:1.5 for reacting for 2 to 3 hours to obtain CaHPO₄Adding CaHPO₄Solid and NH₄HCO₃Adding the solid into a reactor at 600 ℃ according to a molar ratio of more than 3:1 to obtain NH₄H₂PO₄Solution and CaCO₃Solid, NH₄H₂PO₄Introducing equal mass of CO into the solution at 250 DEG C₂Obtaining H required by the circulation process₃PO₄，CaCO₃Calcining the solid at high temperature to obtain CaO, and further obtaining Ca (OH) from the CaO₂For supplementing causticizing process Ca (OH)₂Consumption of (d);

the reaction principle is as follows: