JPS6220319B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing silica from green liquor obtained by concentrating and burning cooking waste in an alkaline pulp cooking chemical recovery process such as the Kraft method or soda method. This relates to a method for removing silica by precipitating it and passing it through. Bamboo, reed, bagasse, etc. have a higher silica content than wood, and silica is dissolved in the cooking waste liquor (hereinafter referred to as black liquor) in the alkaline pulping process such as the kraft method and the soda method. This silica causes various troubles in the subsequent cooking chemical recovery process. The usual kraft pulp chemical recovery process includes a process of concentrating the cooking waste liquid, a process of burning the concentrated waste liquid, and a causticizing process of causticizing the green liquor obtained by combustion to obtain a cooking chemical liquid. If the silica content is high in this recovery process, it will cause troubles such as scaling and clogging problems in the concentration process, poor combustion in the combustion process, and poor calcination of calcium carbonate and a decrease in causticization rate in the causticizing process. The main components of green liquor are Na ₂ S, Na ₂ CO ₃ and NaOH, and other components include Na2SO ₄ , Na2S ₂ O ₃ and Na ₂ SO ₃ , but most of the silica in the cooking waste is green. Go into the liquid. In the causticizing process, slaked lime is added to the green liquor to cause a causticizing reaction and convert Na- _2CO3 into NaOH. The CaCO ₃ precipitate produced at this time is separated, then calcined and slaked to be used as slaked lime in the causticizing process. If silica is contained in the green liquor, the silica will precipitate together with calcium carbonate during this causticizing process. If calcium carbonate contains a large amount of silica, it will cause firing failure in the next firing process. Furthermore, if calcium carbonate containing silica is recycled to the recovery process, it will cause the above-mentioned troubles, so calcium carbonate containing silica has no choice but to be discarded. This leads to increased drug costs and waste disposal costs. As mentioned above, when pulping a raw material containing a large amount of silica, various problems arise in the chemical recovery process, so it is necessary to remove the silica by an appropriate method. Conventionally, methods of removing silica from black liquor and removing silica from green liquor have been proposed as methods for removing silica. In particular, a two-stage causticization method has been proposed as a method for removing silica from green liquor. This conventional method involves adding 1.5 to 2.0 moles of slaked lime to the amount (moles) of silica in the green liquor to produce calcium carbonate and silica as a precipitate, and then over-separating the precipitate, which then produces a liquid that is usually This method regenerates the cooking liquor (usually referred to as white liquor) by causticizing it. In alkaline pulping methods such as the kraft method and the soda method using reeds, bamboo, bagasse, etc. as raw materials, silica is usually contained in the green liquor in the form of SiO ₂ of about 0.2 to 0.3 mol/. This two-stage causticizing method involves adding 1.5 to 2.0 moles of slaked lime to the silica contained in the green liquor to precipitate the silica together with calcium carbonate, and after excessive separation, the liquid is causticized and cooked as before. It regenerates the liquid. The inventors of the present invention conducted intensive studies on this silica removal method, and as a result, they discovered a method for achieving a high silica removal rate by reducing the amount of slaked lime added. The method of the present invention involves adding slaked lime in two or more stages as a step before final causticizing the green liquor for silica removal.This method can reduce the amount of slaked lime added and greatly reduce the lime loss rate. can be reduced to That is, the present invention removes silica from the green liquor by adding slaked lime to the green liquor obtained by concentrating and burning waste water from alkaline pulp cooking such as the Kraft method or the soda method to precipitate calcium carbonate and silica. In the method of This is a method for removing silica from pulp cooking chemicals, which is characterized by excessive separation. In addition, when carrying out the method of the present invention in two stages,
As the first stage, the amount of slaked lime added to the silica content in the green liquor is 0.5 to 1.5 (molar ratio) of Ca(OH) ₂ /SiO ₂
Add slaked lime to the green liquor to double the ratio to precipitate silica, separate the precipitate separately, and use the liquid again as the second stage to add slaked lime to the silica contained in the liquid. ) ₂ /SiO ₂ (molar ratio) is preferably re-added at a ratio of 1.0 to 1.5 times. Furthermore, in the method of the present invention described above, it is preferable to carry out the treatment so that the total amount of slaked lime added is 1.5 times or less in molar ratio to the silica contained in the initial green liquor. When slaked lime is added to green liquor, silica is precipitated and at the same time a causticizing reaction of Na- _2CO3 occurs as described below. Ca(OH) ₂ + Na ₂ CO ₃ →2NaOH + CaCO _3This causticization rate increases as the amount of slaked lime added increases, but as a result of various studies, the molar ratio of the amount of slaked lime added to the silica content in the green liquor is approximately 1.5. When the amount is more than doubled, the causticization rate of Na ₂ CO ₃ suddenly increases. For example, in the case of green liquor containing 0.28 mol of silica, the molar ratio of slaked lime to silica is 1.0
When increasing from 2.0 times to 2.0 times, the causticization reaction rate is approximately 5%.
It increases to about 15% to 20% from before and after. Therefore, in order to reduce the causticization rate of Na ₂ CO ₃ and increase the silica removal rate in the silica removal process, the addition of slaked lime is divided into multiple stages as shown in the present invention, and the amount of slaked lime added at each stage is adjusted to in molar ratio to
As shown in the examples below, processing at 1.5 or less is extremely economical because the amount of slaked lime required for desilication is reduced overall, and the amount of silica-containing matte discarded is also small, making it easy to dispose of. This is extremely advantageous compared to the conventional method. However, if the molar ratio of slaked lime to silica is less than 0.5 times, the silica removal rate tends to be extremely reduced. Therefore, in each stage of treatment, the amount of slaked lime added is desirably 0.5 times or more and 1.5 times or less based on the molar ratio of the silica contained. Next, as an example of an embodiment of the present invention, a two-stage system will be described with reference to FIG. 1 part green liquor containing silica obtained by concentrating and burning cooking waste liquid
Send it to the stage stirring tank 3, and simultaneously supply slaked lime 2,
The reaction is carried out under stirring in a one-stage stirring tank for 30 to 60 minutes. Next, the first-stage treated green liquor 4 containing the produced calcium carbonate and SiO ₂ precipitate is supplied to the filter 5 .
A centrifugal separator, a vacuum filtration machine, etc. are used for filtration.
It is separated into a silica-containing mud 6 and a first stage liquid 7 by over-separation. Next, this first-stage liquid 7 is sent to a second-stage stirring tank 8, slaked lime is added thereto, and the mixture is allowed to react under stirring for 30 to 60 minutes. The second-stage treated green liquor 10 containing produced calcium carbonate and SiO ₂ is supplied to a filter 11 and separated into a silica-containing mud 12 and a desilicated liquor 13. After the silica has been removed, the liquid 13 is subjected to a conventional causticizing process. [Example 1] SiO ₂ 0.28mol/, Na ₂ CO ₃ 1.39mol/,
Add 0.28 mol of slaked lime powder to 1000 ml of a solution containing 0.84 mol of NaOH, put it in a stirring tank, and heat it at a temperature of 93℃.
After reacting for 30 minutes, it was immediately passed through a filter (equipped with an 8μ silipore filter) to separate the generated precipitate, and the silica in the solution was analyzed.
It was 0.07 mol/. This liquid was put into the stirring tank again, 0.07 mol of slaked lime was added, and the reaction was carried out at 93°C for 30 minutes. After filtering as above, the silica and soda carbonate in the liquid were analyzed.
The total amount of slaked lime added to the initial silica was 0.018 mol/, 1.30 mol/, and the molar ratio was 1.25.
The silica removal rate was 93.6%. Also
The causticization rate of Na ₂ CO ₃ was 6.5% in total in the first and second stages. [Comparative example] On the other hand, the above 0.28 mol/SiO ₂ ,
93 by adding 0.56 mol of slaked lime powder to 1000 ml of a solution containing 1.39 mol of Na ₂ CO ₃ and 0.84 mol of NaOH.
Analysis of silica and sodium carbonate in the solution obtained by separating the precipitate produced by reacting at ℃ for 30 minutes revealed that the silica concentration was 0.042 mol/the sodium carbonate concentration was 1.14 mol/
It was hot. At this time, the amount of slaked lime added to silica was 2.0 times the molar ratio, and the silica removal rate was 85%. Moreover, the causticization rate of Na ₂ CO ₃ was 18%. [Example 2] SiO ₂ 0.28mol/, Na ₂ CO ₃ 1.4mol/,
Add slaked lime to 1000 ml of a solution containing 0.84 mol of NaOH as shown below and place it in a stirring tank at a temperature of 93°C.
was allowed to react for 30 minutes. An example of the silica removal rate and causticization rate after the reaction is shown below.

[Table] As shown in the table above, when the molar ratio of slaked lime to silica is less than 0.5, the silica removal rate becomes extremely low. Further, even if the molar ratio is increased from 1.5 to 2.0, the silica removal rate does not increase so much, but rather the causticization rate increases, which is not desirable. Next, the effects of the method of the present invention will be described. Silica removal rate is high with a small amount of slaked lime added.
(Silica removal rate of 90% or more) Because the amount of slaked lime added is small, the amount of silica mud produced is small. The amount of waste generated from silica-containing matud is small.

[Brief explanation of the drawing]

FIG. 1 is a flow diagram of one embodiment of the present invention. 1... green liquor, 2... slaked lime, 3... first stage stirring tank, 4... first stage treated green liquor, 5... filtration machine, 6...
...Silica-containing mud, 7...First stage liquid, 8...
2nd stage stirring tank, 9...Slaked lime, 10...2nd stage treated green liquor, 11...filter, 12...silica-containing mud, 13...2nd stage liquid (desilica-free green liquor).

Claims (1)

[Claims] 1. Slaked lime is added to the green liquor obtained by concentrating and burning alkaline pulp cooking waste such as the Kraft method and the soda method to precipitate calcium carbonate and silica to remove silica from the green liquor. Divide the addition into two or more stages, and set the amount of slaked lime added at each stage to be 0.5 to 1.5 times the molar ratio of the silica contained in the green liquor at each stage to precipitate silica precipitate and over-separate the precipitate. A method for removing silica from pulp cooking chemicals.