FI78327B - BEHANDLING AV I KAUSTICERINGSKRETSLOPPET VID KRAFTFOERFARANDET FOEREKOMMANDE LOESNINGAR FOER SENARE UTNYTTJING AV KALSIUMKARBONATET. - Google Patents

Abstract

1. Method of treating liquors used in the causticisation cycle for digestion of wood by the kraft process allowing obtaining of calcium carbonate which may be used, notably, as a pigment for coating paper, characterised in that it comprises the following operations : dissolving of the saline material in at least part of the weak white liquor, obtained from washing of the calcium carbonate, giving a more concentrated green liquor, filtering of the green liquor at a temperature greater than about 80 degrees C, slaking of the lime at a temperature of about 100 degrees C, causticising the filtered green liquor at a temperature greater than 90 degrees C by addition of the milk of slaked lime obtained, separating the strong white liquor from the precipitate containing calcium carbonate, washing the precipitate with water at a temperature greater than about 80 degrees C to separate the solid calcium carbonate from the remaining white liquor and soluble iron salts at this temperature, recovering the calcium carbonate which has been subjected to treatment, notably hydrocycloning, to give it the desired grain size, less than about 15 mu m, recycling the weak white liquor, notably for dissolving of the saline material.

Description

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Subsequent treatment of solutions in the Kraft process causticization cycle for the utilization of calcium carbonate

The invention relates to a new process for the treatment of solutions present in a causticization cycle in the production of kraft pulp; this method makes it possible to recover calcium carbonate, which can be used in particular as a filler and a pigment, in particular as a pigment for coating paper.

In the cooking of wood using the kraft method, the paper mill uses chemicals that are usually recovered or returned to the cycle. The combustion of black liquors produces salts consisting mainly of sodium sulphide Na 2 S and sodium carbonate Na 2 O 2 * which, when dissolved in water, usually washing water, form a green liquor. During causticization, i.e. when green liquor turns into white liquor, a calcium carbonate precipitate forms simultaneously. When a lime kiln is available in the paper mill, this calcium carbonate CaCO 2 is burned to regenerate the lime, which is reused for causticizing. But when there is no lime kiln available in the paper mill, the calcium carbonate sludge is discarded as waste.

On the other hand, calcium carbonate is a widely used substance as a pigment in the production of coated papers in coating compositions, and the idea of recovering calcium carbonate from causticization has already been suggested, as recovered calcium carbonate can of course be used the color of the calcium carbonate recovered from the caustic sludge is grayish green and not white, as would be necessary when used as a coating pigment. The greenish color of gray is probably due to the presence of carbon and ferrous salts, which cannot be eliminated from the calcium carbonate slurry.

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Moreover, the grain composition of the calcium carbonate recovered in this way is far too high for this use.

The commonly used causticization cycle involves the dissolution of salts, consisting mainly of sodium sulphide and sodium carbonate, in all the water with which the calcium carbonate has been washed, after which the resulting green liquor is used for lime quenching and actual causticization according to the following chemical reaction:

Ca (OH) 2 + Na 2 CO 3 -> CaCO> 3 + 2NaOH

After decantation or any method by which white liquor and solids can be separated, most of the white liquor, consisting of sodium hydroxide and sodium sulphide, is filtered for use in cooking wood in the kraft process. The solids, in turn, are washed to make up the remaining white liquor and returned to the cycle in the form of a weak white liquor, leaving a grayish green slurry containing calcium carbonate that can either be incinerated or disposed of as useless waste as indicated above.

In the past, no attempt has been made to prepare pigments that can be used simultaneously with the treatment of solutions in the causticization process by modifying the causticization conditions. U.S. Patent 2,062,255 discloses a process by which pigments can be prepared simultaneously with a causticization process. This patent recommends that the causticization be performed with lime pre-quenched with boiling water, and the causticization is performed at a temperature less than or equal to 75 ° C with vigorous stirring. Finally, the resulting pigment is useful. However, this method cannot be applied industrially. It has to cool lime to 75 ° C, which is first quenched with boiling water, which upsets the thermal equilibrium and makes it difficult to control the reaction temperature.

It 3 78327 The present invention proposes a new process for the treatment of solutions in the cycle of a causticizing process for the production of kraft pulp, which process can recover calcium carbonate with a degree of whiteness of 85-95% (according to standard T 12 030: "Pulp determination of the degree of whiteness "ISO"), and this degree of whiteness varies according to the initial purity of the lime used, and in particular its iron oxide content and bleaching sequences. The granular composition of the recovered calcium carbonate is weaker than that usually obtained by the classical method. These two essential properties, whiteness and grain composition, make it possible to use this pigment, especially as a coating pigment for coated papers.

The method according to the invention comprises the following operations:. the salts are dissolved in at least part of the weak white liquor from the calcium carbonate wash to give a more concentrated green liquor,. the green liquor is filtered by any suitable method: by static decantation or, preferably, by centrifugation, or by filtration through a pre - layer at a temperature slightly above 80 ° C, and preferably above 90 ° C, in order to avoid thermal losses and also to facilitate filtration,. the lime is quenched with stirring at a temperature of approximately 100 ° C to obtain quenched lime milk,. causticizing the filtered green liquor by adding slaked milk at a temperature above 90 ° C and preferably as high as possible, and this causticisation is carried out with stirring,. treating the calcium carbonate, by hydrocycloning in a strong white liquor at the highest possible temperature, preferably above 80 ° C, with a granulometric cut to the desired value, for example to about 15 μm. The surverse mainly contains very fine particles with dimensions lower than the indicated grain composition of about 15, and at least 50% of said fine particles have a grain composition of less than 2. Alite (sousverse) 78327 contains mainly larger grains, i.e. particles with larger dimensions, suspended in a strong white liquor, which can be treated by filtration or decantation, and which can be advantageously used to form a filtration precursor, e.g. separating the strong white liquor from the precipitate containing solid calcium carbonate by all appropriate means, such as filtration under vacuum, filtration under pressure to recover a strong white liquor at a concentration required for the active substances used in wood cooking,. wash the precipitate at a temperature above 80 ° C so that

OF

separating the solid calcium carbonate which is recovered from the remaining white liquor and iron salts which are soluble at this temperature.

. returning to the cycle a weak white liquor to dissolve the salts.

In one variant, the hydrocyclonation of the calcium carbonate can be carried out in a second step, for example during bleaching in an aqueous medium, as described below.

These steps can be used to eliminate most of the carbon and soluble iron salts of calcium carbonate-containing slurries and thus to obtain calcium carbonate, which can be used especially as a pigment in the coating of coated papers.

According to typical features of the invention, the lime is quenched with a portion of a weak white liquor derived from the calcium carbonate wash, and a second, higher volume portion is used to dissolve the salts.

In this case, about 1/3 of the volume of weak white liquor from the calcium carbonate wash is used to turn off the lime.

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In one variant, the lime is quenched with water. The advantage of quenching with water is that any weak white liquor from the calcium carbonate wash can be used to dissolve the salts and remove lumps and gravel from the lime by conventional means used in lime quenchers. But bringing this water into the shutdown disturbs the hydraulic balance. It is then necessary to concentrate the desired concentration of white liquor by all suitable methods such as partially reducing the volume of water introduced into the solvent device or concentrating the slaked lime obtained, etc.

Additional steps may be included in the method of the invention.

Depending on the color of the calcium carbonate recovered after washing the precipitate, which color is related to the ferric oxide content of the lime used, it may be advantageous to bleach with an oxidizing agent such as hydrogen peroxide H2O2 or NaOCl to convert the remaining green ferric ions to Ferri ions. Optionally, an agent that converts iron to a complex compound, such as triethanolamine TEA, may also be used. Bleaching can reduce variations in the degree of whiteness of calcium carbonate associated with variations in industrial processes and the raw materials used.

It is preferred to use a bleach with which it is not necessary to carry out a subsequent washing of the calcium carbonate. In this regard, hydrogen peroxide is preferred.

It may be advantageous to carry out a partial causticization, which is usually limited to 10% green liquor before it is filtered. About 5% causticization, i.e., approximately 5% of the total calcium carbonate in the reaction, makes the final filtration of the green liquor more efficient, as does the removal of carbon and some of the iron salts. Of course, the calcium carbonate formed in this or 6,78327 hees is not subsequently recovered.

According to a preferred aspect of the invention, the calcium carbonate recovered after washing the precipitate can be ground to give the desired grain size, especially when a very fine grain size is desired, for example one in which 90% of the granules have less than 2 microns and 50% of the granules have less than 1 microns.

In that case, it is preferable to perform bleaching during grinding. Embodiments of the causticization process according to the invention, in which calcium carbonate is recovered, are described according to the figure. In this figure, there are successive frames, each corresponding to one step of the method.

Example 1

The reported flows correspond to an average production of about 8.2 tonnes per hour of bleached pulp using the kraft method.

In panel A, the salts 1 are formed mainly of 200200 and Na 2 S and are dissolved using 30 m 2 / h of a weak white liquor 22 which is recovered: in this way green liquor 3 is formed at a temperature of approximately 95 ° C. The green liquor is then filtered in box C at 95 ° C to remove solid impurities 4 at this temperature, such as carbon and some of the iron salts in the form of Ferro salts. In box D, causticization is performed at approximately 100 ° C by allowing the green liquor calcium carbonate to act on the lime 5 previously slaked in box E. The quenching is performed by reacting 2.5 tons / h of the weak white liquor 2 returned to the cycle, i.e. 10 m with a mixture 6 containing 90% quicklime at a temperature of 100 ° C to remove solid impurities 7.

3.

Approximately 25 m / h of green liquor is used, resulting in a mixture 8 containing a strong white liquor of about 35 m / h leaving 3 causticizing sites.

7 78327 This mixture 8 is hydrocycloned in panel F at 85 ° C, whereby calcium carbonate particles with a grain size of less than about 15 μm are separated from those with a larger grain size.

Part 9, which contains coarser grains, can be removed by running after washing, in which the maximum amount of product, i.e. white liquor, is drawn from it, or possibly used for filtration in box C as long as there is provision for filtration on the top layer. A portion 10 containing very small particles of calcium carbonate and a strong white liquor is filtered in box G, and the strong white liquor 11 is recovered for subsequent use in wood cooking, and the precipitate 12, which is calcium carbonate and the remaining white liquor, is washed in box H. It is washed in this box. sakka 3.

using 40 m / h of water 13 at 80 degrees. The resulting liquid phase 3, about 40 m / h, consists of a weak white liquor 2, 3 which is taken back into the cycle, a second part 2, i.e. 10 m / h for slaking the quicklime as described above for box E, and a second part 22, i.e. 30 m / h for dissolving salts and forming green liquor in box A.

The precipitate 12, washed in box H, consists mainly of calcium carbonate 14. The degree of whiteness of the recovered calcium carbonate is already over 85%, which makes it suitable for use as a coating pigment in paper coating. In this way, about 4 t / h of calcium carbonate is recovered.

If an even higher degree of whiteness or reduction of whiteness variability is desired, calcium carbonate bleaching can be performed in panel I by allowing an oxidizing agent such as NaOCl 3 or hydrogen peroxide to act, thereby converting the ferrous salts to ferric salts. This bleaching may optionally be followed by washing in pane J with water 16 at a temperature of about 20 ° C to remove residual oxidizing agent, if necessary.

Example 2

The procedure is the same as in Example 1 except that the hydrocyclonation in panel F after causticization is omitted and replaced by hydrolysis in panel F-, which separates the recovered calcium carbonate 141 from the coarse grains 9 ^.

Example 3

Work in the same way as in Example 1 except that causticization is performed in box B on about 5% of the green head before it is filtered. In this way, a calcium carbonate precipitate is formed, which facilitates filtration in box C.

Example 4

Work in the same way as in Example 1, except that the 3 limescale is carried out with water 17 using it at 30 m / h and that all the weak white liquor, i.e. 33 m / h, is used to dissolve the salts. The slaked lime milk 5 is concentrated in box E ^, removing 23 m / h of water 18.

Example 5

The procedure is the same as in Example 1, except that the recovered calcium carbonate 14 is ground wet in box K before or after bleaching and preferably during bleaching.

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Claims (13)

A process for treating salts which occur in the cauterization cycle in boiling wood according to the power method and the agent which calcium carbonate can be obtained for use especially in coating paper mad pigments, characterized in that the process comprises the following stages: part of the white liquor which is derived from the washing of the calcium carbonate, whereby a higher concentration of green liquor is obtained, the green liquor is filtered at a temperature, which is slightly above 0 ° C, 0. the quay is leaked at a temperature of about 100 ° C, the filtered green liquor is causticized at a temperature of above 90 ° C by the addition of quenched lime milk, the ethereal white liquor separating from the calcium carbonate precipitate, and the precipitate washed with water, or above 0 ° C, for separation of the calcium carbonate from the remaining white liquor and the iron alts, which are soluble at this temperature, the calcium carbonate, which is treated and particularly hydrocyclonated, is recovered to obtain a desired grain composition of about 15 µm. the evaga white liquor before to the cretaceous for loaning the ellites. Process according to claim 1, characterized in that the quenching of the whey is carried out with a portion of the white liquor which results from the washing of the calcium carbonate. Method according to claim 2, characterized in that the part of the white liquor used for sealing the whey is equivalent to about 1/3 of the eva white white liquor derived from the washing of the carbonate. Method according to claim 1, characterized in that the quenching is carried out by means of water and that