FI64671B - BEREDNING AV VITLUT - Google Patents

Description

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Patent and Registration Office .... . . (y * (44) Date of publication of the publication and of the publication— _

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Sweden-Sweden (SE) 7804239 ~ 7 (7l) (72) Alf Ove Andersson, Kofallsvägen 38, S-710 Fr0 Frövi, Sweden-Sweden (SE) (7¼) Oy Kolster Ab (5¼) Manufacture of white liquor - Beredning av vitlut This the invention relates to a process for the preparation of white liquor for pulp production by the sulphate process. According to the invention, the production is carried out in a closed plant at a pressure exceeding the normal pressure. There are several benefits to this.

In pulping according to the sulphate process, it is necessary to recover cooking chemicals for the satisfactory economy of the process. This is done by evaporating the black liquor obtained from pulp cooking and burning it in a recovery boiler, whereby the inorganic constituents of the liquor are obtained in the form of a melt. This is dissolved in water to give a soda liquor containing e.g. sodium carbonate, which is not involved in the cooking process. It is therefore desired to convert as much of the carbonate as possible into sodium hydroxide, which is present as an active ingredient in the cooking liquor. For this purpose, the caustic soda is causticized with calcium hydroxide.

64671

When burnt lime (calcium oxide) is added to the soda liquor in a lime extinguisher, the burnt lime reacts with the water in the soda liquor to generate heat to give slaked lime (calcium hydroxide) according to the following reaction:

CaO + ^ 2 0 -> Ca (0H) 2 + heat

Slaked lime is partially water-soluble and gives lime milk, which is passed to a causticizing vessel where the potassium in the lime milk reacts with the carbonate in the liquor to give the desired sodium hydroxide and lime slurry (calcium carbonate):

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

The lime sludge is filtered off, washed and stored later for incineration and returned as burnt lime to the process. The white liquor obtained is passed to the soup.

Until now, the production of white liquor has always taken place in a more or less open system and at normal pressure in continuous equipment. However, according to the present invention, the process is carried out in a system which, starting from the lime extinguisher and ending in the storage tank for the finished white liquor, is under a pressure higher than normal pressure. This represents a substantial departure from previously known methods for preparing white liquor and brings considerable advantages, as will be shown in more detail below.

The invention is further illustrated by the accompanying drawing, which schematically shows the structure and execution of the process.

The drawing shows how the soda liquor 4 is passed through the pump 1 to the lime extinguisher 2. The burnt lime 3 is fed to the lime extinguisher 2 through a feed device adapted to supply lime to a vessel under overpressure. Such feeding devices are known to the person skilled in the art. The soda liquor pump 1 operates as a transfer pump red for the soybean liquor, lime milk and white liquor-lime sludge mixture throughout the system and maintains a pressure higher than normal pressure of 64671 up to the white liquor storage tank 15.

The lime fire extinguisher 2 basically works like a fire extinguisher of a known type. The burnt lime 3 and the soda liquor 4 are fed with vigorous stirring by the stirrer 5 and react with each other to generate heat. The bottom mixer 6 is used to homogenize the formed lime gravel, which is scraped towards the outlet barrier to be periodically discharged from the system point 7.

The lime milk is passed to a causticization vessel (or vessels) 8, where it is reacted with sodium carbonate of soda liquor with vigorous stirring 9 to form sodium hydroxide and calcium carbonate (lime sludge).

The mixture is further compressed into a white liquor filter 10, which includes a filter replacement part known in the art. Here, lime sludge is separated, which sediments into the lower cylindrical part of the vessel and settles towards its bottom, where it is homogenized by a stirrer 12 and pressed into the vessel 13 for washing, after which the lime sludge is stored in silo 14. This is possible, as has long been known, to recover the thermal energy that is released when the pressure drops.

The lime sludge from the silo 14 is pumped to the wash filter 18 to partially thicken, partially to release certain contaminants that negatively affect the combustion process and the surrounding environment. The lime sludge from the filter is finally fed to the lime sludge incinerator for delivery as burnt lime back to the lime extinguisher.

The pump 16 pumps the ready-made white liquor 17 as a cooking liquid to the digester.

As mentioned above, an essential feature of the invention is that the production of the white liquor is carried out at a pressure which exceeds the normal pressure. The upper limit of the pressure used is not critical, but is primarily determined by the technical aspects of the equipment and process. A plant that must be able to withstand very high pressures will naturally become more complex and expensive because more durable structures and materials are required. In practice, it has been found that an overpressure of about 1 MPa can be advantageously used, and an overpressure of 200-500 kPa is particularly suitable.

4,64671

The heat generated during the slaking of the lime raises the temperature of the soda liquor above the boiling point. In a conventional type of causticizing plant at normal pressure, the water evaporates and the normal temperature in such an extinguisher is 102-103 ° C. In a closed plant under overpressure, the temperature rises higher because water vapor cannot escape and normally the temperature then becomes about 105-110 ° C. The heat is stored in a white liquor-lime-sludge mixture until both of these components are transferred to their non-pressurized storage tanks.

In some cases, it may even be appropriate to operate at an even higher temperature, in which case only the critical water temperature (374.2 ° C) sets the upper temperature limit. 105-170 ° C has proven to be a suitable temperature range. However, it is not necessary for the invention that the temperature is above the boiling point, but the most important point is, as already mentioned, that the pressure is above normal pressure.

In causticization, calcium carbonate is formed and precipitates as lime sludge. At higher temperatures, the crystals become larger in a constant reaction time. This is because the remodeling of the crystallization sprouts decreases at elevating temperature and the smaller crystals readily redissolve. This means that the probability of precipitation of the larger crystals already formed increases, which means that larger crystals are obtained, which in turn facilitates the separation of the lime sludge. In practice, this means that the causticization time can be shortened somewhat. Thus, the growth of calcium carbonate crystals at a higher temperature in a closed system has a positive effect.

At higher temperatures, the solubility of calcium carbonate increases, while that of calcium hydroxide decreases. Theoretically, therefore, the concentration of carbonate ions in white liquor should increase with increasing temperature. However, this is counteracted by the fact that a higher temperature means that the equilibrium is settled faster.

The equipment used in the process, such as a fire extinguisher, a causticizing vessel, a filter, etc., are of conventional construction for carrying out chemical reactions under overpressure, and can be readily prepared by one skilled in the art. He can also easily select suitable materials for different parts of the equipment.

64671

By carrying out the quenching and causticization processes in the production of white liquor at a pressure higher than the normal pressure, the following excellent advantages are achieved: 1. The plant can be built compact, since the principle of self-casting does not have to be exploited.

2. The required piping system becomes very simple compared to a conventional plant.

3. Only a minimum number of moving parts is required, namely 1 soda-liquor pump, 5 agitators, 1 inlet feeder for lime and 1 outlet feeder for lime-gravel in the basic structure.

4. It should be possible to reduce the building of the plant by volume to approximately half of what is currently required.

5. Due to the closure of the process, the system becomes advantageous from the point of view of environmental protection.

6. The heat of reaction generated in the extinguisher can be stored throughout the system and recovered, for example by means of a hot water scrubber, in the event of a pressure drop in the white liquor tank and the lime sludge silo.

The invention has been described above as being applied to a complete white liquor manufacturing plant. It should be noted, however, that the invention can be applied only to the extinguisher and thus in one way to produce slaked lime (calcium hydroxide). Here it is not necessary to use soda liquor specifically for extinguishing, but instead water or some other aqueous liquid. Here, too, the above-mentioned advantages are obtained in an analogous manner by carrying out the process at a pressure which exceeds the normal pressure.