SE528163C2 - Removal of water-soluble compounds from wood chips before cooking - Google Patents

Abstract

Processes and systems for the treatment of wood chips prior to cooking remove substantially (e.g., at least about 30%) water-soluble compounds therein, especially those compounds which disassociate in water to form potassium and/or chloride ions and/or water-soluble metal compounds such as manganese, calcium and potassium. In preferred embodiments, wood chips entering the cooking system containing water-soluble compounds, such as those compounds which disassociate in water to form potassium and/or chloride ions and/or water-soluble metal compounds such as manganese, calcium and potassium, may be treated prior to cooking by being brought into contact with an aqueous treatment stream (e.g., water or steam condensate) at a temperature (e.g., between about 20° C. to about 120° C.) and for a time (e.g., between about 2 to 200 minutes) so as to reduce substantially (e.g., at least about 30%) such water-soluble compounds.

Description

528 163 forms potassium and/or chloride ions. According to the present invention, the wood chip is treated prior to cooking by contacting it with a water-soluble solvent (e.g., water or steam condensate) to substantially reduce (e.g., at least 30% by weight) such water-soluble compounds containing potassium or chloride. By treating the wood chip according to the present invention, water-soluble compounds in the chip, particularly those containing water-soluble potassium or chloride solutions, are displaced from the interior of the chip and collected in the present aqueous treatment liquid. The aqueous treatment liquid containing the water-soluble compounds can then be drained from the chip before the chip enters the cooking system. The wood chip can be treated (washed) several times to gradually increase the amount of water-soluble compounds. especially potassium and/or chloride-containing compounds, which have been removed from the solution.

These and other aspects and advantages will become more apparent after a thorough consideration of the following detailed description of the advantageous embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS In the following, reference is made to the accompanying drawings. where like reference numerals in the various figures designate like structural elements, and where: Figures 1 - 3 show in general a possible method for treating wood chips according to the present invention; Figures 4A and 4B are graphical representations corresponding to the following examples of the potassium and chloride ion concentrations (mg/l) in the chip filtrate as a function of the extraction time for unbased and basic wood chips of size 2 - 3 mm; Figures 5A and 5B are graphical representations corresponding to the following examples of the potassium and chloride ion concentrations (mg/l) in the chip filtrate as a function of the extraction time for unbased and basic wood chips of size 4 - 6 mm; ' Figures 6A and 6B are graphical representations corresponding to the following examples of potassium and chloride ion concentrations (mg/L) in the filtrate as a function of leaching rate for unbased and based wood chips of size 8-10 mm, respectively; and Figure 7 is a diagram of the total potassium removed from wood chips of three different size categories according to the following examples. 15 20 25 528 163 DETAILED DESCRIPTION OF THE INVENTION The aqueous wash stream that can be used in carrying out the present invention can be almost any stream that can be treated by the pulp mill's existing whitewater treatment system. Such streams include, but are not limited to, water, condensate from evaporators (and/or other facilities), collected rainwater, and bleaching plant wastewater. The washing can advantageously be carried out at temperatures between about 20°C and about 120°C (e.g. advantageously between about 30°C and about 90°C), at pressures up to the downstream pressure of the digester. The residence time during which the fluid can be in contact with the aqueous washing solution can be from about 2 to about 200 minutes. The washing is most advantageously carried out at a pressure between about 2 and 12, more advantageously between about 4 and about 10. It is considered particularly advantageous that the washing can be carried out at elevated pressures, which as mentioned above can rise up to the pressure conditions in the downstream digester.

According to the present invention, various techniques can be used. For example, as shown in the accompanying Figure 1, a system 10 includes a steaming vessel 12, as shown in U.S. Patent No. 5,500,083 (the entire contents of which are expressly incorporated by reference) and which is commercially available from Andrllz Inc. of Glens Falls, New York under the name Diamondback® Steaming Vessel. Steam can be injected into the steaming vessel 12 via line 12-1. The steaming vessel 12 is equipped with a feed tube 14 so that condensate can be drained via line 16. This drained condensate can then be passed on to the existing system (not shown) for treating white water. The vessel 12 shown in Figure 1 could. if so desired. be without a basin, but with a feed screw and a horizontally or vertically arranged washing tank (not shown) downstream from the feed screw where washing water can be fed. The effluent from the washing tank could thus be taken out of the washing tank and treated in the pulp mill's existing whitewater treatment system.

The system 20 shown in Figure 2 includes a similar flash bin 12 as shown in Figure 1, in that steam can be introduced to the wood chips in the bin through line 22-1, and that it is equipped with a screw 24. In addition, the flash bin 22 is equipped with a plurality of horizontal or vertical successive washing tanks 25, 26 downstream from the bin. to which soap liquid can be fed. The chips can be washed with water or some other suitable water-containing stream via line 27. The liquid can be drained from the first washing tank 25 via line 25-1 and then passed on to a second downstream washing tank 26, where the chips are washed a second time. Liquid drained from the second wash tub can be divided into respective portions, a portion of which is used as a white water treatment via line 26-1, and a second portion of which is used to provide a feed stream via line 26-2 to the feed in the first wash tub 25. With the series connection of the wash tubs 25, 26 as shown in Figure 2, it is thus possible to collect the drained wash liquid and pass it to the previous wash tub, or it can be passed on to the pulp mill's existing white water treatment system.

The last washer in the series connection could be, for example, an in-line separator, while an intermediate washer could be a holding washer to increase the time the washer material is in contact with the wood chips. Such a system is depicted in the embodiment in the accompanying figure 3. Alternatively, the last chip water could physically be in the top separator of the digester instead of in a special washer.

.More specifically, as shown in Figure 3, the feeder 32 contains a screw 34 and is equipped with a line 32-2 which allows the injection of steam to the wire ice in the furnace. A series of washing vessels 35, 36 and 37 are arranged to enable washing of the chips in successive stages, in a countercurrent system, by means of false washing water fed via lines 39-1, 39-2 and 39-3 respectively. As briefly mentioned above, the last washing vessel 37 in the series could be, for example, an in-line separator. In the flow between the first washing tub 35 and the intermediate washing tub 36, a holding tank 40 is advantageously placed. The holding tank's task is to extend the time the washing material is in contact with the thread.

Liquid drained from the intermediate vessel 36 and the final vessel 37 can be divided into respective portions, with a portion of it being used as white water treatment via lines 36-1 and 37-1, respectively, and a second portion of it being used for countercurrent flow via lines 36-2 and 37-2 when the chips are in the first and intermediate vessels 35, respectively. 35. In the same way as in the series connection of the washing vessels 25, 26 shown in Figure 2, the washing vessels 36, 37 in the system 30 shown in Figure 3 can collect the drained washing liquid and return it to the previous washing vessel or to the existing system for treating white water in the waste water treatment plant.

It has been found that it is possible to remove the water-soluble compounds. In a particular embodiment of the invention, it has been found that water-soluble potassium can be removed from traffic jams. More specifically, in laboratory experiments, etc. carried out at atmospheric pressure and at temperatures between 30°C and 90°C together with a contact time of the washing liquid of about 30 minutes, it was possible to significantly reduce the potassium-containing compounds by an amount of at least about 30 weight percent (e.g. between about 30 weight percent and about 40 weight percent). The operating conditions used in the laboratory experiments were chosen on the basis of practical limitations existing in the laboratory, and thus do not limit the present invention. In other words, other operating conditions can be utilized within the scope of the present invention. Based on the results of the laboratory experiments, it can be expected that potassium can be removed in greater quantities at higher temperatures and/or operating pressures. It can also be expected that commercial operating conditions will differ from those used in the laboratory. In particular, with higher temperatures, higher pressures and longer residence times.

The present invention is better understood by the following non-limiting examples.

EXAMPLE The moisture content of the wood chips was tested before any treatment was carried out. The chips were divided into two portions - i.e. one portion for such chips to be pre-treated with steam before the leaching treatment, and another portion for such chips to be treated by the urine treatment alone (i.e. without any pre-treatment with steam). The condensate formed during the basting was drained and tested for potassium content.

The chips to be steam treated were separated into three different categories with respect to chip size - i.e. chips with an average size of 2-3 mm, 4-6 mm and 8-10 mm respectively. A plurality of bags containing 50 g of raw chips were prepared for each size category of chips. The respective bags were then subjected to three different temperature ranges (i.e. 30 °C, 60 °C and 90 °C) and five different groups of residence times (i.e. 1 min, 2 min, 5 min. 10 min, and 30 minutes). A total of 45 bags of chips were prepared for the basified chips. The same procedures as those described above were followed in the preparation of the bags containing wood chips without steam pretreatment.

A container of water was heated to the desired temperature (i.e. 30 °C, 60 °C, or 90 °C). Once the temperature was reached, a bag of wood chips was added and held at the temperature for a desired holding time. Once the desired holding time was reached, heating was stopped and the filtrate was tested for chloride and potassium. For chlorides, Standard Method 4500 (Standard Methods. 18th Ed., Mercury Nitrate Method 4500-Cl. Section C, 1992) was used, and for potassium, Standard Method 3111 (Standard Methods. 18th Ed., Metals by Flame Atomic Absorption Spectrometry 3111 A & B, 1992) was used.

The concentrations of potassium and chloride in the filtrate were plotted as a function of the urate clotting time for each unbased and based size category of the test, and these are shown in Figures 4A ~ 4B, 5A-5B and 6A-6B, respectively.

The percentage of potassium removed by basification and leaching was also calculated, and these data are shown in Figure 7.

Although the invention has been described in connection with what is presently considered to be the most practical and advantageous embodiment, it must be emphasized that the invention should not be limited to the embodiment shown, but that on the contrary it is intended to be susceptible to numerous modifications and equivalent arrangements which are included within the spirit and scope of the appended patent claims.

Claims (7)

10 15 20 25 30 528 163 PATENT REQUIREMENTS 1. l. Process for treating wood chips before cooking the chips at elevated temperature and pressurized in a downstream boiler, characterized by the sequential steps a) arranging a chip bin upstream of the boiler, b) introduction of untreated wood chips containing water-soluble potassium and / or chloride-containing compounds in the chip bin, c) injecting steam into the wood chips fed into the chip bin to thereby form an amount of wood chips and condensate as an aqueous treatment liquid, d) transferring the amount of wood chips and condensate to downstream washers and inserting washbasins in an aqueous wash liquor such as an additional aqueous treatment liquid, e) maintaining the aqueous treatment liquid in step d) in contact with the wood chips at elevated pressure not exceeding the pressure of the digester and at a temperature of between about 30 ° C and 90 ° C, a pH of between about 4 and 10 and for a period of between 2 and about 200 minutes, sufficient to significantly reduce va potassium- and / or chloride-containing compounds therein containing at least 30% by weight to thereby form an amount of treated wood chips in a waste treatment liquid containing the removed water-soluble compounds, f) separating the treated wood chips from the waste treatment liquid, and g) feeding the treated wood chips to the digester, wherein step d) comprises washing the treated wood chips in an equivalent series of washing vessels arranged between the chip bin and the digester, and that the treated wood chips are washed in a countercurrent system by feeding washing liquid in as a counterflow in relation to the flow direction of the wood chips, furthermore liquid is drained from the washing vessels, whereby at least a part of drained washing liquid from a downstream washing vessel is returned to an upstream washing vessel. Process according to claim 1, wherein step e) is carried out so that the water-soluble compounds are reduced by at least about 50% by weight. _ The process of claim 2, wherein step e) is performed so that the water-soluble compounds are reduced by at least about 70% by weight. Process according to any one of the preceding claims, wherein step d) is carried out using a washing liquid which is at least one selected from the group consisting of hot water, filtrate from bleaching plant, collected rainwater, evaporator condensate and other factory condensates. Process according to one of the preceding claims, in which an in-line separator is used as the last washing vessel. Process according to one of the preceding claims, in which an intermediate holding vessel is used during washing to increase the time during which washing liquid is in contact with the wood chips. A process according to claim 6, wherein a holding vessel is used after a first and a subsequent washing vessel.