Methods for producing pulp and treating black liquor

Abstract

A method is provided for treating black liquor particularly derived from non-wood pulp, by heating with an alkaline earth metal oxide in a toroidal fluidised bed reactor at a temperature of above 650° C. The method may be used alone or as part of a method of converting graminaceous raw material to pulp for paper or board, said method comprising (a) digesting said raw material with a white liquor based on sodium hydroxide and further comprising calcium hydroxide in an amount effective to substantially convert silica of said raw material to calcium silicate; (b) recovering pulp and black liquor substantially free of uncombined silica; (c) heating the black liquor in a fluidized bed reactor containing calcium oxide for catalysing conversion of organic content of said black liquor to gas and for providing recovered solids including sodium values of said white liquor and calcium oxide; and regenerating said white liquor using said recovered solids. The use of the above mentioned white liquor permits treatment of wheat straw, rice straw and other high-silica materials without resulting in a black liquor that is difficult to treat.

Description

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 10 / 773,870, filed Feb. 6, 2004, which claims priority to PCT International Application No. PCT / GB2002 / 003641, filed Aug. 7, 2002, published as WO 2003 / 014467 on Feb. 20, 2003, which claims priority to GB 0119237.6, filed Aug. 7, 2001, to each of which priority is claimed, and each of which is incorporated by reference in its entirety herein. This application is also a continuation of PCT International Application No. PCT / GB2004 / 050023, filed Nov. 3, 2004, published as WO 2005 / 045126, which claims priority to UK Patent Application No. 03 25578.3, filed Nov. 3, 2003, to each of which priority is claimed, and each of which is incorporated by reference in its entirety herein.FIELD OF THE INVENTION [0002] The present invention relates to a method for producing pulp from graminaceous fibres and also to a method of treatment of black liquor that may be a by-product of said pulp production method or may ha...

AI Technical Summary

Benefits of technology

[0033] The advantage of a higher reaction temperature is the enhanced rate of reaction for black liquor processing and hence throughput may be increased, whilst maintaining output quality. We have now discovered that a temperature of above 650° C. can be used in the above mentioned black liquor recovery process. Recent experiments have shown that the loss of inorganic species from the black liquor when heated to between 650 and 700° C. or even 725° C. in the fluidised bed, was minimal, i.e. the losses were not economically significant, and hence no additional equipment was required to ensure their recovery.

Problems solved by technology

Although useful recovery of chemicals and energy can be achieved in commercial operation, the use of a Tomlinson furnace presents a number of problems.

For example, inadvertent contact between water or dilute black liquor and the inorganic smelt may result in an explosion.

Also, high char bed temperatures lead to increasing emission of sodium salts and excessive fouling of the steam pipes in the upper part of the furnace.

Furthermore, the technology currently used to treat black liquor effluent is, depending on local economic conditions, only viable on a scale of not less than 60,000 tonnes of pulp production per annum, which may be compared with the typical scale of a modern wood pulp mill which is over 360,000 tonnes of pulp production per annum.

Treatment of straw and other gramminaceous materials is, of course, on a much smaller scale inter alia because long-distance transport of bulky agricultural residues such as straw is uneconomic.

Although it has been reported in the patent literature that there have been attempts to commercialise the Copeland process, the inventors are aware that it is prone to severe bed agglomeration, especially when treating black liquor of relatively low calorific value from the cooking of straw, and that the process has since fallen into disuse for lack of technical and commercial viability.

The experience of the inventors is that simple fluidized beds of the kind disclosed by Copeland are subject to unacceptable agglomeration, which makes operation impractical for anything beyond a short start-up period.

Again all three beds are of the simple bubbling type, and the intermediate bed is subject to unacceptable agglomeration for the reasons already given.

As previously mentioned, these raw materials are bulky, so that transportation costs and logistics mean that they are best pulped locally and therefore on a relatively small scale of around 10-100 tonnes of pulp production per day.

Pulp mills generate black liquor effluent that if discharged to watercourses causes severe pollution.

Lack of economically viable technology to deal with black liquor effluent under 60,000 tonnes per annum of production has meant that many existing small pulp mills have been forced to close to stop pollution of watercourses.

This lack of suitable technology has also prevented the establishment of new small pulp mills, in particular new mills that might have used agricultural residues.

The subsequent lack of demand for small pulp mills has meant that little research and development of small pulp mill technology has been carried out.

Consequently small pulp mill technology and straw pulping in particular has not advanced as far as large-scale wood pulping technology.

However, a process for recycling NaOH / Ca(OH)2 black liquors has not been described and such black liquors have in the past merely been discharged untreated.

The relatively high content of silica in straw and other non-wood cellulosic agricultural products presents difficulties for chemical recovery.

A modified wood-based recovery system may be used if the silica content of the cereal straw is less than 5 - 6 wt %, but at higher capital and operating costs.

However, for products of higher silica content, especially for rice straw, there has been up to now no process that is technically and commercially viable.

When these species are in the vapour phase, additional process equipment is required to recover them at increased overall cost.

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