This invention relates to a process of regen¬ erating spent liquor from kraft or sulfate pulping of lignocellulosic materials (black liquor) and reutiliz- ing it in a delignification process.

Australian patent 473,185 discloses a method wherein kraft white liquor is oxidized and reutilized in another part of a kraft plant or in oxygen bleaching processes. In more detail patent 473,185 discloses a cyclic process for utilizing sodium values in kraft pulping, in which sodium losses normally are less than sodium additions to the process, thus tending to build up a sodium surplus, and which includes the steps of pulping cellulosic material with a pulping liquor com- prising sodium hydroxide and sodium sulfide, separat¬ ing spent pulping black liquor, evaporating and combust¬ ing the black liquor to recover sodium values as sodium .sulfide and sodium _, carbonate, dissolving the sulfide.and sodium carbonate in water to form green liquor, caust-- icizing the green liquor with calcium hydroxide to form white liquor, and recycling white liquor to form pulp¬ ing liquor and is characterized by the improvement which comprises maintaining sodium balance at least in part by removing sodium values as white liquor, oxidizing the white liquor with air at an elevated temperature, and utilizing the oxidized whiteliquor as a source of alkali in another cellulose pulp treatment process.

It is an object of the present invention to utilize oxidized white liquor in other delignification processes.

The kraft process is widely used, due to the excellent properties of the pulp it aff-ords . However, for some purposes such as the manufacture of corrugating paper and some component pulps for- linerboards, the high strengths obtainable with kraft pulping are not necessary and the relatively low yield and consequent high cost of kraft pulps is a disadvantage. Accordingly, vari¬ ants such as "high yield kraft" and various sulphite

processes have been recommended and used for these purposes. The most widely applied of these higher yield processes is the so—called neutral sulphite semichemical (NSSC) process which is capable of giving pulps with

-* ^* yields in the range 65-857o, and with properties suitable for use as the principal component in the manufacture of corrugating paper and as an important component of linerboards and bag and wrapping papers.

A significant proportion of kraft pulp is pro- 0 duced for use in linerboards, most of which is used in the manufacture of corrugated fibre containers. The latter are constructed in sandwich fashion from an inner and outer liner with a corrugated paper layer between c them. The pulp most commonly used for manufacture of 15 corrugating paper is of the semi-chemical type, e.g. neutral sulphite semi-chemical (NSSC) pulp and it is common practice for the pulp manufacturer to produce both types of pulp on the same site, i.e. to Operate both a kraft mill and a NSSC mill-within the same 20 complex.

The kraft pulping process is largely self con¬ tained as far as effluent is concerned, since the waste liquor (black liquor) is concentrated and burned in a furnace designed for recovery of the pulping chemicals ^•** -' viz. sodium hydroxide and sodium sulphide. The kraft recovery furnace is capable of burning the waste liquor from a NSSC mill, but the main product of such com¬ bustion is sodium sulphide and not sodium sulphite i.e. the recovered chemicals are suitable for re—use in the

30 kraft mill but not in the NSSC mill. Systems are avail¬ able for conversion of the sodium sulphide to sodium sulphite, but these are complex and ver} ^τ expensive and have not achieved widespread use.

The result of this situation is that the waste 5 liquor from the NSSC mill can, in most cases, be used only as make-up in the kraft system, i.e. it serves to make good the inevitable losses frcir. the kraft pulping

system cf. loss of sodium with the pulp, which is unavoidable even with the most efficient washing, and loss of volatile sulphur compounds which, in modern mills are largely collected and burned. The losses incurred in a well-run modern kraft mill with strict pollution control are low and the need for make-up correspondingly small. The waste liquor produced by a NSSC mill may be sufficient to provide make-up for a kraft mill with six to seven times the pulp output cf. a NSSC mill producing, say, 150 tonnes per day of pulp may produce waste liquor sufficient for a kraft mill of ca. 900-1100 tonnes per day. In most pulp mills this relationship between the size of the kraft and NSSC mills does not hold, the amount of waste liquor produced by the NSSC mill frequently exceeding and in many cases greatly exceeding, the make-up requirements of the kraft mill. In such cases a serious environmental problem arises, since the excess waste liquor must be disposed of by other means. One such method is to burn the NSSC waste liquor in the kraft recovery furnace, but to withdraw the unwanted excess in the form of sodium sulphate from, for instance, the electrostatic precipitators. Unfortunately, the sodium sulphate produced in this way does not have a ready market and it m y be necessary to store to dump it. A more satisfactory alternative is to withdraw green liquor (a solution of sodium sulphide and sodium carbonate) from the kraft recofery system and use it as cooking liquor in the semi-chemical mill. However, the pulps produced in this way are inferior in properties to NSSC pulps, i.e. they give paper of drak colour and appreciably poorer strength.

It is an object of the present invention to provide a pulping process which will allow a simplified kraft chemical recovery while obtaining results similar to those of the NSSC process, in terms of high yield, good colour and adequate strength properties.

Semichemical pulps can be prepared by the soda

process which employs sodium hydroxide as the active pulping chemical but this is rarely, if ever, used because pulp strengths and colour are not as good as for the NSSC process and chemical cost is higher than

5 for the sodium carbonate, or sodium carbonate-sodium hydroxide semichemical processes.

The importance of colour cannot be over-stress¬ ed as this factor at present hinders or prevents the use of semichemical soda pulps in products such as liner

^** O boards and bag and wrapping papers.

One aspect of the present invention resides in a discovery that good colour or lightness in chemical or semi-chemical pulps .is dependent on a high residual content of alkali in the spent liquor resulting in a

¹ high residual pH at the end of the cooking period. It is already known that there is a correlation between good colour and yield but the present invention resides in the discovery that at the same yield an excess of alkali results- in better colour. Likewise where yields 0 are different, a pulp of lower yield can have equivalent colour properties to a pulp of higher yield if excess alkali has been used in the production of the lower yield pulp. Although an excess of alkali will result in a high residual pH, in buffered pulping systems a large 5 difference in the amount of alkali may not be reflected in the residual pH values particularly where the pH is high. Previous processes using sodium sulphite, sodium carbonate or sodium carbonate-sodium hydroxide could not ^* have obtained the high residual pH nor did the high 0 yield soda pulp processes give a high residual pH as the first two do not use a sufficiently strong alkali while the latter two normally use insufficient alkali. This is especially true of the high-yield or semichemical process in which the aim has been to consume as much as 5 possible of the applied alkali and leaves a low residual pH. In the present invention the use of excess strong alkalis gives a high residual pH of from 9 to 14 preferably 12 to 13.

Oxidized white liquor contains principally sodium hydroxide, sodium carbonate and sodium thiosul- phate. If the white liquor is only partially oxidized some sodium sulfide will also be present. However, the total content of sulphur in the oxidized or partially oxidized white liquor as sulfide or thiosulfate is about half the sulphur content of an N.S.S.C. pulping liquor.

The present invention therefore provides a semichemical pulping process of delignifying lignocellu- losic material which comprises digesting the lignocellu- losic material with an excess of oxidized kraft white liquor to ensure that the pH of the spent liquor at the end of the delignification treatment is more than 12.

The present invention provides a means of avoi- ding the problem of mis-match between kraft and NSSC mills. A system is disclosed whereby a mill may pro¬ duce both kraft and semi-chemical pulp while maintaining 'a correct balance of pulping chemical, indeed, ^* as will- be explained in what follows, the new method not only allows of the disposal of semi-chemical pulping waste liquor but also permits increased flexibility in the operation of the kraft mill itself. In the practice of the disclosure, spent liquor is concentrated and burned in a kraft type recovery furnace in the traditional fashion to give a smelt which is afterwards converted to green liquor (a mixture of sodium carbonate and sodium sulphide) . The latter is then causticized to form white liquor (a mixture of sodium hydroxide and sodium sulphide) in the normal fashion. The major part of this white .liquor is used as cooking chemical in the kraft mill while the remaining part is oxidised to con¬ vert the sodium sulphide to a mixture of sodium hydrox¬ ide, sodium thiosulphate etc. The oxidised white liquor is then used with or without an additive (e.g. anthraquinone) as cooking liquor to make semi-chemical pulp in place of the traditional NSSC pulp.

In another aspect the present invention pro¬ vides in the cyclic process for utilizing sodium and

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sulfur values in kraft pulping, in which sodium and sulfur losses normally are less than sodium and sulfur additions to the process, thus tending to build up a sodium and sulfur surplus, and which includes the steps of pulping cellulosic material with a pulping liquor comprising sodium hydroxide and sodium sulfide, separat¬ ing spent pulping black liquor, evaporating and combust¬ ing the black liquor to recover sodium values as sodium sulfide and sodium carbonate, dissolving the sulfide and sodium carbonate in water to form green liquor, caustic- izing the green liquor with calcium hydroxide, to form white liquor, and recycling white liquor to form pulping liquor, the improvement which comprises maintaining sodium and sulfur balance at least in part by removing sodium and sulfur values as white liquor, oxidizing the white liquor with air at an elevated temperature and then digesting lignocellulosic material with an excess of said oxidized white liquor, so that, the pH of the spent liquor of said delignification treatment is more than 12, as part of a semichemical delignification process.

Preferably the digestion temperature of the oxidized white liquor digestion is 120°C to 150°C.

The process is applicable to all types of wood including both hardwoods and softwoods and to non-woody materials ^uch as straw, bagasse etc. Where wood is used it may be barked or unbarked and may include branch¬ es, roots, twigs and leaves as in the case of so-called "whole tree chips". Conventional semi-chemical soda pulping pro- cesses usually employ temperatures of 165-180°C and the pH of the spent liquor is normally in the range of 9-10. Also, U.S. Patent No. 3,954,553 which relates to a (similar) process designed to produce pulp similar to that from the conventional semichemical sulphite process¬ es, discloses a process of using mixtures of sodium hydroxide and sodium carbonate in which a cooking temper¬ ature of 375°F (190°C) is used.

In another aspect of this invention it has been discovered that pulping with oxidized white liquor at lower temperatures than normal can result in improved strength of the pulp. In the process according to this aspect of the invention cooking temperatures in the range of up to 160°C can be used although temperatures above 100°C are preferred and temperatures up to 120 -150°C are most preferred. Operation at temperatures up to 160 C has number of advantages. The unexpected advantage is that the strength of the resultant pulp is higher when lower cooking temperatures are used. There is also an indication that in some cases colour is improved and of course the lower cooking temperatures results in an overall energy saving. A preferred form of the invent¬ ion combines the use of excess alkali and low process temperatures to give pulps of high residual pH having good .colour and strength properties .

^' This process of the invention ha's been found' to give pulps with properties, including colour, similar to those of NSSC pulps. In employing this process we have found in a further preferred aspect of the invention that further acceleration of the rate of cooking, and thereby either decrease in reaction temperature or reaction time can be obtained by addition to the chips or to the cooking or makeup liquor of small quantities of quinonoid or hydroquinonoid compounds. Such additions may be made as the chips and/or liquor are being introduced into the digester or as a separate pretreatment. The addition of the quinonoid or hydroquinonoid compounds is particularly useful where low cooking temperatures are used as increased strength due to the lower temperatures can be obtained in shorter cooking times due to the addition of the compounds.

A further benefit of the use of the additives described is an additional improvement in the strengths of the pulps produced. As may be seen from the examples

JB _

the strengths of pulps prepared by the process of the invention are similar to those of the conventional NSSC pulps at similar yields. It can also be seen that the selectivity of delignification is improved, as shown by the lower Kappa numbers obtained with the process of the invention compared with the NSSC process.

The quinonoid or hydroquinonoid compounds which we have found beneficial include anthraquinones (AQ's) and anthrahydroquinones and their tautomers and derivatives, naphthoquinones and naphthohydroquinones and their tautomers and derivatives and benzoquinones and benzohydroquinones and their tautomers and derivatives. Examples of a tautomer of anthrahydroquinone are 10-hydroxyanthrone and 1, 4 dihydroanthraquinone and an example of a tautomer of a naphthohydroquinone derivative is 1, 4, 4a, 9a - tetra- hydroanthraquinone-..

A more detailed. escription of ^* such compounds is given in Australian specification 25519/77. The practical examples set out in the Table below demonstrate the preferred process of the invention. In all cases, a charge of 400 g od chips was pulped in a rotating electrically heated digester using a liquor to wood ratio of 3.5 to -4. Other conditions are as shown in the table and individual examples. Pulp properties were determined using Appita Standard Methods after beating in the Lampen Mill to 200 Canadian Standard freeness.

Examples 1 and 2 use NSSC pulping liquor instead of NaOH. liquor and the liquor used in the examples is 11.0% sodium sulphite plus 2.6% sodium bicarbonate on O.D. wood.

In the table and elsewhere in this specifica¬ tion colour means tristimulus green reflectance factor, otherwise referred to as "lightness".

Table 1 - Semichemical Soda and Oxidised White Liquor Pulping of Mixed Species Eucalypt Wood

Example 10

Process NSSC Semichemical Soda-AQ Semichemical soda-AQ (Caustic soda) (Oxidized white liquor)

NaOH(% on OD wood) 15 15 15 15 15 15 15 15

AQ (1 on OD Wood) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

Cooking temp ( C) 170 170 140 140 140 140 140 140 140 140

Time at cooking temp (hr) 0.5 1.0 0 0.25 0.25 0.5 0 0.25 0.5 0.5

Yield (% OD/OD) 74.5 70.6 74.2 70.2 70.8 67.6 73.6 72.3 70.2 69.4

Kappa number 146 148 141 134 . 139 125 142 139 132 133

Spent liquor pH 7.1 6.9 13.1 13,.0 ^* 13.0 11.7 13.0 12.8 13.0 12.8

Colour 32.4 30.1 33.6 31.5 29.2 27.8 34.1 31.3 27.1 27.3

Burst index (kPa m /g) 3.5 3.8 3.1 3.5 3.4 4.0 3.6 3.4 3.8 3.9

Breaking length (km) 5.9 6.4 5.5 5.9 5.7 6.4 5.9 5.7 6.3 6.5

Concorra Crush (N) 334 338 303 333 315 349 322 346 331 354

From these examples-it can be seen that semi¬ chemical soda-AQ pulping with excess oxidized- white liquor to maintain a high residual pH (examples 8 to 12) gives pulps with strengths and colour similar to NSSC

5 pulping (examples 1 and 2) or semichemical soda-AQ pulp¬ ing in which sodium hydroxide is used as the alkali source (examples 3 to 7).

The advantages of the process of the invention will be readily apparent to those skilled in the art.

10 Both the kraft and semi-chemical pulp mills make use of the same recovered chemicals, with the minor modifica¬ tion that the chemical solution to be used in the semi-chemical process undergoes a simple oxidation process. The need for a complicated multi-step

-- ^■ 5 conversion of sodium sulphide to sodium sulphite is eliminated and a self-contained chemical recycling system results, with no need for any kind of chemical bleed of excess chemical. Since both the kraft and. semi-chemical mills are using essentially the same 0 chemicals, the problem of a mis-match or imbalance does not arise and the relative size of the two mills can be fixed at any desired ratio. As in all pulping processes, some loss of chemical is unavoidable and must be made good by addition of make—up. A further advantage of 5 the invention consists in the fact that chemical make-up can utilise either sulphur-containing or sulphur-free chemicals or both e.g. sodium sulphate and/or sodium carbonate. If sodium sulphate is used as the sole make-up chemical, the sulphidity (i.e. the percentage of 0 the total of active sodium compounds which is present as sulphide) will rise, since the effective sulphidity of sodium sulphate is 100%, due to its virtually complete reduction to sodium sulphide in the recovery furnace. Conversely, the effective sulphidity of sodium carbonate 5 is zero and use of this compound as sole make-up chemical will cause the sulphidity of the pulping chemical solution to fall. By varying the proportion of sodium sulphate to sodium carbonate in the make-up

is therefore possible to adjust the sulphidity to any level likely to be of practical interest. This provides a mechanism controlling this critical pulping parameter quite unavailable in the kraft/NSSC system where the make-up chemical (NSSC spent liquor) is of fixed composition determined by the requirements of the NSSC pulping process. Furthermore, the only practicable way of withdrawing excess chemical from the system is in the form of sodium sulphate, again a material of fixed sulphidity.