US2398420A - Bleaching and loading of papermaking fiber - Google Patents

Description

, Patented Apr. 16,1946

. 2.398.420 BLIEACHING AND LOADING OF PAPER- MAKING FIBER Horace Freeman, Three Rivers,

asslgnor to Consolidated Paper Quebec, Canada. Corporation Limited, Montreal, Quebec, Canada No Drawing.

Continuation of application Serial No. 412,599, September 2'7, 1941. This application May 8, 1945, Serial No. 592,727. In Canada November 22, 1940 2 Claims.

This invention relates to methods for improving the appearance and properties of vegetable fiber to be used in the manufacture of printing paper. More commonly, it is to be used for the improvement of paper derived from mechanical wood pulp, otherwise termed groundwood. This application is a continuation of application Serial No. 412,599, filed September 27, 1941.

It has long been known that both animal and vegetable fibers may be bleached or partly bleached by subjecting the fiber to the action of soluble metal hydrosulphite solutions at somewhat elevated temperature (British Patent #8351, Graham and Cope, 1904). It is also well known that said hydrosulphite solutions are unstable and, therefore, better results are obtained if said solutions are used almost immediately after their preparation or if they are prepared in situ with the fiber. Further it has been shown that advantage may be had in preserving said hydrosulphite solutions if they are maintained alkaline when bleaching vegetable fiber (U. 8. Patent 1,027,953, Wickham), but if alkaline hydrosulphite solutions are used against ground wood, the bleaching effect is lost owing to the action of alkali upon the groundwood which is thereby causedto be darker than in its-original state.

To overcome Snyder, U. S. Patent #l,99l,824, to bleach the groundwood in acid solution of hydrosulphite and then later to add an alkali such as finely divided calcium carbonate and carbonate may remain in the groundwood and act as a filler in the paper made from it. In carrying out tests of this process, I have found that it is difllcult to handle acid solutions of hydrosulphite in large scale equipment unless this is made of expensive acid-proof materials. Further, if the neutralization of the acid is efiected by any form of alkali in contact with the pulp, it is necessary to restrict the addition of said alkali so that the pH of the pulp does not exceed 6.5. In attempting to add alkali to large quantities of pulp so that the final mass ends with a slightly acid condition, I have found that parts of the pulp mass become over-alkaline. The result is that said portions of the pulp become yellow and brown and this color is not subsequently removed when the whole mass is thoroughly mixed and brought to the proper end point, consequently a substantial part of the bleaching effect is lost. Further, I have found that when calcium carbonate is used as the alkali, it reacts only with the free uncombinedacid in the solution and it this, it has been proposed by over metallic zinc in a box,

it is said that the I does not react with any bisulphites which may be present, at least within the space of time which is practicable for commercial operation, and for this reason, it is necessary to use calcium hydrate, otherwise termed milk of lime as the neutralizing agent.

I have found also that in order to carry out the bleaching operation and in order to obtain control, it is not only better, but necessary to add the alkali in the form ofmilk of lime to the bleaching solution of soluble hydrosulphite before use and to bring the bleaching solution to a pH of approximately 5 to 6.5 before adding it to the groundwood pulp which normally will have pH.- of between 5 and 6, owing to the organic acids which are contained in the wood. When this procedure is followed, the bleaching solution may i be handled in the slightly acid condition without resort to the use of acid-proof equipment and the final result is that the whole mass of the pulp is maintained slightly acid and no reversion of the bleaching effect is encountered.

In preparing the solution of the soluble hydrosulphite, it is preferable to utilize what is known as sulphite cooking acid which is prepared by passing sulphur dioxide and water through towers containing limestone. Said cooking acid, in its raw state, may contain 4.5% total sulphur dioxide of which 1.7% is combined and 2.8% is free sulphur dioxide. $uch acid solutions of calcium bisulphite are caused to pass tank or tower whereby zinc enters into solution through combination with the free acid and its eflect is to reduce a considerable part of the solution to the form of soluble hydrosulphite which may be partly zinc hydrosulphite and partly calcium hydrosulphite accompanied in solution by sulphltes of zinc andcalcium maintained in solution as bisulphites by a residue of free sulphurous acid which still remains after the treatment with zinc. This clear solution is caused to react immediately and continuously with a finely divided suspension of calcium hydroxide whereupon the free sulphur dioxide is neutralized and there is effected a precipitation of the now insoluble sulphites of calcium and zinc while the hydrosulphites still remain in solution.

In carrying out this operation, 'to later success of the bleaching process to insure that the milk of lime is of sufiicient degree of fineness, which may be obtained by passing it through a screen of 200 mesh to the inch and also to insure that an excess of the lime is not added, so that the resultant suspension of insoluble sulit is essential phites inthe solution of hydrosulphite will not pass the neutral point and therefore will not become alkaline. Preferably, it should remain slightly acid at a pH of 6.5 or somewhat less. Said carefully prepared suspension can then be used immediately in ordinary equipment for bleaching vegetable fiber, notably groundwood. This operation is effected by bringing the suspension into contact with the groundwood at a consistency of about 3% or higher and by maintaining the mixture with agitation at a temperature in excess of 100 F. for a short period of time which may not exceed 40 minutes. The effect of this operation is the removal of color from the fiber, in which process it is probable that the soluble hydrosulphites are oxidized to the form of insoluble sulphites and, therefore, there is an increase in the amount of insoluble sulphites of zinc and calcium. These insoluble compounds are allowed to remain with the fiber in the process of making the latter into paper wherein they perform a useful function as whitening agents and therefore to heighten the effect of the bleaching, to increase the opacity and to assist in producing a smooth finish on the surface of the paper.

I am aware that it previously has been shown (Haywood U. S. Patent #2,090,002, 1937) that insoluble calcium sulphite can be produced by neutralizing sulphite cooking acid with milk of lime and that the said insoluble sulphite has been used in the manufacture of paper, but said disclosure does not involve the production of the mixed insoluble compounds of zinc and calcium, which are the result of the present invention, nor doe it disclose a method of bleaching the fiber simultaneously through the use of the well-known hydrosulphites.

As an example in carrying out this procedure, I cause sulphite acid containing 4.7% total sul phur dioxide of which 1.7% is combined with lime as calcium bisulphite to pass continuously over metallic zinc until it has absorbed approximately 0.3% of its weight of zinc. The reduced solution containing zinc and some remaining free sulphurous acid is then caused to mix continuously with a suspension of milk of lime containin approximately of calcium hydroxide. The amount of lime added is controlled to give the resulting mixture a slight acid end point in the range of pH 5.0 to 6.0. It is found that approximately 187 gallons of the acid calcium bisulphite solution yield about 220 gallons of bleaching solution after the addition of the lime. The effect of the lime addition is to precipitate a part of the zinc hydrosulphite as insoluble zinc salts and practically all of the calcium bisulphite as insoluble calcium sulphite. Thus, the insoluble compounds in the bleaching solution at this stage are calcium and zinc salts, both possessing a high degree of whiteness. The hydrosulphite remains in solution. If the solution is now immediately mixed with groundwood having a natural pH of approximately 5.5, bleaching is rapidly effected at a temperature over F. The temperature need not exceed F. In the course of the bleaching operation, there is further precipitation of insoluble calcium and zinc salts as sulphites, the end result being that the pulp is rendered nearly white by bleaching while it is simultaneously loaded or filled with the greater part of the now insoluble white compound of calcium and zinc. The above quantity of 220 gallons of bleach suspension is usually sufilcient to bleach one ton of groundwood and to increase its weight by the insoluble filler to the extent of 4 to 5%.

When said bleached and filled pulp is utilized in the manufacture of paper, it is found that the operation of bleaching and loading has not substantially altered the natural acidity of the stock which remains slightly acid in the range of pH 6.0 to 6.5 and therefore the operation of forming paper from the stock is not upset; whereas if the stock is allowed to become alkaline, the bleaching effect is lost and the mechanical operation is .upset. Thus, the product of my process is bleached paper of a high degree of brightness containing white insoluble compounds of calcium and zinc. The groundwood fiber s'o prepared may be mixed with other paper making materials without loss of the properties imparted by this process.

I claim:

1. The process of filling and bleaching paper making stock which consists in forming an acid hydrosulphite solution of calcium and zinc by treating calcium bisulphite solution containing free sulphur dioxide with metallic zinc, neutralizing the solution to a pH of substantially 5 to 6.5

by the addition of milk of lime, thereby forming a white precipitate containing zinc and calcium compounds, adding the suspension of said white precipitate in the slightly acid solution of hydrosulphite to the paper making stock and thereby effecting the bleaching and loading of said stock at a temperature between 100 and 180 F.

2. The process of bleaching and loading paper making stock which comprises treating a calcium bisulphite solution containing free sulphur dioxide with metallic zinc to form an acid hydrosulphite solution, adding calcium hydrate to said acid solution to give it a pH of 5 to 6.5 and precipitate insoluble sulphites of calcium and zinc in the solution of hydrosulphites, and adding the solution containing said precipitate to groundwood pulp at a temperature in excess of 100 F.

HORACE FREEMAN.