US2290601A - Bleaching of ground wood pulp with preliminary bisulphite treatment - Google Patents

Description

Patented July 21, 1942 BLEACHING OF GROUND WOOD PULP WITH PRELIBIINARY BISULPHITE TREATLIENT Joseph S. Reichert, Samuel A. McNeight, and

Howard L. Potter, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., "a corporation of Delaware No Drawing. Application February 16, 1940, Serial No. 319,238

7 Claims.

This application is a continuation-in-part of our patent application Serial No. 297,150, filed September 1939, now Patent No. 2,199,376, issued April 30, 1940.

The invention relates to the bleaching of paper pulp and, more particularly, to the bleaching of ground wood pulp. In present commercial practice, ground wood pulp, usually with other paper pulps, is employed in the manufacture of papers such as book and magazine paper and newsprint paper. Paper prepared from ground wood pulp, in addition to containing the relatively inexpensive ground wood pulp, generallypossesses improved printing properties and a better opacity as compared with paper prepared solely from the so-called chemical pulps as sulphite pulp. Improved printing properties, increased absorbency, and smoother surface all result when ground wood pulp is utilized in the manufacture of paper.

Unfortunately, there is no method now known to the paper pulp industries by which ground wood pulp of high brightness and uniform high quality can be prepared. In view of the inherent advantages of ground wood as a raw material in the manufacture of paper, paper manufacturers have for some time been interested in a commercially practical method for preparing ground wood pulp of high brightness under conditions wherein the wood fiber would not be deleteriously weakened or the opacity or printing qualities of the resulting paper impaired by the treatment. The principal object of this invention is, therefore, the development of a practical commercial method for the preparation of ground wood pulpof high brightness, which pulp is suitable, either alone or in admixture with other pulps, for use in the preparation of paper of satisfactory light color and increased whiteness retentivity.

While ground wood pulp has, to some extent, been subjected to single stage bleaching proc-- esses in which a reducing agent is employed, these attempts to secure satisfactory bleaching have been generally unsuccessful. -Any increased brightness in the ground wood pulp resulting from the usual acid sulphite bleaching operation, wherein reducing agents such as hydrosulphites and bisulphites are employed, has generally not been carried over into the paper manufactured therefrom. At best, only a temporary whiteness has been imparted to the paper. Moreover, ground wood pulp bleached by the acid sulphite method, the single stage bleaching process uticloses a new and improved method for the' bleaching of ground wood pulp which yields an improved wood pulp of high quality and satisfactory brightness. The single stage bleaching process disclosed in that application is, forthese reasons, a distinct improvement over the single stage bleaching methods now known to the art. We have found, however, that even better bleaching results are obtained and a ground wood pulp of high brightness and improved properties is secured, when there is employed a two-stage bleaching process, the first stage being the usual acid sulphite reducing bleaching treatment now known to the art and the second stage involving the use of our improved oxidizing bleach. Our improved process is, therefore, directed in its broader aspects to a two-stage bleaching operation which includes, as the first step, treatment of the ground wood slurry with the usual acid sulphite bleaching liquor, i. e., sulphur dioxide in acid solution, followed by a second step wherein the pulp is treated with an alkaline solution containing a peroxide. The acid sulphite treatment of the first stage may, of course, be carried out by treating the ground wood pulp slurry with a bisulphite or a hydrosulphite in a solution rendered acid, if necessary, by the addition of a mineral acid such as sulphuric acid, or of an acidic substance such as alum, these agents forming sulphurous acid in the solution. The second stage, or oxidizing bleaching step, employing an alkaline solution of hydrogen peroxide or its equivalent, is substantially identical with the single stage bleaching process described in our co-pending patent application and is carried out under the conditions described in that patent application.

In order that our two-stage process for the bleaching of ground wood pulp may be entirely clear, we will describe the individual steps thereof in some detail. The first step. involves treatment of the ground wood pulp in acid solution with sulphur dioxide or some agent yielding sulphur dioxide under the conditions of treatment. In accordance with the usual practice in the paper and paper pulp industries, this is hereinafter reierred to as the acid sulphite treatment.

Ordinarily, any reducing bleaching treatment employing sulphur dioxide or its equivalent now lizing an agent yielding sulphur dioxide in acid known to h art may beutilized as the first step in our two-stage process for the bleaching of ground wood pulp. Thus, the bleaching agent may be a bisulphite such as sodium bisulphite, or a hydrosulphite such as sodium hydrosulphite, employed in acid solution. The acid employed may be a mineral acid such as sulphuric acid or an acidic substance such as alum, or other suitable acidic material. The pH of the solution during the acid sulphite bleach should be between 3 and 6, the suspension of ground wood in the bleached liquor being acid in reaction during the entire step. Generally, it is to be preferred that the pH fall somewhere within the range 4.0 to 5.5, and we regard a pH of 5.0 or close thereto as the optimum pH during this step.

As is now well-known in the wood pulp art, the amount of the acid sulphite bleaching agent employed is not critical since the bleaching action is of the instantaneous or flash type, at most requiring but a short period of contact, If desired, however, the pulp may be allowed to remain with the acid sulphite bleaching agent for a period of several hours. It is frequently desirable to allow the ground wood pulp to remain in contact with the acid sulphite bleaching liquor for a period of time suflicient to result in the decomposition and elimination of the major portion of the acid bleaching solution. This is especially advantageous when washing of the ground wood pulp at the conclusion of the acid sulphite bleach is not contemplated. Sulphur dioxide and a peroxide such as hydrogen peroxide or alkali metal peroxide will react when brought together, Under these circumstances, if a large amount of residual bisulphite or hydrosulphite remains in the wood pulp, a certain quantity of the hydrogen peroxide or equivalent peroxide is destroyed without effecting any substantial bleaching by reaction with the sulphur dioxide, if the pulp is directly treated in the second stage without intermediate washing. While intermediate washing between the two stages of our process is desirable, we do not ordinarily employ such washing for the reason that it introduces an additional step and further expense. We therefore ordinarily operate so that but a small excess of the bisulphite or hydrosulphite, or none at all, remains in the pulp at the time that pulp is contacted with the solution containing the peroxide, so that the loss of peroxide by reaction is relatively small in extent and well within permissible limits.

The second stage of our process, involving treatment of the ground wood with an alkaline peroxide, will now be described in some detail, this step constituting substantially the same procedure described in conjunction with the single stage bleaching process of our pending application, Serial No. 297,150, now Patent No. 2,199,376.

The active agent in the second stage is a peroxide in alkaline aqueous solution, this solution ordinarily also containing a stabilizing and buffering agent such as sodium silicate. An alkaline solution of hydrogen peroxide can be employed with entire success. However, we prefer to use an alkali metal peroxide such as sodium peroxide. A solution of an alkali metal peroxide has been found to be equivalent in action in our process to that of an alkaline solution of hydrogen peroxide, and since sodium peroxide on an active oxygen basis is relatively less expensive than hydrogen peroxide, we prefer to use the alkali metal peroxide. In place of sodium peroxide or hydrogen peroxide, 9. solution ofa peroxygen compound such as sodium perborate may also be utilized. As all these agents in alkalin solution are equivalent in action in our process, the following description of the preferred procedure employed in the second stage of our improved bleaching method will refer specifically to sodium peroxide as the bleaching agent. When other oxidizing bleaching agents such as hydrogen peroxide or alkali metal perborates are used, the amount of those bleaching agents necessary is readily determinable from the amounts herein specified for sodium peroxide. The use of a bleaching agent other than sodium peroxide will involve adjustment of the pH of the bleaching bath within the limits hereinafter specified, as well as the utilization of such quantities of the respective active oxygen-yielding material as are necessary to yield an active oxygen concentration substantially equivalent to the active oxygen concentration resulting from the use in our process of the specified amounts of sodium peroxide.

Our extensive experiments in the bleaching of ground wood pulp have been carried out principally with commercial aqueous suspensions of ground wood which are of from 3 to 7% pulp concentrations. In the description of the second stage of our process which follows, we have in mind pulps of approximately those consistencies. Of course, during the first stage of our process, it is not necessary to maintain any specified concentration of pulp in the aqueous bleaching solution, the acid sulphite treatment being carried out under conditions now well known in the single stage acid sulphite bleaching process as now known to the paper and pulp industries. During the second stage of our process, while we prefer to employ a pulp consistency of from 3 to 7%, there is, of course, nothing inherent in our process which restricts it to treatment of ground wood slurries falling within the specified consistencies. It may be noted that wood pulps of from 3 to 7% consistency are generally prevalent in the paper pulp and paper industries.

When utilizing sodium peroxide as the bleaching agent in the second stage of our process we have found that it is desirable to utilize an amount of sodium peroxide not over approximately 7.5% of the weightof the dry ground wood being treated. In specifying the amounts of chemical agents utilized, the ordinary practice of the paper pulp industry is followed, the amounts being expressed as a percentage based on the weight of the dry ground wood pulp being treated. For example, bleaching with 5% sodium peroxide means that there will be present in the bleach bath five pounds of sodium peroxide for every one hundred pounds of ground wood being treated. Similarly, bleaching with 2% sodium peroxide means that the bleach liquor contains two pounds of sodium peroxide per one hundred pounds of dry ground wood present in the slurry being treated. This method of expressionis customary in the paper pulp industries, and will be followed here.

While ordinarily we prefer to use about 2 or 3% sodium peroxide or two to three pounds of sodium peroxide per one hundred pounds of the dry weight of the ground wood present in the 'pulp being treated, it is possible to utilize higher percentages of sodium peroxide to give generally a greater degree of whiteness in our two step process. Similarly, lower percentages of sodium peroxide may be employed, but result, generally, in a less increase in brightness. For securing improvements in brightness satisfactory for commercial operations, we have found it inadvisable to utilize less than 0.25% sodium peroxide (onequarter pound of sodium peroxide per one hundred pounds of ground wood present in the pulp being treated). Amounts of 0.50 pound of sodium peroxide per one hundred pounds of ground wood (0.50%) have been found in many instances to yield a satisfactory bleached pulp when employed as the second bleaching agent in our process where the pulp has first been treated with the acid sulphite bleach. While the higher concentrations of sodium peroxide, concentrations up to 7.5% sodium peroxide based on the dry weight of the pulp or even higher, as for example 10% sodium peroxide, when utilized in the second step of our two-stage process will generally result in greater whiteness and to some extent a higher degree of whiteness retentivity, we have generally found it inadvisable to exceed the figure of 7.5% sodium peroxide concentration, i. e., 7.5 pounds of sodium peroxide per One hundred pounds of dry ground wood. Percentages in excess of 10.0% will practically always result in but slight increase in whiteness when utilized in our two-stage process, and will tend to increase the cost of the bleaching-treatment to one which will no longer render the process attractive from the been utilized and temperatures as high as 120 F.

are also practical. The temperature range that We have found desirable, 80 to 120 F., does not differ materially from ordinary room temperatures and the use of such relatively low temperatures in bleaching the ground wood pulp constitutes a distinct departure in the art. In view of the inherent difiiculty of securing a relatively permanent bleach, we have found that the maintenance of a temperature preferably in the neighborhood of 100 F., and in an event within the range 80 to 120 F., is essential.

Another essential factor which must be strictly regulated in order to secure the desired degree of bleaching during our second step, and the desired whiteness retentivity, is the pH of the mixture of ground wood pulp suspension and bleaching agent. The regulation of this pH during the major portion of the oxidizing bleaching operation within a very narrow range is essential if a satisfactory product is to be secured. We have found that the pH must fall strictly within the range 10.0 to 11.0 during the major portion of the bleaching if results of commercial acceptability are to be obtained. Generally speaking, at the lower temperatures, such as temperatures of 80 F. or thereabouts, relatively higher pH values such as a DH value up to 11.0 or even somewhat higher can be maintained without the danger of excessive injury to the pulp.

Ordinarily we prefer to maintain the pH value within the range 10.5 to 10.8, although at temperatures as low as 80 F., as previously stated, the pH may safely be as high as 11.0 or even higher without interfering with the desired bleaching. If the pH value is allowed to go too high, not only is the bleaching not satisfactory, but there is the possibility of a distinct yellow color being formed in the pulp, which yellow color is very difficult to bleach out or remove. We have observed, on the other hand, that pH values below 9.5 will generally result in inconsequential and unsatisfactory bleaching and will not produce a pulp of the desired brightness. For most purposes, therefore, it is essential to maintain a PH of not below about 10.0 in order to secure i and bleaching agent during the second step of our process is very desirable if not essential as it may provide part of the alkalining agent, and also acts to some extent as abuflering material to maintain the pH value within the desired range 10.0 to 11.0 during the major portion of the bleaching. Surprisingly enough, we have found that the amount of sodium silicate which should be present for satisfactory bleaching depends largely on the concentration of sodium peroxide or equivalent agent utilized. Thus, in employing 1 or 2% sodium peroxide, i. e., 1 to 2 lbs. of sodium peroxide per 100 pounds dry weight of the ground wood pulp, it is desirable to have present at least 6% of sodium silicate. (The sodium silicate concentration is specified in the same manner as the sodium peroxide concentration, and a 6% concentration means 6 pounds of sodium silicate per 100 pounds dry weight of the ground wood being treated.) If amounts of sodium silicate less than 6% are present when the sodium peroxide concentration is from 1 to 2% NazOz, the degree of brightness of the pulp at the conclusion of the process has been found to be somewhat less than that obtainable by the use in the second stage of our bleaching process of the proper amount of sodium silicate. Similarly, when the sodiumperoxide concentration utilized in the second step is in excess of 2%, we

have found that the minimum quantity of sodium silicate which should be present if maximum brightness is to be secured should be about 10%. For example, when treating ground wood pulp at 100 F. durin the second stage of our process with a bleach liquor containing 3% sodium peroxide, there should also be present in that bleach l quor about 10 pounds of sodium silicate per 100 pounds dry weight of the ground wood. It may be stated that the values given for the sodium' silicate content, if maximum improvement in brightness is to ,be secured during the second stage, are based on certain standard commercial sodium silicates, such as 42 B. Grasselli No. 30.

It is, of course, understood that the improved process is in no sense restricted to the use of this silicate, or to sodium silicate of any specified gravity or manufacture.

The sodium silicate concentration in the bleaching liquor will ordinarily not exceed 10% even for the higher concentrations of peroxide, i. e., concentrations up to 10.0% NazOz. Even for very low concentrations of peroxide, concentrations as low for example, as 0.50% or 0.025%, it has been found desirable for most satisfactory bleaching and improvement in brightness during the second stage of our process to have substantially 6% sodium silicate present. The sodium silicate range may, therefore, be said to extend from 6% sodium silicate to' 0% sodium silicate, the particular amount selected being determined by the concentration of sodium peroxide or m ivalent bleaching agent utilized in our process.

The presence of sodium silicate is also desilable as it furnishes protection for the metallic equipment against corrosion by alkaline compounds present in the bleach liquor, when that liquor comes into contact with metals. Moreover, it serves the important and primary function of assisting in maintaining the pH within the range given, to some extent'serving as both an alkalining and buffering agent, and as a stabilizing agent.

We have found that a period of contact betwee the ground wood pulp and the oxidizing bleaching solution of from 2 to 4 hours or even longer is desirable in order that satisfactory brightening may be secured. As previously specified, the pH should fall during the greater portion of the bleaching at some value within the range of 10.0 to 11.0. However, it is not detrimental to the quality of the ground wood pulp if the pH drops during the latter portion of the step as low as 9.5, and in some instances even as low as 9.0,.as the major portion of the bleaching will have been substantially completed by this time. The major portion of the bleaching may, for example, be complete within a very few minutes, even though in order to secure a more eiiective utilization of the peroxide, it may be desirable to allow the wood pulp to remain in contact with the bleach liquor for periods of four hours or even longer. As long as the pH is maintained within the range of 10.0 to 11.0 during the major portion of the bleaching, it is immaterial if the pH value falls below 10.0 even for a considerable period of time, as long as the major portion of the bleaching has been completed before the decrease in the pH value of the suspension occurs.

In order to secure satisfactory contact between the ground wood suspension and the alkaline oxidizing agent during the second step of our process, it is desirable to subject the suspension to initial stirring to an extent sufiicient to insure thorough mixing; Continuous stirring is not essential, but agitation in some manner sufilcient to secure thorough mixing at the start of the bleaching process may be regarded as a desirable element in our process. In place of stirring-by means of a mechanical stirrer, contact can, of course, be secured by other means well known in the industry such as the discharge of gas bubbles in the suspension, shaking the container, etc. A At the conclusion of the second step or our improved bleaching process, the pulp is immediately ready, without the necessity for further treatment, for use in the manufacture of paper or for other utilization in the paper industry. It may sometimes be desirable to subject the ground wood pulp at the conclusion of the process to washing, with water or with aqueous liquids, but washing is not an essential step in our process and need not be carried out at any phase thereof. Generally, the pH of the mixture which is converted into paper, called the "furnish" in the paper-making industry, is adjusted by the addition of acid or acid substances as part of the process of paper manufacture.

By the use of our two stage bleaching process, it is possible to secure an improvement in brightness not attainable by the use of the single stage bleaching process described in our previously rei erred to co-pending patent application. Our improved bleaching method possesses the added advantage that it renders permanent at least part of the color improvement securable by use of the acid sulphite treatment, a result that has not previously been attained when ground wood pulp has been subjected to the bleaching action of reducing bleaching agents. Moreover, the impermanent character of the improvement in color characteristic of the acid sulphite bleach is not evident in pulp bleached by the two stage method, since the final step in our process, involving the use of the oxidizing alkaline bleaching agent, serves to fix and render permanent at least part of the improvement in brightness secured during the acid sulphite treatment. Since not only a high degree of'brightness, but also a high whiteness retentlvity is thus secured, ground wood pulp bleached in accordance with our two-stage process is of quality much superior to any that can be attained by the use of bleaching agenm of equivalent cost in any single stage method, including the single stage oxidizing bleaching method of or co-pending application. The degree of brightness attained is one that, of course, has never been previously secured with any acid sulphite bleaching treatment. To those skilled in the art who are familiar only with the transitory and impermanent brightness secured with sulphur dioxide in acid. solution, the whiteness retentivity of the bleached ground wood pulphas been universally regarded as remarkable.

As an example of our improved two-stage process for the bleaching of ground wood pulp, the following may be given:

Example Ground wood, having a brightness as measured by the General Electric reflectance meter of 59.5 was treated in accordance with the following twostage process.

The ground wood pulp suspension of 5% consistency was treated with approximately 0.5% sodium bisulphite in acid solution, in accordance with the usual acid sulphite bleach utilized in the paper pulp industry. The pH of the suspension during this step was 4.0. The suspension was allowed to stand for approximately one hour in order that the greater portion of the bisulphite might be used up during this step.

The ground wood pulp was now treated with 3% of sodium peroxide, i. e. 3 pounds of N820: per pounds of ground wood pulp, based on the dry weight of the ground wood. The pulp consistency was maintained at approximately 5%, and the pH was adjusted to 10.5 by the addition of sulphuric acid and of sodium silicate. The amount of sodium silicate added was approximately 10%, based on dry weight of pulp. The temperature was maintained at 100 F. and the second step bleaching operation was continued until substantially all of the sodium peroxide had been used up.

The ground wood pulp, at the conclusion of the two-stage process, had a brightness of 69.5 as measured by the General Electric reflectance meter. The pulp was suitable for immediate use in th manufacture of paper.

As a comparison, the second step of the process was repeated with a sample of ground wood pulp which had not been previously subjected to the first stage treatment with the acid sulphite bleach liquor. All other conditions maintained were identical with those specified. The brightness of this ground wood pulp at the conclusion of the single stage oxidizing bleach was found to be 67.0 as measured by the General Electric reflectance meter. It is evident that by the use of the acid sulphite first stage bleach, the brightness of the pulp was increased 2.5 points.

To those skilled in the art, many modifications and widely different embodiments and applications of our invention in the general field of bleaching ground wood pulp will be readily apparent. It should be understood that the details of our preferred process as described, including the concentrations, conditions under which the bleaching is carried out and other factors involved, may be subjected to some change without departing from either the spirit or scope of our invention. It is intended that our invention is not to be restricted to specific preferred embodiments, minor details, or specific modes of operation, other than as necessitated by the prior art and appended claims.

We claim:

1. The process of bleaching ground wood pulp in order to prepare a ground wood pulp of high brightness for use in the paper making art which comprises: subjecting said ground wood pulp to an acid sulphite bleach; and thereafter subjecting said ground wood to the action of an aqueous alkaline oxidizing bleach liquor containing a peroxide, said aqueous alkaline liquor having a pH within the range of 10.0 to 11.0 during the major portion of said oxidizing bleach step, the temperature during said latter oxidizing bleach step being maintained within the range, 80 to 120 F.

2. The process of preparing a ground wood pulp of high brightness for use in the paper making art which comprises: subjecting said ground wood tothe actionof an acid sulphite bleach; and thereafter subjecting said ground wood to the action of an oxidizing bleach liquor containing sodium peroxide, said oxidizing bleach liquor having a pH falling within the range 10.0 to 11.0 during the major portion of said oxidizing bleaching step, the mixture of ground wood and oxidizing bleach liquor being maintained at a temperature within the range 80 to 120 F. during the period wherein the major portion of the bleaching during said oxidizing bleach step occurs.

3. The process of bleaching ground wood pulp in order to secure a product of high brightness suitable for use in the paper making industry which comprises: subjecting said ground wood to the action of an acid sulphite bleach bath; and thereafter treating said pulp in an aqueous suspension of from 3% to 7% pulp concentration with an alkaline ozidizing bleach liquor containing sodium peroxide and sodium silicate, said sodium peroxide being present in amounts up to 10 parts by weight per 100 parts dry weight of said ground wood, the temperature of said aqueous suspension of ground wood and alkaline bleach liquor falling, during the major portion of said oxidizing bleaching step, within the range 80 to 120 F., and the pH of said suspension, during that portion of said oxidizing bleaching step wherein the major portion of said oxidizing bleach occurs, falling within the range 10.0 to 11.0.

4. The process of bleaching ground wood pulp in order to prepare a ground wood pulp of high brightness for use in the paper making art which comprises: subjecting said ground wood pulp to an acid sulphite bleach; and thereafter subjecting said ground wood to the action of an aqueous alkaline oxidizing bleach bath containing a peroxygen compound selected from the group which consists of hydrogen peroxide, the alkali metal peroxides, and the alkali metalperborates, said aqueous alkaline liquor having a pH within the range 10.0 to 11.0 during the major portion of said oxidizing bleach step, the temperature during said latter oxidizing bleach step being main tained within the range to F.

5. The process of bleaching ground wood pulp in order to prepare a ground wood pulp of high brightness for use in the paper making art which comprises: subjecting said ground wood pulp to an acid sulphite bleach; and thereafter subjecting said ground wood to the [action of an aqueous alkaline oxidizing bleach liquor containing a peroxide and sodium silicate, said aqueous alkaline liquor having a pH within the range 10.0 to 11.0 during the major portion of said oxidizing bleach step, the temperature during said latter oxidizing bleach step being maintained within the range 80 to 120 F.

6. The process of preparing a ground wood pulp of high brightness for use in the paper making art which comprises: subjecting said ground wood to the action of an acid sulphite bleach; and thereafter subjecting said ground wood to the action of an oxidizing bleach liquor containing an alkali metal peroxide, said oxidizing bleach liquor having a pH falling within the range 10.0 to 11.0 during the major portion of said oxidizing bleach step, the mixture of ground wood and oxidizing bleach liquor being maintained at a temperature within the range 80 to 120 F. during the period wherein the major portion of the bleaching during said oxidizing bleach step occurs.

7. The process of preparing a ground wood pulp of high brightness for use in the paper making art which comprises: subjecting said ground wood to the actionof an acid sulphite bleach; and thereafter subjecting said ground wood to the action of an oxidizing bleach liquor containing hydrogen peroxide, said oxidizing bleach liquor having a pH falling within the range 10.0 to 11.0 during the major portion of said oxidizing bleaching step, the mixture of ground wood and oxidizing bleach liquor being maintained at a temperature within the range 80 to 120 F. during the period wherein the major portion of the bleaching during said oxidizing bleach step occurs.