US1768822A - Method of bleaching - Google Patents

Description

Patented July 1, 1930 UNITED STATES PATENT OFFICE LINN BRADLEY, OF MONTCLAIR, NEW JERSEY, AND EDWARD P. MGKEEFE, OF PLATTS- BURG, NEW YORK, ASSIGNORS TO BRADLEY-MCKEEFE CORPORATION, OF NEW YORK, N. Y.,. A. CORPORATION OF NEW YORK METHOD or BLEAGHING No Drawing.

This invention relates to improvements in the bleaching and purification of fibers and fabrics or other materials, especially cellulose from wood for use in the manufacture of pulp and paper products, etc. In the common 'methods of bleaching pulp with chlorine -or chlorine-bearing compounds, such as bleaching powder, the treatment tends to injure or weaken it. Wood pulp bleached with chlorine compounds frequently has a natural wood tint rendering it inferior for the better qualities of product and its ash content may be greatly increased by the bleaching operation particularly 1 when bleaching powder is used.

The improved bleaching process of the present invention is a multiple-stage process in which-the pulp is subjected-first to the action of a suitable oxidizing bleaching agent present in an amount insufiicient to complete the bleaching treatment, and in which the bleaching operation is completed by a subsequent treatment with bleaching materi al. Each stage may employ the same and/or different bleaching reagents and any or all of the stages be either acid or alkaline, according to. the reagents chosen and the results desired.

According to the present invention. we

avoid some of the usual objections to bleaching with chlorine or chlorine-bearing compounds, and we produce a high white cellulose product, low in ash, and of good strength, thus making it suitable for. many purposes for which cellulose bleached by former methods has been found less satis factory.

According to the present invention one or more stages of the bleaching operation may be carried out with a manganate or permanganate as described in our co-pending application Serial No. 449,245, filed March 2, 1921, of which this application is a continuation in part. Or the invention may be practiced by the use of bleaching solutions containing an active chlorine-bearing reagent, as exemplified by our co-pending application Serial No. 544,322, of which this application is a continuation in part.

ccording to one embodiment of the pres- Application filed March 14, 1928. Serial No. 261,697.

ent invention, we may employ a manga: nate or permanganate in thebleaching operation. When a solution of a manganate or permanganate is added to the material to be bleached or oxidized, not only does the manganate or permanganate exert an oxidizing action on the bleachable or oxidizable constituents of the material, but these constituents exert a reducing action which results in the precipitation of a compound or compounds of manganese, so that the material is given a brown color. This production of a compound of manganese takes place not only when the manganate or permanganate is alkaline, but even when sufficient acid is added to give a mildly acid reaction, unless a relatively large excess of acid is used or the material is heated to a high temperature or allowed to stand a prolonged .time with a moderate amount of acid. The compound of manganese thus precipitated is to a considerable extent deposited directly on the fibers,

and this production of a compound of manganese appears to take place coincidently with the action of the manganate or permanganate upon the materials associated with the fibers; although the formation of a compound of manganese may take place in the solution where the solution contains 0x1- dizable constituents.

After the treatment of the material to be bleached with the manganate or permanganate (if a sutliciently large excess of acid and high temperatures have not been used ,or the mix allowed to stand for a long time while moderately acid) the material is, as above stated, of a brown color. It is now treated to dissolve away or remove a manganese compound, preferably by the addition of a sufficient amount of sulfur dioxide or a solution of sulful dioxide, or of a bi-sulfite, or a sulfite and anacid, or any other suitable reagent. When sulfur dioxide or other suitable reagent is added to the material to be thus treated, the brown color promptly disappears and the material is left white. The manganese: compound is thus rendered soluble and can be washed free from the bleached material.

We have found that manganates can be successfully used for the bleaching process of the present invention by using an amount larger than whenpermanganates are used and we include the use of manganates as well as permanganates. However, their bleaching effect is less efficient than that of permanganates; and we regard the use of permanganates as more advantageous. The permanganates can be produced from manganates by oxidation with suitable agents; and this production of permanganates and their subsequent use according to the present invention can advantageously be combined. For example, sodium manganate, admixed with an excess of alkali, may be treated in solution with chlorine to oxidize sodium manganate to sodium permanganate and to form sodium hypochlorite with excess alkali; and the resulting mixture can then be utilized as a composite bleaching or oxidizing agent, as hereinafter more fully described.

According to the present invention, instead of adding the bleaching liquor, e. g., including manganate or permanganate, all at one time in sufficient amount to complete the desired bleaching treatment, we have found that an economy in the required amount of bleaching material can be effected by carrying out the bleaching in stages, i. e., by adding part of the bleaching liquor and permitting it to act upon the fibrous material, then removing soluble compounds (resulting from this partial bleaching) by removing the spent liquor from the fibrous material washing etc., and then adding to the partially bleached fibrous material some of the same or a different kind of bleaching liquor. Two or more similar treatments, as desired, may be employed until the required bleaching result is attained. When onlya small or moderate amount of bleaching material is used in this way, i. e., in the first or preliminary partial bleaching treatment, there appear to be produced compounds which are water-soluble and may be substantially colorless, some of which are apparently not fully oxidized, i. e., are capable of consuming permanganate in becoming oxidized to a further degree. We have found that these compounds can be removed from the partially-bleached fibrous material by the washing or liquor removal operation, or otherwise, so that they will not be present 1n a subsequent stage to consume bleaching material, such as for example, a manganate or permanganate.

The amount of sulfur dioxide or other sulfurous material required for converting the brown material, if any produced, by

. action of amanganate or permanganate,

may vary somewhat with the amount of manganate or permanganate consumed. Ordinarily only a small or moderate amount is required to form a soluble manganese compound and bring the material to a white color, provided a sufficient amount of bleaching material has been consumed and the material previously washed, but sufficient sulfur dioxide should be used to insure dissolving all the undissolved manganese compound remaining upon the fibers.

It is one advantage of the present invention that the bleaching material, when an alkali metal compound is used, is sufficiently soluble so that the finished bleachedand washed fibrous material is low in ash as well as of good white color and excellent texture. The bleaching process, moreover, is one which can be carried out at a moderate temperature.

As above stated, the permanganate which may be used in the present invention, may advantageously be produced from manganates. Alkali metal manganates are commonly made by fusion methods which leave an excess of alkali, such as sodium hydroxide, with the manganate, when economic yields are desired. Instead of neutralizing this excess of alkali with an acid, or otherwise removing it, we have found that it is advantageous to leave it with the manganate, and then treat the mixture of manganate and excess alkali in aqueous solution, or in suspension in part, with chlorine, either after or before adding the mixture to the fibrous material to be bleached. Chlorine will act upon the manganate to form permanganate and it will also act upon the excess alkali to form hypochlorite, so that if a proper amount of chlorine be added there will be produced a composite solution containing both. permanganate and hypochlorite. This composite solution may be used in one or more stages for bleaching the pulp. in the multi-stage process of the present invention, or it maybe used in only one of the stages, and a different solution, such, for example, as a solution containing a chlorine-bearing bleaching reagent, may be used in another stage. The hypochlorite in such composite bleaching liquorappears to act in a different and more advantageous manner than when used alone, while the action of the permanganate is supplemented by the action of the hypochlorite, so that a lesser amount of permanganate need be used thanwhen permanganate alone is relied upon for obtaining a similar degree of bleaching.

The composite bleaching liquor may be directly made in solution at the time and place of use. For example, if sodium manganate is made by a fusion method and contains an excess. of alkali, this can be shipped in a solid state to a laundry, textile factory, paper mill, etc., dissolved in water, and the solution treated with a suitable amount of gaseous chlorine sufficient to oxidize the manganate and produce hypochlorite from substantially all available alkali, and this solution (after filtering if necessary) can 'in hypochlorite liquor, etc.

then be added directly to the pulp or other material to be bleached. If the manganate or permanganate contains an insuflicient amount of excess alkali to produce the desired amount of hypochlorite, suitable alkali or alkaline compounds can be added before the treatment with chlorine, and the com-.

posite solution will be obtained.

Instead of producing the composite solution containing permanganate and hypochlorite and adding this solution to the pulp or other materials to be bleached, the manganate or permanganate and excess alkali or other suitable compounds can be added to and mixed with the pulp or other material to'be bleached and the proper amount of chlorine then introduced, for example, in the form of a chlorine solution or preferably in gaseous form, care being taken to secure good distribution and absorption of the chlorine.

The proportions of permanganate and hypochlorite in the composite bleach solution can be varied and regulated, for example, by varying and regulating. the amount and proportion of free alkali in admixture with the manganate which is to be treated with chlorine. So also, a composite bleach solution of regulated permanganate and hypochlorite content can be made by admixing separately produced permanganate and bypochlorite solutions 0 known permanganate and hyprochlorite content, or the desired amount of permanganate may be dissolved Such solutions can be produced and admixed prior to the addition of the composite bleaching solution to the pulp or other material to be bleached, or the mixture may take place, for example,

- in the pulp by separately adding the solutions thereto, with appropriate agitation of the pulp to bring about uniformity of dis-' tribution of the reacting materials. The regulation of the proportions of permanganate and hypochlorite makes possible the regulation of the bleaching operation ina simple and advantageous manner, as will be readily appreciated.

When the pulp or other material 'to be bleached has been treated with the composite bleaching agent, it can be subsequently treated with sulfur dioxide solution or other suitable reagent in the manner above described. By washing the'pulp or other material to be bleached, after the manganate or permanganate and hypochlorite treatment, the pulp, etc., can be freed from the greater part of the compounds in solution, prior to the sulfur dioxide solutionorother treatment for-dissolving a manganese compound.

. Instead of using sulfur dioxide for converting oxide or other compounds of manganese into a soluble form, other agents which con- Vert the oxlde into colorless and soluble niaperiod of time, etc. the time of the cooking operation makes terial can be similarly used. Acids alone can be used for this purpose, such as hydrochloric acid or sulfuric acid, but a consider able amount of such acid will be required unless the action is carried out or finished at an elevated temperature, or allowed to stand for considerable time and the fibers may be thereby injured. The treatment with sulfur dioxide or sulfurous acid has the advantage that it takes place readily and quickly at ordinary temperature.

We have found that pulps obtained, for example, by the soda, sulfate and sulfite processes, so-called, are not only given; a white color, but retain their white color much better and longer when bleached according to the present invention with a manganate or a permanganate, either alone or assisted by chlorine or a hypochlorite, than when bleached by chlorine or a hypochlorite in single stage, according to the common methods of bleaching, i. e., without the use of a manganate or permanganate. This advantage of increased permanence of whiteness of the bleached pulp is a characteristic advantage of the present invent-ion, as compared to pulps bleached by bleaching powder in the customary single-stage manner.

It is a further advantage of the present invention that it makes possible the satisfactory bleaching of chemical pulp from wood produced by what is commonly known as the short-time or quick-cook method, that is, by carrying out the cook with cooking liquors of various degrees of strength or of concentration, pressure, temperature and time, and terminating the cook without carrying it to completion. Such pulp, produced by the short-time or qulck-cook method, so-called, cannot be readily and satisfactorily bleached by ordinary bleaching methods, and usually results in a heretofore would be considered under-- cooked and difiicult to bleach by common bleaching methods can advantageously be bleached by the process of the present invention and brought to a good white color with materially less injury to the stock than with methods of bleaching heretofore commonly used. The present invention makes possible therefore the production of a satisfactory bleached pulp from under-cooked stock, thus avoiding the necessity in many cases of continuing the cooking operation for the normal The cutting down of possible a material increase in the number of digestions which can be carried out in a given apparatus, and correspondingly increases the producing capacity of the digesters over the usual practice in the art.

We accordingly include and claim as a part of our invention the production and bleaching of such under-cooked pulp by'treating such pulp' in the manner herein described.

111 addition to the bleaching of under-.

cooked pulp, the process of the present invention is also of particular value for bleaching pulps which are difficult to bleach by common bleaching methods, and enables a relatively superior bleached pulp to be obtained thereby.

Thereare large supplies of forest products, etc., which, due to their inherent composition and properties, have been unsuited to the production of high grade cellulose material by the processes heretofore commonly practiced. We have found that many such materials can advantageously be treated for the production of pulp or cellulose materials by the use of cooking liquor containing an alkali metal sulfite, as more fully described in companion applications. The

' bleaching process of the present invention is of special value in conjunction with such a cooking process, and gives a high grade white pulp from cellulose material. In this way certain forest products, bagasse, reeds, grasses, bamboo, corn stalks, straws, etc., which are not commonly considered suitable for production of high grade pulp or cellulose material, can be converted into white products of relatively high grade. We, therefore, have increased the available supply of raw materials available for production ofhigh grade pulp or cellulose products. Even when such products are subjected to the usual methods of cooking, the pulp obtained can be satisfactorily subjected to the bleaching process of the present invention to give a bleached pulp of higher grade and whiter in color than is obtained with the usual bleaching processes, i. e., employing bleaching powder in the customary manner and amounts.

The process of the present invention, as

L above stated, is of more or less general application to the bleaching of fibrous material and fabrics, etc., of a bleachable character and willgive whiter bleached materials and with less injury than with any of the usual bleaching methods of. which we are aware.

"While we have described the invention more particularly in connection with the use of alkali metal and alkaline earth metal manganates and permanganates, the invention is not limited thereto, but other suitable manganates and permanganates can be used which are capable of exerting a bleaching or oxidizing action, and such manganates or permanganates can be used either alone, as hereinbefore described, or in conjunction with other agents, e. g., hypochlorites, etc.

The invention will be further illustrated by the following more detailed examples, but it will be understood that the invention is not limited thereto. WVhen using a liquor containing manganate or permanganate alone, the treatment will vary somewhat, depending upon various factors, such as the kind and amount of organiomaterial to be oxidized, the kind of manganate or permanganate used, etc. Some chemical pulps derived from woods may require the use of from about'one to about two percent sodium permanganate or its equivalent, based on the air-dry weight of. the pulp (i. e., pulp containing ten percent water) and the liquor maybe heated to around 30 to 40 C., more or less, until all or nearly all of the permanganate has been consumed in oxidizing organic matter. The resultant liquor, which may still contain some oxidizable organic matter in solution, is removed from the fibrous material and some suitable amount of manganate or permanganate solution is added to the partially bleached fibrous material. An improved result is obtained by such multiple-stage process and the amount of permanganate required for effecting the desired-degree of bleaching will ordinarily be somewhat less than when all of the permanganate is employed in one stage. Thus about one-half the entire amount of permanganate used ordinarily may be added to the unbleached fibrous material in the first stage, or less than one-half of such amount may be employed in the first stage, while in some cases somewhat more than one-half of such amount may be employed in the first stage. After the final oxidizing treatment, the fibrous material may be subjected to a treatment to remove manganese-bearing material, as indicated above. In some cases the bleaching liquor used in the oxidizing treatment may advantageously be slightly or moderately acid, but, if preferred, a neutral or moderately alkaline liquor may be used for the oxidizing treatment. life have found that it is advantageous, when employing bleaching liquors capable of oxidizing non-fibrous organic matter associated with fibrous material derived from wood, for example, chemical pulp, to so conduct the first or preliminary bleaching stage that all or substantially all of the oxidizing power of the bleaching agent or agents added in regulated amount (less than suflicient for the entire amount of oxidation to be effected) is "consumed before the resulting liquor and its dissolved constituents are removed from the resulting fibrous material, and further, so that such liquor when removed from such partially bleached material (fibrous), contains organic matter which is capable of reacting with oxygen derived from a permanganate such as potassium permanganate. For example, by treating such unbleached chemical pulp with chlorine alone, or with a hypochlorite alone, or with a manganate or permanganate alone, or with various of these reagents simultaneously or in conjunction or in any other manner, but with the amount of such reagent or reagents employed in the first stage or preliminary bleaching treatment being less, such as for example, one-half, more or less, than the entire amount of bleaching agent or agents needed for the desired total degree of bleaching, and consuming all or substantially all of such reagent or reagents (supplied in the first stage) in reaction or.

reactions with organic matter thereof, the resulting liquor will in most cases, if not all, contain some organic matter which is capable of reacting with oxygen from a dissolved oxidizing agent such as potassium permanganate. By removing such a liquor from the partially bleached fibrous material, and particularly if the fibrous material is washed before the second treatment, such oxidizable organic matter is removed from the fibrous material before the second or subsequent addition of bleaching agent or agents is made thereto. Thus we have made it possible to reduce the amount of bleaching agent or agents required for the complete bleaching process.

As another example of how our invention may be practiced, chlorine, or hypochlorite may be used to bring about a partial bleaching of such fibrous material and the oxidizing bleaching treatment can then be completed with a'manganate or permanganate.

Instead of using a permanganate solution, bleaching solutions can be used containing chlorine-bearing compounds alone or in combination withother reagents.

A pulp may be prepared from jack-pine or loblolly-pine, or pines of a similar character, by cooking the pinewood chips with a cooking liquor containing a preponderating amount of caustic soda and a lesser amount of sodium sulfide, for example, 15 parts by weight of caustic soda and 5 parts by weight of sodium sulfide per 100 parts by weight of wood (figured on the air-dry basis) at a temperature corresponding to a saturated steam pressure of 110 pounds for a period of about 6 hours. The resulting pulp may be freed from the residual liquor and washed in the usual way. It may be then .diluted with sufficient water to enable it tobe readily handled (e. g., containing 1 to 5 of fibers) and then treated with chlorine by admixing a hypochlorite solution (e. g., bleaching powder) therewith and liberating the chlorine or hypochlorous acid in place by means of added mineral acid, such as sulfuric acid. Instead of producing the chlorine or hypochlorous acid in this way, chlorine gas can bedirectly introduced into and absorbed by the pulp. Only a moderate amount of chlorine is necessary, for example, 3 to 15 pounds, more or less, of bleaching powder per 100 pounds of pulp (air-dry basis). Sufiicient mineral acid is thereafter admixed to give an acid reaction. This chlorine treatment can be carried out at ordinary atmospheric temperature and requires but a few minutes for its completion, ordinary care being taken to uniformly treat the pulp and utilize the chlorine upon the fibrous material in an economical manner.

After such a preliminary acid bleaching treatment, the excess liquor may be removed, for example, by means of a suitable pulp thickening device, either with or without previous addition of soda or lime to fix any free acid, and the pulp is preferably washed. with water to remove ,a considerable part of the dissolved organic matter that may be present with the pulp. The pulp is then subjected to a further bleaching operation with an alkaline hypochlorite bleach, e. g., chloride of lime or bleaching powder, although the amount of such bleaching agent can be somewhat decreased from that commonly used, when a single bleaching treatment with this reagent is relied upon. The amount of hypochlorite solution, may, for example, be the equivalent of from 3 to 15 pounds of bleaching powder per 100 pounds of pulp (air-dry basis), and the bleaching can be promoted by heating the pulp to a moderate degree. When the bleaching operation is thus completed, the pulp is washed to free it from the residual solution, and is then ready for use as a bleached pulp.

The bleaching of such pine pulp in the manner. above described has been found to give a good strong white pulp which holds its color satisfactorily even upon prolonged exposure to air. When the pulp is made with a cooking liquor, such as a cooking liquor containing a preponderating amount of normal sodium sulfite with a moderate amount of other digesting material, such as sodium sulfide or caustic soda, which is without seriously objectionable action upon the fibers, the resulting pulp will contain fibers of a substantially uninjured condition such that the pulp will contain strong fibers which maybe thus bleached to a satisfactory white color, without undue injury.

The preliminary treatment of the pulp with chlorine can be effected by introducing chlorine or chlorinated water or an acid hypochlorite solution into the pulp or producing such compound therein, e. g., on 1ts way to a thickening device preferably of acid-rethe thickening and the removal of the residual bleach liquor takes place at the same time. The preliminary acid bleaching operation appears to alter or remove certain resistant organic matter or otherwise dissolve or change certain of the constituents present so that the subsequent bleaching, for example, with chloride of lime, can be readily carried out with possible saving in time and in steam for heating, while giving an improved bleached pulp which has not been subjected to the injurious action of a stronger alkaline bleach liquor at high tempera tures. The bleached pulp accordingly will have an improved color as well as improved properties in other respects, and the bleached pulp obtained from pines will be different from such pulps bleached by the usual method.

According to another application of the invention, the preliminary bleaching operation can be carried out by adding chloride of lime solution or sodium hypochlorite solution to the unbleached pulp and also adding a limited and regulated amount of acid sufficient to give the pulp an acid reaction and to liberate chlorine or hypochlorous acid, and, after the first bleaching treatment, a predetermined amount of alkali can be added sufiicient to fix the free acid and enough additional alkaline hypochlorite (e. g., bleaching powder or sodium hypochlorite solution) may be added to finish the bleaching operation, the pulp being moderately heated during the second bleaching.

treatment which is advantageously carried out under non-acid conditions.

We consider it more advantageus, however, to subject the pulp to a washing operation after the preliminary acid bleaching treatment and before the subsequent bleaching treatment, in order to remove certain organic matter from the pulp and free it from any remaining free acid, and from compounds rendered soluble by the acid bleaching treatment, before adding alkaline hypochlorite for the second bleaching treatment.

Pulp which is diflicult to bleach with an alkaline hypochlorite solution alone (e. g., a solution, of bleaching powder) can nevertheless be readily bleached by the improved multi-stage treatment of the present invention. For example, we have found that Kraft pulp, produced from spruce by the.

sulfate process so-called and which was diflicult to bleach with bleaching powder alone, could be readily bleached to a strong white pulp by the present process. \Ve have also found that pulp from Canada jack-pine, Minnesota jack-pine, Virginia loblolly-pine,

yellow pine, etc., (produced by means of a cooking liquor made up essentially of a limited amount ofcaustic soda and a lesser amount of sodium sulfite) could be readily bleached by our new method to a beautiful strong bleached pulp with a moderate amount of bleaching agent.

The second stage of the multi-stage bleaching processican be carried out at ordinary temperature if a somewhat increased time is permitted for the bleaching operation, especially with pulp such as poplar pulp made by the soda process and spruce pulp made by the so-ca'lled (acid) sulfite process, although it is better to warm the liquor to a moderate degree. Bleached pulps of excellent white color can be thus obtained with relatively decreased injury to the pulp in that the process avoids the severe treat ment with alkaline hypochlorite at an excess temperature such as tends to oxidize the pulp with resulting production of oxycellulose or to modify the pulp with resulting production of high amounts of beta and gamma cellulose, etc. The second stage of the bleaching process can be carried out by the addition of sodium perborate, sodium peroxide or by permanganates, for example, by sodium or potassium permanganate, using either an alkaline permanganate solution or one which is made acid by the addition of a suitable acid in sufficient amount. When an alkaline permanganate solution is employed, the pulp preferably should be warmed somewhat to facilitate or expedite the bleaching operation. When an acid solution is used, the bleaching takes place readily at ordinary temperatures although a moderate heating may be employed. The manganese compound deposited on the fibers can be readily removed by a solution supplied with sodiunr sulfite or bisulfite and 'sulfuric acid, or by a solution of sulfurous acid, and the pulp can then be freed from dissolved reagents by washing.

The following example shows another application of the invention;

Short leaf pinc was cooked with cooking liquor containing caustic soda and sodium sulfite for a period of about four hours, the amount of caustic soda being about 15 parts by weight and the amount of the sodium sulfite 25 parts by Weight in 450 parts of liquor used for treating 90 parts by weight of wood (calculated on a. bone-dry basis). The pulp obtained was of excellent quality, with long and strong fibers, but it did not bleach readily to a white pulp in a single stage with a calcium hypochlorite solution. It was readily bleached, however, when treated with chlorine in the presence of Water at ordinary room temperature for about 15 minutes, then washed and treated with calcium hypochlorite (bleaching powder) solution and moderately warmed. The resulting pulp was a good strong white pulp.

Cotton linters, black gum, red gum, tupelo gum, chestnut, white oak, black oak, red oak, hickory, birch, beech, maple, bamboo,

jute and other pulps have been bleached by chlorite bleaching liquor, in which the acid bleaching liquor is preferably and advantageously removed, for the most part, from the pulp, before the second stage of the process. The preliminary stage can be carried out with the use of chlorine gas added to the pulp or by the use of.added bleaching powder or sodium hypochlorite and with added acid such as sulfuric acid to give an acid reaction. The second bleaching operation can then be carried out at ordinary temperature for sufiicient time or with moderate warming with a solution of calcium hypochlorite orbleaching powder or sodium hypochlorite. The second bleaching operation can also be carried out with sodium perborate, sodium peroxide or with sodium or potassium permanganate and with or without sulfuric acid with subsequent treatment of the pulp with sodium bisulfi'te' and sulfuric acid, or a solution of sulfurous acid, followed-by a washing operation.

Instead of using an active chlorine reagent alone or an oxidizing manganese compound alone, a composite liquor may be used containing both of these oxidizing agents, for example, a liquor may be prepared containing from about one-fourth part to about three parts, more or less, of sodium permanganate, or its chemical equivalent, and about one-fourth to fifteen or twenty parts, more or less, of sodium hypochlorite or its chemical equivalent, such amounts of bleaching reagents being based on the air-dry weight of the pulp, such aschemical wood pulp. Suitable ratios of such reagents within the limits named may be chosen as required to effect the desired degree of bleaching and a portion of the liquor, ordinarily about one-half, will be added to the pulp for the first treatment and the remaining portion added in one ormoresubsequent treatments after the removal of spent liquor from the resulting fibrous material.

In many cases, as for example where the pulp has been produced by an alkaline cooking treatment suclf' as the so-called sulfate process, or where permanganate is used, it isadvantageous to use a bleaching liquor which isslightly acid. However, since an acid reacting liquor requires the use of special acid-resisting apparatus, such advantages may be overridden by other practical considerations and our process is equally applicable in cases where alkaline liquors are used in one or all-of the steps, as where acid liquors are used in any or all.

In such cases an alkaline manganate or permanganate solution may beused in both stages, or composite liquors as describedabove may be used in one or both stages, or the alkaline liquor in one or both stages may be a solution of an alkaline hypochlorite.

The following is an example of how the two-stage process may be performed using an alkaline hypochlorite bleaching solution. The well-washed and unbleached pulp, e. g., spruce pulp obtained by the conventional acid-sulfite process, composed of around three to seven pounds of fiber to around 97 to'93 parts of water, more or less, may be treated with a bleaching powder liquor prepared by dissolving calcium hypochlorite in cool water and such bleaching liquor may have a small amount of calcium hydroxide in it so as to render it moderately alkaline. The amount of bleaching liquor added to the pulp for the first-stage treatment may vary somewhat. Thus an amount of bleaching liquor which contains around five to ten 7 parts, more or less, of bleaching powder (35% available chlorine) for each 100 parts, by weight, of fibers, may be added to the pulp and thoroughly mixed therewith; then the mixture mav be gradually heated while it is being well agitated and circulated, by

the addition of steam in regulated amount,

for example, such as will not overheat one portion to an objectionable temperature, and the temperature of the mixture gradually brought up in a substantially uniform manner to around 30 to 60 degrees centigrade, more or less.

The treatment may be thus continued until the liquor shows by test that substantially all of its available bleach has been exhausted; then the mixture may be subjected to a dewatering operation by means of any suitable device, and the resulting pulp may with advantage be given a washing treatment by means of water so as to remove a large amount or substantially all of the bleach residues from the pulp. Then the dewatered and preferably washed pulp is admixed with an additional charge of bleaching liquor of the kind described above, the amount thus added being s'ufficient or somewhat more than sufficient to carry the bleaching to the desired degree. The mixture of pulp and added bleaching liquor is circulated and gradually heated and thoroughly mixed, the heating being continued until a temperature of around 30 to 60 0., more or less, has been reached.

hen the fiber has been bleached to the desired degree, the bleach residue is removed in anysuitable manner and the bleached fiber then well washed with water.

The amount of bleaching liquor added for the second-stage treatment may vary somewhat, depending upon the material to be bleached, the degree of whiteness desired, the amount of bleaching reagent utilized in the first-stage treatment, etc. Generally,

an amount of bleaching liquor carrying from five to ten parts of bleaching powder (35% availablechlorine) is sufiicient for the second-stage treatment; and the larger the amount of bleach used on the first-stage treatment, the lower the amount required for the second-stage treatment.

In case the bleach residue removed from the secondstage treatment in any of the above examples, carries a substantial or rather large amount of unconsumed bleach ing reagent, which can be determined by suitable tests, such residue can be utilized, at least in part, for a preliminary bleaching treatment of the unbleached pulp, and a regulated additional amount of bleaching material may be added thereto so that the bleaching liquor for the first-stage treatment may be composed (in part) of bleach residue from the second-stage treatment and (in part) of freshly supplied bleaching material. Thus a fair or moderate excess of bleaching material, over that actually required for the secondstage bleaching treatment, may be added in the second-stage, and

then when the color of the pulp has come up to the desired point, the surplus may be removed along with the liquor removed from the bleached pulp and later employed in apreliminary bleaching treatment of un bleached fibrous material. When operating in the latter manner, care should be taken to avoid heating the liquor in the secondstage to such a temperature that an objectionable amount of available chlorine is lost from the liquor owing to such heating treatment.

In the accompanying claims we have used the term a hypochlorite of an alkali-forming metal as including hypochlorites of alkali metals such as sodium hypochlorite and the hypochlorites of alkaline earth metals such as calcium. hypochlorite or chloride of lime.

e claim:

1. The method of bleaching fibrous material which comprises adding thereto an oxidizing bleaching liquor in an amount insufficient to produce the complete bleaching result, permitting such liquor to remain in contact with the fibrous material until its bleaching action is substantially exhausted, removing spent liquor and soluble compound from the fibrous material and subsequently completing the bleaching to the de sired degree by means of another addition of oxidizing bleaching liquor.

2. The method of bleaching fibrous material which comprises adding thereto a bleaching liquor containing oxidizing material in an amount insuflicient to produce the complete bleaching result, permitting such liquor to remain in contact with the fibrous material until its bleaching action is substantially exhausted, removing spent terial in excess of the amount required to.

complete the desired bleaching result, separating liquor from the bleached fibrous material and utilizing unconsumed bleaching material in the liquor for treating unbleached fibrous material. a

3. The method of obtaining fibrous material suitable for being converted into white paper, which comprises digesting fibrous material, by means of liquor including a sulfite of an alkali, separating residual liquor from the fibrous material, washing the fibrous material and subjecting the washed fibrous material to the action of a bleaching liquor including an oxidized compound of manganese, said liquor being added in successive portions and spent liquor being removed between such additions, and thereafter treating the resulting fibrous material to remove colored compounds of manganese and to obtain a white fibrous material.

a. The method of bleaching cellulosic fibrous material, which comprises subjecting such fibrous material to the action of a reagent capable of exerting an oxidizing bleaching treatment upon non-fibrous organic material associated with the fibers, in the presence of water, removing resulting liquor from the resulting fibrous material, and thereafter subjecting the resulting fibrous material to the action of a reagent capable of exerting an oxidizing bleaching treatment upon non-fibrous organic material associated with the fibers, one of such reagents being a reactive oxygen-bearing compound of manganese.

5. The method of bleaching cellulosic material, which comprises adding thereto such an amount of bleaching material including an oxygen-bearing compound of manganese,

' capable of exerting a bleaching action, as is required for bleaching the cellulosic material, in a series of additions, and removing, between such additions, soluble compounds from the fibrous material after a part of such bleaching material has exerted its ac tion and before the last addition of such bleaching material is made.

6. The method of bleaching fibrous material derived from wood, which comprises adding thereto such an amount of bleaching material including an oxygen-bearing-compound of manganese, capable of exerting a bleachin action, as is required for bleaching the brous material, in a series of two or more additions, and removing, between such additions, soluble compounds from. the fibrous material after a part of such bleaching material has exerted its action ant before the last addition of such bleaching material and subsequently treating the fibrous material to remove a water insoluble compound or compounds of manganese.

7. The method of bleaching undercooked wood pulp, which comprises adding thereto such an amount of bleaching material including an oxygen-bearing compound of manganese, capable of exerting a bleaching action, as is required for bleaching the fibrous material, in a series of two or more additions, and removing, between such addie tions, soluble compounds from the resulting fibrous material after a part of such bleaching material has exerted its action and before the last addition of such bleaching material. I

8. The method of bleaching cellulosic fibrous material, which comprises subjecting the fibrous material to the action of liquor containing a manganate of an alkali metal in amount less than sufficient to complete the bleaching treatment, removing soluble oxidizable compounds from the fibrous material, and subsequently subjecting the fibrous material to a bleaching treatment under conditions adapted to bleach remaining organic matter.

9. The method of bleaching cellulosic fibrous material, which comprises subjecting the fibrous material to the action of liquor containing sodium permanganate in an amount less than sufiicient to complete the bleaching treatment, removing resulting liquor and soluble oxidizable compounds from the fibrous material, subsequently treating the fibrous material with sodium permanganate, and subsequently treating the fibrous material to remove a manganese compound therefrom.

10. The method of bleaching fibrous material derived from wood by means of a digesting treatment, which comprises treating such material by means of oxidizing material in such amount and under such conditions as are adapted to effect at least onehalf of the desired total bleaching, removing soluble material from resulting fibrous material, and subsequently subjecting such re sulting fibrous material to. an oxidizing treatment adapted to complete the bleaching.

11. The method of bleaching cellulosic fibre-bearing material, which comprises treating such material by means of oxidizing bleaching material in such amount and under such conditions as are adapted to effeet a partial but incomplete bleaching thereof and to produce a liquor which contains dissolved matter capable of reacting with available oxygen contained in sodium permanganate, removing such dissolved matter from resulting partially bleached fibrous material, and substantially subjecting such resulting fibrous material to an oxidizing treatment adapted to complete the bleaching.

12. The process according to claim 11, in

which a chlorine-bearing compound is employed in at least one of the bleaching treatments.

13. The process according to claim 11, in which a chlorine-bearing compound, capable of exerting an oxidizing treatment upon organic matter present in unbleached chemical pulp derived from wood by means of a digesting treatment, is employed in at least two oxidizing treatments.

14. The multiple-stage process of bleaching chemical wood pulp which comprises the following steps: (a) mixing the unbleached fibrous material with a liquor containing an oxidizing bleaching material; (6) treating the mixture so as to react a portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the bleaching material supplied in step (a) (c) removing residual liquor containing soluble reaction products from this partially-bleached fibrous material; ((Z) admixing with this partially bleached fibrous material additional bleaching liquor including an oxidizing bleaching reagent and thereafter treating the fibers so as to convert them into bleached chemical pulp, the amount of available bleaching ma terial supplied to the fibrous material in the first-mentioned mixture being less than Would be sufficient to convert the unbleached pulp into high white bleached chemical pulp in a single-stage bleaching treatment.

15. The process according to claim 14, in which the total amount of available bleaching material supplied to the fibrous material is less than would be sufficient to convert the unbleached pulp into high white bleached chemical pulp in a single-stage bleaching treatment.

16. The process according to claim 14, in which at least one-half of the total amount of available bleaching material supplied to the fibrous material is supplied to the fibrous material prior to the step a.

17. The process according to claim 14, in which substantially the total amount of available bleaching material supplied to the fibrous material prior to step 0 is consumed in the bleaching treatment prior to step 03.

18. The multiple-stage process of bleach ing chemical wood pulp, which comprises the following steps: (a) mixing the unbleached fibrous material with bleach liquor including an oxidizing'bleaching material; (6) treating the mixture so as to react a portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the bleaching material supplied in step (a) and so as to form a residual liquor which includes dissolved organic matter; (c) ,removing such residual liquor containing soluble reaction products from the partially-bleached fibrous material; ((1) admixing with this partiallybleached fibrous material additional bleach liquor including an oxidizing bleaching reagent and thereafter treating the fibers so as to convert them into bleached chemical pulp, the total amount of available bleaching material supplied to the fibrous material in the process being less than would be sufficient to convert the unbleached pulp into fibre-bearing material,

high white bleached chemical pulp in a sin gle-stage bleaching treatment. 19. The multiple-stage process of bleaching chemical wood pulp, which comprises the following steps: (a) mixing the unbleached fibrous material with bleach liquor including an oxidizing bleaching material; (6) treating the mixture so as to react aportion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the bleaching material supplied in step (a) and so as to form a residual liquor which includes dissolved organic matter; (c) removing such residual liquor containing soluble reaction products from the partially-bleached fibrous material; (d) admixing with this partially bleached fibrous material additional bleach liquor including an oxidizing bleached reagent and thereafter treating the fibers so as to convert them into bleached chemical pulp, at least one half of the total amount of available bleaching material supplied to the fibrous material being supplied to the fibrous material prior to step 0 20. The process according to claim 19, in which nearly all of the bleaching material supplied prior to step c is exhausted prior to step d. t

21. The process according to claim 14, in which a mixture including fibrous material and available bleaching material in step d is heated to a moderate temperature.

22. The process according to claim 14, in

which a bleach liquor mentioned in step d as an alkaline liquor.

23. The multiple-stage 'process of bleaching chemical wood pulp, which comprises the folloyving steps: (a) mixing the unbleached fibrous material with bleach liquor including an oxidizing bleach material; (b) treating the mixture so as to reacta portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the bleaching material supplied in step (a) and so as to form a residual liquor which includes dissolved organic matter; (0) removing such residual liquor containing soluble reaction products from the partially bleached fibrous material; (d) admixing with this partially-bleached fibrous material additional bleach liquor including an oxidizing bleaching reagent and thereafter treatingthe fibers so as to convert them into bleached chemical pulp, one of said mixtures being treated with bleach liquor at low consistency.

24. The process according to claim 14,.

in which the partially bleached fibrous ma- -ing bleach material in such amount and under such conditions as are adapted to effeet at least one-half but less than all of the desired total bleaching removing soluble material from resulting fibrous material, and subsequently subjecting such resulting partially bleached fibrous material to an oxidizing treatment adapted to complete the bleaching.

26. The method of bleaching chemical pulp from wood which comprises subjecting the pulp to the action of a liquor containing an oxidizin bleaching reagent in amount less than su cient to complete the bleaching treatment in a single stage and in amount such that soluble oxidizable compounds are present, removing soluble oxidizable compounds from the pulp, and subsequently subjecting the resulting pulp, without intermediate cooking at temperatures above 100 C., to a bleaching treatment with a solution containing an oxidizing bleaching agent.

In testimony whereof we aflix our signatures.

LINN BRADLEY. EDWARD P. MCKEEFE.

. CERTIFICATE OF CORRECTION.

Patent Nb. 1,768,822. Granted July 1,- 1930, to

LINN BRADLEY ET AL.

It is hereby certified that error appears in the printed speciticationol the above numbered patent requiring correction as follows: Page 8, lines 53 and 54, claim 1, for the word "compound" read compounds; page 9, line bl, claim ll, for "substantially" read subsequently; page 10, line 28, claim 19, for "bleached" read bleaching, and line 45, claim 22, for "as" read is; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record oi the ease in the Patent @ilice.

Signed and sealed this 21st day of October, A. 0. 1930,

M; J. Moore, 7

(Seal) Acting Commissioner of Patents.