US3481826A - Process for bleaching wood pulp with fluorine,hydrofluoric acid,and oxygen difluoride - Google Patents

Description

F'IPSSQ? United States Patent Int. (:1. D21 9/12 US. Cl. 162-63 ClalmS ABSTRACT OF THE DISCLOSURE A method of bleaching wood pulp with fluorine, hydrofluoric acid, or oxygen difluoride.v The plup is contacted This invention relates to the process of bleaching wood pulp and natural and synthetic fibers, such as are used the paper making industry, the preparation of cloth fabrics and films. In one particular embodiment, it relates to improvement in the method of bleaching wood pulp used in the preparation of paper to give an improved bleached product.

The bleaching of wood pulp and other cellulosic fibers to a high brightness with yaryingbleaching agents such as chlorine or hypochlorite is frequently accompanied by degradation during bleaching. In spite of the large amount of work that has been done in the field, and the large number of 'multi-stage bleaching processes that have been developed, there is no completely satisfactory answer to the problem of bleaching without degradation. Chlorine dioxide does have the property of bleaching without substantial degradation, but it is quite costly and requires specialhandling due to its explosive tendency.

The principal object of my invention is to provide a process for the bleaching of fibrous materials through chemical action by employing fluorine either alone or as its compounds. Another object of my invention is to provide improved bleached brightness over the use of conventional bleaching chemicals. Another object of my invention is to provide improved bleaching with lesser reduction in strength, greater permanence and ease of handling than with the use of conventional bleaching chemicals. A further object of my invention is to replace or supplement the currently employed bleaching chemicals by the use of fluorine or fluorine compounds to attain an improved quality and/ or economy.

I have discovered that the reaction product of various fluorine compounds produces an effective bleaching agent. Thus, I have discovered that a very substantial increase in brightness can be secured in a chemical wood pulp by treating it with fluorine compounds or gaseous fluorine and that the increase in brightness is accomplished by very little degradation in the strength of the cellulose.

"The dispersing viscosity and the bursting and tear strength of the pulp are not changed appreciably by bleaching using my novel process, whereas when chlorine or hypochlorites are employed, there is usually a substantial loss in strength. The treatment can be carried out using the conventional mechanical procedures and equipment'for bleaching with chlorine, or chlorine compounds.

Current bleaching technology, particularly applied to the bleaching of chemical wood pulp for paper, uses chlorine, caustic soda, calcium and sodium hypochlorites, chlorine dioxide, sodium and hydrogen peroxide. With ground wood pulp, reducing agents, such as zinc or 3,481,826 Patented Dec. 2, I969 "ice sodium hydrosulphite may be used, with or withoutthe use of hydrogen peroxide. In the usual conventional processes, several chemicals are used in series or stages, with residues from the reactions being washed out between the stages. A conventional bleaching sequence, for example, uses a series of stages that have been designated by certain letters. For example, C is used to designate chlorination, E for extraction using caustic soda, H for sodium or calcium hypochloride, D for chlorine dioxide and P for hydrogen peroxide. Thus, for example, a

bleaching sequence for a sulphite pulp might be designated as CEH; C for chlorination, E for caustic soda extraction, and H for hypochloritefiA bleaching cycle for sulphate pulp might be designated as C EI IED; i.e. chlorination, extraction, hypochlorite, extraction and chlorine dioxide. Another cycle might be six stage as CEHEHD; i.e. chlorination, extraction, hypochlorite, extraction, hypochlorite and chlorine dioxide. Another cycle might be CEDED. It is apparent that many other combinations may be used according to the properties desired in the bleached pulp and the starting material used in the process. v

The C stage is used mostly for purification of the pulp by chlorinating the non-cellulosic materials present such as lignin. The E stage is used to solubilize and washout chlorinated substances. The H stage is used to bleach and white the pulp and the D stage is added to further whiten the pulp without decreasing its strength. The P stage is sometimes used at the end to reduce loss of brightness of the pulp upon aging.

In each stage it is important to control the concentration or consistency of the pulp, the percentage of the chemical, the temperature, the contact time, sometimes known as dwell time, and the pH. Typical chlorine bleaching systems and the modified system of my invention are set out in the examples below.

In the first stage of my process, I use a fluorine gas previously dissolved in water or passed directly into the pulp suspension. The gaseous fluorine may be generated in the presence of the pulp. Another source of fluorine is the anhydrous hydrogen fluoride of commerce which may be added directly to the pulp suspension alone or in combination with gaseous fluorine or sodium fluoride. The effects of the fluorine or its compounds upon the pulp are completed before the pulp is exposed to human contact. The fluorine bleaching treatment of my invention can be carried out by contacting the pulp with fluorine or a suitable fluorine compound in an amount equal to 0.5 to 9 percent by weight based on the weight of the pulp at a temperature of about to F. for a period of 1 to 4 hours. Suitable bleaching agents which can be used in any process include fluorine, hydrofluorine acid and oxygen difluoride.

I prefer to designate my fluorine stage as F so that it might be written in a bleaching sequence as FEH mean ing: fluorine, extraction and hypochlorite. For a sulphate pulp, it might be a fluorine stage in place of the chlorine stage.

Particularly useful forms of fluorine compounds are the oxy compounds. For example, oxygen difluoride (OR) is prepared in a continuous process by bubbling flnorineinto a 2% solution of sodium hydroxide. The efiluent is a gas that can be passed directly into the pulp at any stage of the process, particularly as the final superbleaching stage usually carried out by the use of chlorine dioxide. In some cases where chlorine dioxide is used in a stage other than the final stage, oxygen difluoride may be used to replace it in that stage. Oxygen difluoride is a particularly suitable source of fluorine for the purpose of my invention since it decomposes into the components oxygen and fluorine at the temperatures of operation of the normal 3 bleaching system, and where it is used I prefer to designate it as the stage.

The amounts of the fluorine compounds to be added in my novel process would closely parallel the amounts where chlorine compounds are used. Thus, if fluorine is added, it is used in the amount of 4 to 9%, usually about 7% on the pulp in the first stage, based on the gaseous fluorine, added as such or generated in the pulp. The pulp would normally be preferably about 3% consistency. The reaction would be carried out at the temperature of preferably about 80 to 90 F. and the retention time would be from 1 to 4 hours, usually about 1% hours. The extraction stage with caustic soda would be carried out with a solution of /2 to 3% caustic soda, preferably about 2 /2% caustic soda at a consistency of preferably about 12% and a temperature of about 120 F. to 180 F. for a period of preferably about 1 hour and an alkaline pH. In the dioxide stage of the process, the reaction would be carried out preferably with about 0.5% of the fluorine compound at preferably about 12% consistency, at a temperature about 160 F. for a period of about 1 hour. In normal operation, the pulp would be washed between stages. If the pulp were to be treated with a peroxide, it would be a final treatment of the bleached pulp at about 1% usually without subsequent washing. More broadly this embodiment of my invention comprises the steps of: fiuorination of the pulp with fluorine or a suitable fluorine compound; washing and extracting the pulp with a sodium hydroxide solution; treating the pulp with oxygen difluoride; and then washing and recovering the bleached product. A more specific form of this general embodiment the fluorination is carried out using a gaseous hydrogen fluorine concentration of 2 to 9% andthe oxygen difluoride treatment is carried out with .5 to 15% of the compound (i.e. oxygen difluoride) and a pulp consistency of 3 to 12%.

Considerable variation is practiced in the amount of chemicals, consistency of the pulp, temperature, dwell time and the number and sequence of steps as may be arranged to obtained the desired results as to pulp quality in brightness, opacity, strength, viscosity, permanganate number and the like, within the limits of the original pulp and the effectiveness of the chemical employed.

With ground wood pulps a single stage uses about 1% sodium or zinc hydrosulphite at 2 to 5% consistency for 1 to 2 hours and 110 to 150 F. at pH 4.5 to 6 for the zinc and pH 6-7 for the sodium salt. It may be supplemented by a stage of 1% sodium or hydrogen peroxide (stabilized with sodium silicate and magnesium sulphate) at 10 to consistency and alkaline pH for 3 to 4 hours at 100 F.

My invention is further illustrated by the following specific but non-limiting examples.

EXAMPLE I 100 grams of a southern pine sulphate pulp having a viscosity of 58.0 centipoises at 3% consistency is treated with 5% fluorine gas, added as such, at a temperature of 75 F. for a retention time of one hour. The pulp is then washed. The second stage is a treatment with 2.0% caustic soda for one hour at 12% consistency and 160 F. The pulp is then washed. The third stage is a treatment with 1.5% sodium hypochlorite for 2 /2 hours at 12% consistency, pH 10.0 and 95 F. The pulp is then washed. The fourth stage is extraction using 1.0% caustic soda for 1 hour at 12% consistency and 160 F. The pulp is then washed. The fifth stage is a treatment with 1% chlorine dioxide for 4 to 5 hours at 12% consistency and 160 F. The pulp is then given its final washing. The brightness is 90 and the viscosity is 28.0 centipoises.

In this example, oxydifluoride might be used to replace chlorine dioxide in the fifth stage.

4 EXAMPLE II grams of a soft wood sulphite pulp is bleached by the FEH sequence with washings between the stages and at the end. In the F stage 3% fluorine is used at 3% consistency for /2 to 1 hour at 60 F. with a pH of 2-3, and the E stage 0.5% castic soda is used at 8% consistency for one hour at F., and in the H stage 3% available chlorine is used at 12% consistency for 2 /2 hours at 80 F. The pulp brightness is 87.

In making dissolving pulp, where a caustic soak stage is used, a final treatment with peroxide is precluded by the presence of silicates in the peroxide solution. In place of such a P stage and 0 stage mayv be employed.

EXAMPLE III 100 grams of an aspen semi-chemical pulp cooked by the cold soda process with a brightness of 39 is treated with 2% zinc hydros'ulphite followed by 1% oxydifluoride to obtain a brightness of 63.

EXAMPLE IV 100 grams of a ground wood pulp with a brightness of 58 is treated with 1% oxydifluoride to obtain a brightness of 65.

What is claimed is:

1. A process for bleaching wood pulp which comprises contacting the pulp with a quantity of oxygen difluoride equal to about 0.5 to 9% by weight, based on the weight of pulp, for a sufficient time to bleach said pulp.

2. The process of claim 1 wherein said pulp'is contacted with said oxygen difluoride for from 1 to 4 hours at a temperature of about 90 to 160 F.

3. A process for bleaching wood pulp which comprises contacting the pulp with a quantity, equal to about .5 to 9% by weight based on the weight of pulp, of a bleaching agent selected from the group consisting of fluorine, hydrofluoric acid, and oxygen difluoride, for from 1 to 4 hours at a temperature of about from 60 to 160 F., whereby said pulp is bleached, washing said bleached pulp and extracting said bleached pulp with /2 to 3% caustic soda solution at a temperature of about to 180 F., washing the extracted pulp and then treating the pulp with oxygen difluoride in an amount equal to about .5 to 15% by weight based on the weight of pulp.

4. The process of claim 3 wherein the first bleaching treatment is carried out using gaseous hydrogen fluoride at a fluorine concentration of 2 to 9% and the oxygen difluoride treatment is carried out with 0.5 to 15 of the oxygen difluoride and with a pulp consistency of from 3 to 12%.

.5. The process" of claim 3 wherein the oxygen difluoride treatment is carried out by contacting the pulp with about .5% oxygen difluoride at a temperature of about F. for about 1 hour and using a pulp consistency of about 12%.

References Cited Kirk-Othmer, Encyclopedia of Chemical Technology 2nd ed., vol. 9., Interscience, N.Y. (1966), p. 527.

Clark, J. T., Determination of Lignin by Hydrofluoric Acid in Tappi, vol. 45, No. 4, April 1962, pp. 310-314.

Casey, I. P., Pulp and Paper, vol.'I, Interscience, New York (1960), p. 520.

S. LEON BASHORE, Primary Examiner R. D. BAJEFSKY, Assistant Examiner US. Cl. X.R.