US2195302A

US2195302A - Bleaching pulp

Info

Publication number: US2195302A
Application number: US250169A
Authority: US
Inventors: Hampel Clifford Allen
Original assignee: Mathieson Alkali Works Inc
Current assignee: Mathieson Alkali Works Inc
Priority date: 1939-01-10
Filing date: 1939-01-10
Publication date: 1940-03-26
Anticipated expiration: 1957-03-26

Description

Patented Mar. 26, 1940 UNlTED STATES PATENT OFFICE 2,195,302 BLEACHING rm Clifford Allen Hampel, Niagara Falls, N. Y., as-

signor to The Mathieson Alkali Works, Inc., New York, N. Y., a corporation of Virginia No Drawing.

pplication January 10, 1939, v

; Serial No. 250,169

' 3 Claims. (Cl. 8-105) lo losic materials with the use of chlorites as bleaching agents which involve the formation of a high density pulp having a pulp density in excess of about 15% under conditions 'such that the chlorite is substantially inactive, and subsequent ll activation of the chlorite by increase of temperature, or the addition of acid or acidic materials, or both. Among the specific processes included is that of forming such a high density pulp containing the chlorite, and activating the pulp by pass- 20 ing hydrochloric acid gas into contact with it, advantageously with'heating to temperatures in excess of 50 C., for example, from 80 C. to 100 C. This invention relates to improvements in this process. In accordance with the present invention, a high density pulp containing the chlorite with a pulp density above about 15% is formed under conditions such that the chlorite is substantially inactive, and the chlorite is subsequently acti- 30 vated by bringing into contact with the pulp a gaseous mixture containing about 25% or less hydrochloric acid gas by volume, for example,

from to 25%, advantageously while heating the pulp above 50 C. The hydrochloric acid gas is 35 absorbed by the aqueous pulp, and activates thechlorite, so that the bleach is relatively rapid and complete. By the use of such a gas mixture, containing less than about 25% of hydrochloric gas by volume, the concentration of hydrochloric acid 40 in the aqueous pulp is readily maintained at the optimum concentration for eflective and rapid bleaching, that is, at a concentration corresponding to a pH of about 3.5 to 5.0. This use of the diluted hydrochloric acid gas has important advantages over the use of undiluted hydrochloric acid gas in that it'avoids the formation of local high concentrations of acid which might cause local reactions to take place so rapidly that available chlorine is lost before the bleaching is com- 0 pleted, or might seriously afiect the strength of the cellulosic material by degradation due to concentrated acid. The use of the diluted hydrochloric acid gas mixture also has important advantages over the use of aqueous acids for the bleaching of high density pulp. Where aqueous strong acids are used, the concentration must be so low, to maintain the acidity at the optimum range and to avoid degradation of the fibre, that the aqueous acid includes sufiicient water to lower the density of the pulp substantially, with the're- 5 sult that the-bleaching is a low density bleaching, without the important advantages of high density bleaching with respect to cost, etc.

The chlorites which may be used in bleaching in accordance with this invention include the 10" alkali metal chlorites, such as sodium chlorite, and the alkaline earth metal chlorites, such ,as calcium or magnesium chlorite. These chlorites are substantially inactive with respect'to the bleaching of cellulosic materials at temperatures below about 50 0., e. g., room temperature, particularly in neutral or alkaline solution, and high density pulps containing the chlorite in proper proportions for bleaching under conditions such that substantially no bleaching occurs are readily prepared. Such high density pulps containing the chlorite may be prepared in various ways, as by forming a low density pulp containing chlorite and removing water from it by filtration or expression, or both, by spraying pulp with chlorite g5 solution, etc.

In accordance with the invention, the high density pulp, having a density in excess of about 15%, e. g., from 15-50%, containing the necessary amount of chlorite to effect the desired bleach, is

first prepared under conditions of temperature and acidity at which substantially no bleaching occurs, and the pulp is then treated with hydro- 'chloric acid gas diluted with an inert gas, such as air or nitrogen, advantageously while heatedgto temperatures in excess of 0., for example, from -100 C. or even somewhat higher, to activate the chlorite. The proportion of hydrochloric acid gas in the gas mixture should not exceed about 25% by volume, and is advantageouso lyirom 1025% by volume. The amount of the. gas mixture so'used is advantageously regulated so the pH of'the pulp is maintained from about 3.5 to 5.0. With many high density pulps it is difiicult or impossible to measure the pH value; 5 and with such pulps, it is advantageous to supply the gas mixture at a rate such that the desired bleach is obtained in about 30-60 minutes, or at a rate such that during the bleaching period, an

amount 01 hydrochloric acid equal to about A 50 the weight of the available chlorine or thechlorite is supplied.

- Cellulosic materials generally may be bleached by the process of this invention, including paper and paper pulp, such as kratt, sulfite and sulfate 5 pulp, pulped wood; cotton linters, hemp and cellulosic derivatives, such as rayon. Such cellulosic products may be partially bleached by the use of chlorine, hypochlorites, etc., prior to bleaching athigh pulp densities with chlorites and dilute hydrochloric acid gas.

The invention will be illustrated by the following. specific example, but it is not limited thereto. Example-150 parts (dry basis) or kraft paper pulp in the form of damp crumbs were stirred into 3000 parts of water containing 16.36'parts of available chlorine-as sodium chlorite. The pulp was filtered and pressed to a density of 21.4%. It then contained 2% of available chlorine based on the dry pulp. The pulp was shredded, placed in an insulated rotary container heated by live steam at 100 C., and a mixture of hydrochloric acid gas and air, containing about'12% hydrochloric acid gas, was passed into contact with the pulp at a rate such that about 0.0129 part of hydrochloric acid were supplied per minute. The

pulp was maintained for about 1 hour at 100 C.

I 1. In thehigh density pulp bleaching of cellulosic material by chlorites, the improvement which comprises activating the chlorite after formation of a high density pulp having a pulp density in excess of about 15% and containing the chlorite under inactive conditions by bringing into contact with the pulp-a mixture of ,an inert gas and hydrochloric acid gas.

2. In the high density pulp bleaching of ce1- .lulosic material by chlorites, the improvement which comprises activating the chlorite after formation of a high density pulp having a pulp density in excess of about 15% and containing the chlorite under inactive conditions by bringing ,.into contact, with the pulp a mixture of an inert gas and hydrochloric acid gas containing up to about 25% of the hydrochloric acid gas by volume -3. In the high density pulp bleaching of kraft pulp by chlorites, the improvement which comiprises activating the chlorite after formation of a high density kraft pulp having a pulp density in excess of about 15% and containing the chlorite under inactive conditions by bringing into contact withithe pulp a mixture of an inert gas and hydrochloric acid gas containing up to about 25% of the hydrochloric acid gas by volume.

CLIFFORD ALLEN HAMPEL. a0