Erco Industries Limited

Erco Envirotech Ltd.

W Howard Rapson

Douglas W Reeve

Gordon Rowlandson

Maurice C Fredette

Norman W Meyers

Allan F Gilbert

Chlorine dioxide, particularly for water treatment but useful in other applications of chlorine dioxide, such as the bleaching of pulp, is produced in an enclosed reaction zone filled with chlorine dioxide-generating reaction medium under sufficient pressure to prevent chlorine dioxide from forming a continuous gaseous phase and the resulting aqueous solution of chlorine dioxide in spent reaction medium is discharged to a recipient aqueous medium, such as a flowing water body to be treated, without the formation of gaseous phase chlorine dioxide.

Small scale generation of chlorine dioxide for water treatment

The adverse effects of dissolved organic material in the aqueous phase of pulp suspensions during bleaching using mixtures of chlorine dioxide and chlorine are overcome by applying the chlorine dioxide and chlorine serially to the pulp in two steps without an intermediate wash. An aqueous solution of chlorine dioxide and chlorine in which the chlorine constitutes from about 6 to about 10% of the total available chlorine of the solution is used in the first step and chlorine is used in the second step. The aqueous solutions are formed by dissolving in water chlorine dioxide and chlorine formed in a chlorine dioxide generator.

Bleaching procedure using chlorine dioxide and chlorine solutions

The accumulation of potassium values in a closed cycle pulp mill process usually using sodium hydroxide as a pulping chemical is prevented by removal of potassium values as potassium chloride by crystallization. A variety of procedures are described, including coprecipitation from white liquor with sodium chloride followed by separation of the codeposited salts, precipitation from white liquor separate from sodium chloride precipitation, and intermittent precipitation from green liquor with intermittent precipitation of sodium chloride from white liquor.

Process for removal of potassium values from pulp mill liquors

A pulp mill bleach plant operation having a low effluent volume, a low consumption of water, energy and chemicals, and yet provides efficient bleaching, caustic extraction and washing is described. Water conservation is practised by controlling the use of wash water in the bleach plant, controlling the design and operation of washers, deckers and other mechanical devices used in the bleach plant and controlling the inflow of water with chemicals. An aqueous polluting effluent-free pulp mill water utilization system is also described.

Low effluent pulp mill, bleach plant operation

Bleach plant operation

Chlorine dioxide is produced in a continuous process by reaction of sodium chlorate and hydrochloric acid. The reaction medium is maintained at its boiling point under subatmospheric pressure and the resulting steam is used to dilute the chlorine dioxide and chlorine and remove the same from the reaction zone. Sodium chloride is deposited from the reaction medium in the reaction zone. The mole ratio of chlorate to chloride in the reaction medium, and the temperature of the reaction medium are controlled within narrow limits to provide high efficiency of production of chlorine dioxide.

Production of chlorine dioxide at high efficiency

Acid and alkaline filtrates are separated from the bleach plant of a bleached pulp mill for use in washing unbleached pulp free from entrained pulping liquor. Acid effluent is neutralized with sodium hydroxide solution prior to use in the washing as the last washing step while alkaline filtrate is used in the remainder of the washing. Part of both the acid and alkaline filtrates are not used in the washing to allow bleed of calcium value and caustic extraction stage solids which otherwise would build up on the washed pulp mat passing to the bleach plant.

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Scarborough, Canada

W Howard Rapson