Kamyr, Inc.

Ahlstrom Machinery, Inc.

Hans G Lindberg

A method and apparatus facilitate removal of metals from acidic liquids containing dissolved metals (such as pulp mill bleach plant washer filtrate) by oxidizing the liquid with ozone, hydrogen peroxide, oxygen, and/or air to increase the valence of the metals (like Mn and Fe) so that they become easier to precipitate. Oxidation may be practiced using any suitable reactor, such as a gas-contact reactor, or a plunger type reactor where the oxidizing agent is primarily gaseous, or a static mixer or stirred tank where the oxidizing agent is primarily liquid. After oxidation the pH of the liquid is adjusted (e.g. to over 6, preferably about 8-11) to precipitate the metals. The precipitated metals are then removed by filtration, sedimentation or flotation, and the metals-depleted liquid used elsewhere in the pulp mill.

Method of controlling of metals in a bleach plant, using oxidation

The removal of bleaching chemical-consuming metals from treatment fluids in a cellulose pulp mill is facilitated by treating the pulp in an acidic or neutral stage to dissolve metals, washing or thickening the pulp to produce a filtrate containing dissolved metals, and then using an alkali liquid to adjust the pH of the filtrate to cause dissolved metals to precipitate. The alkali liquid is pretreated with carbon dioxide to reduce the sulfide content and increase the carbonate content thereof, preferred alkali liquids being green liquor and white liquor. The reduced-metal content filtrate is used in the bleach plant of the pulp mill, to make bleaching chemicals, as a wash liquid, etc. Treatment of the alkali liquid with carbon dioxide is typically practiced by scrubbing the liquor with carbon dioxide gas, for example, using a plate column scrubber or a stationary packing scrubber.

Treatment of bleach plant effluents

Liquid effluent from a pulp mill bleach plant having at least one chlorine chemical bleach stage is evaporated and then used in chlorate manufacture. The bleach plant effluent produced by evaporation may be stored and then transported to an off-site chlorate production location, and/or may be subjected to a sodium chloride removal process (such as evaporative crystallization) and only sodium chloride transported to the off-site location. Chlorates produced off site can be returned to the mill (with acid and caustic that are optionally produced) for chlorine dioxide manufacture on site. The chloride-depleted stream from sodium chloride removal may be burned in the mill recovery boiler.

Minimum adverse environmental pulp with feed to chlorate manufacture

Transition metals are removed from cellulose pulp and treatment fluids in a pulp mill. Medium consistency digested pulp is treated in an acidic stage, and then washed to produce a filtrate. The pH of at least a substantial part of the filtrate is adjusted so that it is greater than 9 (preferably greater than 10.5), while a sufficient carbonate content is provided to keep the calcium content low. Adjusting the pH causes dissolved transition metals in the filtrate to precipitate as solids, and the solids can then be filtered out, e.g. by precoat filtering, or clarification followed by polishing filtering. The greatly reduced transition metal content filtrate is then used elsewhere in the pulp mill, as in a washer following an oxygen delignification stage. The acidic stage may be an EDTA stage, an ozone stage, an acid only stage, etc., and the pulp is preferably treated so that it is bleached in at least one Z stage and one P stage, and preferably also an E (e.g. E.sub.op) stage.

Control of metals and dissolved organics in the bleach plant

Chloride dioxide for a cellulose pulp mill bleach plant is produced by concentrating (evaporating) liquid effluents from the bleach plant to a concentration level high enough for incineration, incinerating the concentrated effluents to produce an ash, chemically reacting at least a part of the ash to produce chlorate, and using the chlorate in the manufacture of chlorine dioxide. The ash is purified to produce sodium chloride and the sodium chloride is reacted with oxygen and external energy to produce sodium chlorate. The chlorate is then used in the manufacture of chlorine dioxide. Sulfates produced are used to manufacture acid and/or caustic, and heavy metal hydroxides are disposed of. A part of the ash may be used directly in chlorine dioxide manufacture. At least some of the salt from chlorate manufacture may be fed to the chemical recovery loop, including a recovery boiler, in the pulp mill. Evaporated gases may be returned to the pulp mill and bleach plant.

Growing community of inventors

Glens Falls, NY, United States of America

Hans G Lindberg