Pulp and Paper Research Institute of Canada

Steven Prahacs

Joseph G Dorica

Mahmoud K Azarniouch

Pritham Ramamurthy

Alfred Wong

A process is for removal of suspended solids including resin and fatty acids from pulp mill effluents including Chemi-Thermo-Mechanical Pulp (CTMP) mill effluents. The process involves filtration of untreated or primary treated effluents through a mat of primary sludge, pulp or woodroom sludge. A sludge mat is formed on a wire screen or punched drum and the effluent to be treated is filtered through the mat. Most of the suspended solids and a part of colloidal material and dissolved solids are retained by the mat. Following the filtration stage, the sludge is dewatered and removed. A portion of the sludge is recycled to form the mat for the next filtration cycle, while the excess sludge is purged. The filtration is achieved by the application of vacuum below the filter mat or pressure over mat. The process will produce treated effluent with low suspended solids and sludge at high solids consistency and eliminate the need not only for a clarifier but also for sludge thickening and filtration units often used in pulp mills. Calcium chloride may be added to the effluent prior to primary settling or before filtration, to enhance the removal of suspended and total solids.

Process for removal of suspended solids from pulp and paper mill

Oxidized weak black liquor or effluents from alkaline bleaching stages, particularly oxygen delignification and/or ozone and/or hydrogen peroxide bleaching effluents are treated in process stages which include an electrolytic cell to recover NaOH, lignin, O.sub.2, H.sub.2 and to achieve other important benefits, the most important of these are (i) to unload the evaporator-recovery furnace and lime-kiln causticizing plant, and (ii) to enable a mill to implement low- or no-chlorine bleaching techniques without overloading their recovery furnace and/or lime kiln or requiring a new, larger recovery system, the electrolytic treatment can be carried out with one or a combination of the above types of effluents; it is also possible to increase the conductivity if required and thus the efficiency of the electrolytic cells and the yield/production of NaOH by the introduction of Na.sub.2 SO.sub.4 (make-up or recycle from recovery furnace) and/or NaCl into the feed to the process; the process involves pre-acidification of aqueous alkaline liquid containing lignin to initiate precipitation of lignin, feeding the partly acidified liquid to the anolyte compartment of an electrolysis cell in which the anolyte and catholyte compartments are separated by a cation permselective membrane, and in which the catholyte compartment is provided with a source of hydroxide cations by carrying out electrolysis to effect migration of sodium ions into the catholyte compartment, and simultaneously to acidify the liquid in the anolyte compartment to substantially complete lignin precipitation; sodium hydroxide is recovered from the catholyte compartment and lignin is recovered from the anolyte compartment.

Recovery of NaOH and other values from spent liquors and bleach plant

A multi-step method is provided for the purification of waste water, especially pulp mill effluents. It comprises a number of steps carried out in a specific sequence. The first step involves passing an oxygen-containing gas through the waste water while the waste water is in intimate contact with a finely divided activated carbonaceous material, preferably including, as a preliminary step, the addition of the finely divided activated carbonaceous material to the waste water. The second step involves adding a clarification agent selected from one or more of a coagulant, a flocculant, and a polyelectrolyte to the treated waste water provided by the first step. The third step involves recovering a solids-free effluent by known means, e.g. by a clarification step carried out by gravitational, flotational or centrifugal means to provide the waste water free of the suspended material originally contained therein and a sludge including the suspended material originally contained in the waste water, the added carbon and the added clarifying agent. Optionally, it includes the fourth step of regenerating the carbonaceous material from the sludge, and/or the fifth step of regenerating the clarification agent from the sludge.

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Beaconsfield, Canada

Steven Prahacs