A new freeze concentration process for minimum effluent operation of bleached pulp mills

Freeze concentration involves the volume reduction of an aqueous solution by partial freezing of the water and subsequent separation of the resulting ice crystals from the concentrate. The concentration of the effluents from a bleaching plant of a pulp mill was identified as an area needing the application of this method. A new freeze concentration technology, suitable for such a large effluent volume treatment was developed based on the lessons learned from previous unsuccessful attempts or from successful ones but with units of small productivity. The new process involves a decoupling of the nucleation and the growth of the ice crystals and the use of ultrasounds for the initiation of nucleation in the nucleator. The goal of the process is to help pulp mills to achieve a toxic concentration in effluents from Elemental Chlorine Free (ECF) bleaching as low as that with Total Chlorine Free (TCF). The feasibility of the process for bleaching effluents has been demonstrated in the laboratory where ice of high purity in terms of adsorbable organic halides (AOX) and non-process elements (NPE) was obtained. The ice provides very pure water that can be recycled, allowing for bleach plant closure without the adverse effects of NPE and organic compounds built up in the process. An 100-liter pilot plant is described in this paper that was built and is currently tested. The collected data from this pilot plant will be used for the design of an 1000-liter plant to be operated at the facilities of the Swenson Process Equipment Co. and subsequently transferred to a Weyerhaeuser mill for in-site testing.