a b s t r a c tThe major inorganic solid residues of chemical pulp mills include green liquor dregs, various lime residues, recovery boiler fly ash, and salt cake produced at the chlorine dioxide generator. Only a small proportion of these residues is currently utilized outside the mill area: landfill disposal and reuse at the mill are the most typically applied methods. The situation should be improved in the future for environmental and economical reasons. The focus of this review is on the separation of inorganic pulp mill residues to enhance the possibilities of their utilization. Solid-liquid separation operations, such as filtration and sedimentation, and the associated treatment processes, are described in close detail. The review aims at recognizing the most promising residues for utilization, as well as enhancing the understanding of the material balance of chemical pulp mills. The review also covers the most potential industrial applications for the utilization of residues, as well as the possibilities of using them as raw material in value-added products.

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Filter aids are used in challenging filtration applications, such as the deliquoring of biomass suspensions and separation of fine particles from liquids. Organic filter aids, typically composed of cellulose, may be the preferred option in the case of bioprocesses, because their energy content can be recovered together with the primary filter cake. In this study, a laboratory scale pressure filter was used in order to investigate the effect of various filter aids on the filtration of an enzymatically hydrolyzed biomass suspension. Various organic and inorganic filter aids were used in two modes: as a body feed and as a precoat. According to the results, the average specific resistance of the cakes could be best reduced using the body feed mode. Filter aids also helped to reduce the moisture content of the cakes and to increase the corresponding filtration capacities. Additionally, a process scheme enabling utilization of the solids was proposed.

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The objective of this study is to introduce a treatment sequence enabling straightforward and effective recovery of hazardous trace elements from recovery boiler fly ash (RBFA) by a novel method, and to demonstrate the subsequent removal of Cl and K with the existing crystallization technology. The treatment sequence comprises two stages: dissolution of most other RBFA components than the hazardous trace elements in water in Step 1 of the treatment, and crystallization of the process chemicals in Step 2. Solid-liquid separation has an important role in the treatment, due to the need to separate first the small solid residue containing the trace elements, and to separate the valuable crystals, containing Na and S, from the liquid rich in Cl and K. According to the results, nearly complete recovery of cadmium, lead and zinc can be reached even without pH adjustment. Some other metals, such as Mg and Mn, are removed together with the hazardous metals. Regarding the removal of Cl and K from the process, in this non-optimized case the removal efficiency was satisfactory: 60-70% for K when 80% of sodium was recovered, and close to 70% for Cl when 80% of sulfate was recovered.

The presence of hazardous trace elements (HTE) in the chemical recovery cycle of Kraft pulp mills is the main obstacle for the utilization of the inorganic residues of the process. Electrostatically precipitated (ESP) recovery boiler fly ash (RBFA), consisting mainly of sodium sulfate Na2SO4, is a solid side stream where HTE are concentrated. Unlike most other ashes, RBFA is to a great extent water-soluble. A novel reverse leaching method, based on the dissolution behavior of RBFA in water, is introduced in this paper. The method is founded on the use of an appropriate liquid/solid mass ratio and a favorable pH, which together contribute to the formation of a small and readily settling solid residue where almost all HTE are concentrated. This paper focuses on evaluating the influence of the treatment conditions on HTE removal, energy consumption and material losses. The results of this study show that the removal efficiency of the investigated HTE and other analyzed metals was excellent under alkaline conditions, the apparently suitable pH range for the removal of most of these metals being approximately 11.7-12.2. Lead was observed to be the most difficult HTE to remove: the highest obtained degree of removal of Pb was 89 %. The removal rate of Cd and Zn was approx. 100 % within the mentioned pH range.

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Abstract Green liquor dregs represent the most important inorganic residue of chemical pulp mills. The dregs are usually settled in thickeners, washed and deliquored with lime mud precoat filters, and transported to the landfill. The utilization of dregs is challenging, due to the high concentration of hazardous trace elements (HTE) in their solid phase. There are basically two potential strategies for the reduction of the HTE content of dregs: mechanical classification according to differences in the size and density of particles, and removal of HTE by various chemical treatments. The objective of this study was to evaluate the applicability of straightforward mechanical separation methods for the purification of dregs from HTE. The evaluated separation methods included particle size–based classification by sieving, and classification on the basis of differences in the settling properties of particles in gravitational and centrifugal separation. It can be concluded that all the evaluated separation methods could be used to reduce the HTE content of dregs, although the separation efficiency was not very high in most cases. Centrifugation had clearly the best performance of the investigated techniques. The fractions consisting of large particles contained consistently lower concentrations of HTE, compared to fractions containing a lot of fines.