The aim of this study is to present the design, optimization and modelling of a chemical recovery system for a novel CS2-free viscose-type process that entails dissolution of pre-treated dissolving pulp in a continuous-flow reactor in cold alkali and wet spinning of cellulose in sodium carbonate solutions. Technologies already known to other industries for the recovery and reuse of chemicals, such as causticizing, recalcination, recarbonization and freeze-separation, were used. Chemical equilibria simulations were performed with OLI Studio 9.5, with the purpose to select experimental conditions which avoid undesired precipitations in each unit operation. Synthetic solutions mimicking the spent coagulation liquor were used in the laboratorial experiments. The proposed chemical recovery system was shown to be technically feasible and reduce chemical make-ups to a minimum of 45 kg/ton of NaOH and 4 kg/ton of H2SO4. Small amounts of Zn are expected to precipitate during recarbonization of the coagulation liquor at 30 °C and causticizing at 98 °C. Thus, a filter for ZnO particles should be included in the design of the recarbonization unit and a continuous purge of lime mud and input of fresh lime make-up should be needed to keep burnt lime availability at an acceptable level. Overall, the results presented in this study portray a solution to reduce operating costs and the environmental impact of novel viscose-type processes with alkaline spin dopes and wet spinning of cellulose in sodium carbonate solutions. © 2020, The Author(s).

Research was carried out to investigate hydrothermal carbonisation (HTC) treatment of different effluent sludges from the pulp and paper industry in a Parr-reactor. Sludge samples were evaluated from a thermomechanical paper (TMP) mill and a kraft market pulp mill (NSWBK). The issues studied included HTC treatment time; addition of acid and alkali; dewatering properties of the resulting slurry; and non-process element (NPE) concentration in the original sludge, the filtrate and the filter cake. It was found that HTC improved the fuel quality. Alkali metals were depleted in TMP and NSWBK sludge and in both cases, the sludge was easier to dewater. The yield was better for TMP than NSWBK sludge and the yield losses had a negative impact on the NSWBK sludge. TMP sludge was already a viable boiler fuel and the treatment improved the fuel qualities even more. The HTC treatment of the NSWBK sludge resulted in lower chlorine and potassium, with lower fuel-nitrogen resulting in lower nitrous oxide. Although HTC treatment improved the low heating value of the sludge, this was counteracted by yield loss. Compared with untreated NSWBK, the overall energy impact was negative. HTC treatment offered mixed opportunities from the viewpoint of the recovery cycle.

A set of empirical equations is presented for modelling the precipitation of aluminium, silicon and phosphorous in green liquor plants as a function of initial concentrations of these elements plus magnesium, calcium and alkali in the dissolver. As process parameters, the model includes residence time and temperature. Concentrations, temperature and residence time have been varied according to a factor design plan. Main findings are that in laboratory experiments it was possible to remove aluminium, silicon and phosphorous down to concentrations below their solubility in white liquor thus avoiding accumulation of these elements in the lime cycle. In addition, the experimental work showed that the initial concentrations of aluminium, silicon and phosphorous in raw green liquor have a strong increasing impact on their concentrations in filtered green liquor.

Scaling by Na2CO3-Na2SO4-type solids counts among the most common problems in black liquor evaporation. Previous studies have identified two double salts precipitating from Na-CO3-SO4 solutions: burkeite, Na2CO3·2Na2SO4 and (sodium sulphate) dicarbonate, Na2SO4·2Na2CO3. The latter is believed to be more prone to cause scaling. However, it has long been suspected that other double salts, with largely unknown properties, may form in this system. This work summarizes the results of precipitation studies in the Na2CO3-Na2SO4-(additive) system targeting the transition zone between burkeite and dicarbonate formation. At least one additional double salt has been identified by X-ray diffraction in both bulk crystals and in the scale. The solubility data and the equilibrium liquor compositions have been compared with earlier models

Presteaming is a well-established technique in pulp mills, which improves cooking liquor impregnation by removing air from within and between chips. The aim of the study was to investigate how conditions during steaming affect the subsequent kraft cook and properties of the obtained pulp. It was found that higher pressure and temperature during chip presteaming led to increased degradation and dissolution of hemicelluloses. Lower refinability and tensile index was obtained for pulps cooked after presteaming at high pressure and for a long time.

Optimal performance of the green liquor plant, recausticizing plant, and lime cycle is vital for adequate white liquor availability and quality as well as for a mill's energy efficiency. Recently, various problems in the liquor and lime cycle have been frequently reported by our industrial partners, including poor performance of green liquor filters or sludge filters, decreased filterability of lime mud, increased lime kiln dead load, and poorquality white liquor. Those problems are most likely caused by an accumulation of nonprocess elements in the liquor and lime cycle due to increased mill closure, increased use of biofuels, or both. Data from the literature and earlier studies have been analyzed with regard to the occurrence and concentration of nonprocess elements in various process streams, including filtered green liquor, green liquor sludge, lime mud, and white liquor. The mineral forms in which nonprocess elements often precipitate were also studied, together with the common knowledge and rules of thumb used by mills for dealing with the problems. The literature data are compared with the newest analytical results from a sampling campaign involving several mills with varying process solutions with respect to nonprocess elements. The consequences and possible recommendations for the mills are presented. Application: Kraft pulp mills can use this information to diagnose and possibly solve problems related to nonprocess elements in day-to-day mill operation.

The paper reports conditions suitable for recovering and processing 4-O-methylglucuronoxylans from birch and eucalyptus spent kraft cooking liquors. The black liquor originating from the initial heating-up phase in birch kraft cooking exhibits a high xylan and low lignin concentration. The maximum concentration of polymeric xylan was found in the cooking liquor just before the cook reached its final cooking temperature. For eucalyptus kraft cooking, however, lignin was more abundant than xylan in the cooking liquor throughout the whole cook. Birch and eucalyptus xylan, exhibiting good purity, were produced by employing a process chain involving, redrawing of xylan-rich spent liquor early in the cook, upgrading of the spent liquor employing ultrafiltration with diafiltration followed by precipitation and drying.