This work investigates the potential to integrate modern viscose manufacturing with prehydrolysis kraft pulping in order to improve the economic and environmental feasibility for production of regenerate cellulose fibers from wood. The study is largely based on calculations from literature data, but key stages are also tested experimentally. It is concluded that a kraft pulp mill can supply the acid demands of the viscose plant via acetic acid formed in the prehydrolysis and sulphuric acid for coagulation, with alkali for mercerization and dissolution, and it can also take care of spent liquors from the viscose plant. The pulp used for regenerated cellulose manufacture is delivered as wet pulp from the pulp mill, no drying is needed and a considerable amount of energy is saved. However, in an integrated production the viscose mill cannot use the whole production of cellulose from the kraft mill. One method of removing zinc ions from the coagulation bath effluents, based on precipitation of zinc sulphide via a well-controlled addition of green liquor from the pulp mill, has been studied.

Modern chemical technology is an efficient tool for solving problems, particularly within the complex environment of the pulp and paper industry, and the combination of experimental studies, mill data and mass balance calculations are of fundamental importance to the development of the industry. In this study various examples are presented, whereby chemical technology is of fundamental importance.

It is well documented that under normal conditions the molten salt mixture from the kraft recovery boiler flows down into the dissolving tank without problems. However, in the case of  alternatives to the kraft recovery boiler, knowledge of more precise data of the molten salts is required for the design calculations. In this study the viscosity for the case of sodium carbonate and 30 mole% sulphide has been measured and is of the magnitude 2 – 3 cP at temperatures relevant for a recovery boiler, i.e. similar to water at room temperature.

The presence of non-process elements (NPE) in a typical pulp mill has been investigated. The main input is with regards to the wood, and anticipated problems include; deposits in evaporators, high dead-load in liquor streams, plugging of the upper part of the recovery boiler and decreasing efficiency in the causticization department. Efficient green liquor clarification is of the greatest importance as an efficient kidney for many NPE. Mill data and calculations show that the magnesium added in the oxygen delignification does not form a closed loop.

Integration of a prehydrolysis kraft pulp mill producing dissolving pulp with a plant producing viscose textile fiber could be highly beneficial. The prehydrolysis liquor will contain both sugars and acetic acid. It is however not possible to fully replace the sulphuric acid of the viscose spinning bath with acetic acid of own production. The sulphuric chemicals from the viscose plant can be partly taken care of in the kraft recovery area as well as the viscose plant which can be supplied with alkali and sulphuric acid. Zinc-containing effluents from the viscose plant can be treated with green liquor to precipitate zinc sulphide.

Modern chemical technology is an extremely efficient tool for solving problems particularly in a complicated environment such as the pulp and paper industry. Here, examples are studied during which chemical technology is of fundamental importance.

At normal conditions the molten salt mixture from the kraft recovery boiler flows down into the dissolving tank without hindrance. However, for certain kraft recovery boiler alternatives, knowledge of more precise data of the molten salts is required. The viscosity for the case of sodium carbonate and 30 mole% sulphide has been measured and is of the magnitude 2 – 3 cP at relevant temperatures.

The main input of non-process elements (NPE) is down to the wood, and known problems include deposits in evaporators and decreasing efficiency in the causticization department. Green liquor clarification is an efficient kidney for many NPE. Magnesium added in the oxygen delignification does not form a closed loop.

Integration of a prehydrolysis kraft pulp mill producing dissolving pulp with a plant producing viscose textile fiber could be of significant interest, as the handling of both alkali and sulphuric compounds can be integrated. Problems will however arise as the capacity of the pulping line and the chemical recovery has to be adjusted.

The material balance of magnesium for one mill and some important data from two other mills show that the build-up of magnesium will not be a problem. This conclusion is supported by laboratory studies and thermodynamic calculations which explain the chemical basis for the behavior of magnesium in the mill system. The main input of magnesium are the wood chips and the magnesium inhibitor. The main outputs are the effluent from the conventional bleaching and the dregs and grits. The magnesium concentrationdoes not form a closed loop, and the white liquor contains only neglectible amounts of magnesium.

The distribution, material balances and operational implications of a number of foreign (non-process) chemical elements have been studied with special attentionto the effects of increased systems closure. The elements and compounds discussed include Potassium (K), calcium (ca), silicon (Si), aluminum (Al) and manganese (Mn). The results are based on detailed surveys of two kraft millsof different degrees of closure, on theoretical chemical equilibrium calculations, laboratory studies and material balance modelling.

It is concluded that the chemical elements may be grouped in two classes. One class, including Ca, Mg, P and Mn, may be almost completely removed fromthe liquor system by green and white liquor clarification, i.e. in dregs, lime mud and grits. The other class, including K, Cl, Al and  (with some reservations) Si, is not so removed and will tend to build-up in the liquor system with increased systems closure. Thus, these elements will call for special measures for reduced input, or specific output processes.

The effects of the non-process elements in some mill departments are discussed, mainly evaporator scalingand the effects on recovery furnace operation. Increased potassium (K) levels in the liquor system, for instance, are predicted to increase fly-ash flow and reduce sulfur emissions from the recovery furnace.