INTEGRATION OF A DISSOLVING PULP MILL AND A CELLULOSE BASED TEXTILE FIBER PLANT

Hans Magnusson, Niklas Kvarnlöf and Ulf Germgård

Department of Engineering and Chemical Sciences, Karlstad University, SE 65188 Karlstad, Sweden Email: [email protected]

ABSTRACT

Textile fibers based on cellulose can soon become a very important product for the pulp and paper industry and it is believed that it would be beneficial to locate such a textile fiber production unit at the same site as the pulp mill. By an integration of pulp production and textile fiber preparation, products from the pulp mill could then easily be transported to the textile fiber plant and some of the by-products from the textile fiber plant could be recycled to the pulp mill. The integrated two processes could also share for example the fresh water supply and the effluent treatment plant. The paper discusses different alternatives and their pros and cons.

I. BACKGROUND It is today well-known that environmental protection activities will become very critical for the survival of mankind: the greenhouse effect, emissions of harmful chemicals and the shortage of land and water for food production are examples of the problem areas. However, only one activity cannot solve all problems but each step that is taken in the right direction is beneficial. Research into the development of new technologies can give us environmental benign textile fibers that can replace oil based fibers like polyester and nylon but also cotton due to its problems when growing in the fields. The future textile fibers will be based on cellulose from trees, the target is to produce a similar textile fiber quality as today and at a competitive prize. Preliminary studies have indicated that this goal is not too far away.

Textile production The global textile fiber consumption will increase dramatically in the future and some experts have predicted that the demand in 2050 will be three times the current level [1]. Thus, new types of textile fibers based on cellulose are needed and preferentially should the fiber production be integrated with a dissolving pulp mill. It is also important to know how the chemicals added to the combined production will react and how the chemicals can be regenerated and effluent streams can be taken care of. During the last decades an increasing interest has been concentrated towards the development of new environmental acceptable cellulose solvents which could be used in an economical feasible production process for textile fibers. Different solvent systems for the dissolution of cellulose have been listed and investigated by several researchers [2, 3, 4] but the main problem is that the chemicals used are often toxic, harmful to the environment, expensive, requiring complicated process conditions or a combination of these factors.

It has been shown that cellulose is soluble in aqueous sodium hydroxide in a small window in the ternary cellulose-sodium hydroxide –water phase diagram [5]. This area is somewhat enlarged with for instance urea, thiourea [6] and some other compounds, like zinc oxide or combinations of these. [7]. No systematic studies of the impact on the solubility window in the face diagram cellulose – sodium hydroxide - water seems to have been done.

A process for production of cellulose carbamate was patented by Hill & Jacobson in 1938 [8]. The process is based on impregnation of the cellulose with an urea – alkali solution and treatment at elevated temperature (above 130 °C) and afterwards dissolution in sodium hydroxide.

An alternative way to produce cellulose carbamate [6], includes impregnation of the dissolving pulp (or other suitable cellulose material) with sodium hydroxide/urea at a low temperature (around – 8 °C) and then heating it to a slightly higher temperature. The produced cellulose carbamate solution can be used to produce both films and fibers of cellulose. The chemicals involved in the production of cellulose carbamate are water, urea and sodium hydroxide. Depending on which process is used, also thiourea and/or zinc compounds can be used. For the spinning of fibers a conventional coagulation bath of sulfuric acid and sodium sulfate is used [9].

II. METHODS A simplified flowsheet for the integrated pulp/fiber plants was used as basis for an EXCEL model that calculates the relevant mass balances for different elements and for water. Data for the model was mainly taken from the literature [10, 11].

III. RESULTS AND DISCUSSION The size of a production line for conventional viscose fibers is today of the magnitude 50000 t/year and larger plants consists of more than one line [12]. Therefore 50000 t/year was chosen as the capacity of a theoretical textile plant in our calculation.

One obvious result is the importance of high cellulose content in the spinning dope. In the viscose process a typical amount is 10 % cellulose [11]. For NaOH/ZnO it is reported a cellulose content of the magnitude 3 – 6 % [10], the content is limited by the formation of gel particles if higher cellulose content is used. An increase from 6 % to for example 12 % cellulose corresponds to a decrease of the water volume of 15 to 7 m3/ton fiber which would be very interesting from en economical point of view.

NaOH for pretreatment and mercerization can be supplied from a kraft pulp mill and it can be evaporated to get the specified strength. A better alternative is to use this input as the input to the combined pulp and textile plant system and to use the overflow/effluent from the textile plant as sodium make-up for the pulp mill.

A greenfield mill can be built according to the principles outlined here, but it is important to design the systems in such a way that the two plants also can operate independent of each other. To add a textile fiber plant to an existing pulp mill and convert it to production of dissolving pulp is a challenge, and detailed knowledge of both the pulping and the textile fiber processes are necessary.

The most common spinning bath composition includes sulfuric acid and sodium sulfate as well as additives like ZnSO4 improving the coagulation and increasing the strength of the fibers [13]. It is important to remember that the strength of the fibers, wet or dry is not the same as the strength of the spin yarn or the finished textile.

Zinc compounds are used both in the dissolution of cellulose and in the coagulation baths. For protection of the environment it is important to minimize the zinc content in the effluents from the combined pulp and textile fiber plant. In this case is the chemistry favorable, as zinc sulfide has a very low solubility under alkaline conditions. Green liquor from the pulp mill can be used to selectively precipitate zinc sulfide and thus to make it possible to remove this precipitate and send concentrated zinc slurry to recycling and reuse elsewhere.

Such a combined pulp mill/textile plant could be very interesting for many pulp mills in Sweden which today need to change their pulp/paper processes to other products with better future potential.

Figure 1. A pulp mill integrated with a textile fiber plant. EPC, Electrostatic Precipitator Catch, a process to remove potassium and chloride from the recycled electrostatic precipitator dust.

Figur 2 The principal stages in a spinning plant

IV. CONCLUSIONS There are substantial benefits from an integration of a pulp mill and a textile fiber plant which is studied in an on-going project at Karlstad University. The intention is to present an environmental benign combined process with a favorable economy. Detailed mass and energy balances will show the necessary data for dimensioning of the production lines. Research in the areas improved dissolution processes and spinning conditions may change today’s limitations and give new opportunities for the production of textile fibers of sufficient strength at a viable cost.

V. ACKNOWLEDGEMENTS Thanks are due to the Bo Rydin Foundation for the economical support of Hans Magnusson´s travel cost to EWLP 2014, Seville, Spain.

VI. REFERENCES

[1] Johnson, Tim F.N. Current and future market trends, p 273 – 289 In Woodings, C. Regenerated Cellulose Fibres, The Textile Institute, 2001. [2] Turbak, A. Other Processes. p 174 – 198 In Woodings, C. Regenerated Cellulose Fibres, The Textile Institute, 2001. [3] Wang, Y. Cellulose fiber dissolution in sodium hydroxide solution at low temperature: Dissolution kinetics and solubility improvement, Diss. Georgia Inst of Technology, 2008. [4] Heinze, T.; Koschella, A. Solvents applied in the field of cellulose chemistry – A mini review. Polimeros 2005, 15, 84-90 [5] Sobue, H.; Kiessing, H.; Hess, K. The cellulose-sodium hydroxide –water system as a function of the temperature. Physik Chem B 1939, 43, 309–328. [6] Cai, J, et al. Novel fibers prepared from cellulose in NaOH/urea aqueous solution. Macromolecular Rapid Communications 2004, 25, 1558 – 1562. [7] Zhang, S. et al. Dissolution behavior and solubility of cellulose in NaOH complex solution. Carbohydrate Polymers 2010, 81, 668-674. [8] Hill, J.W.; Jacobson, R.A. Chemical process. U.S.Patent 2 134 825, 1938. [9] Mao, Y. et al. Effects of coagulation conditions on properties of multifilament fibers based on dissolution of cellulose in NaOH/urea aqueous solution. Ind. Eng.Chem.Res. 2008, 47, 8676-8683. [10] Kihlman, M., et al. Effect of various pulp properties on the solubility of cellulose in sodium hydroxide solutions. Holzforschung, 2012, 5, 601-606. [11] Söderlund, C-A. Personal communication. 2013. [12] Wright, T. Sateri starts up viscose staple fiber plant in China. Nonwovens Industry Magazine 2014-01-06. www.nonwovens-industry.com/contents/view_breaking-news. [13] Wilkes, A.G. The viscose process, p 37-61. In Woodings, C. Regenerated Cellulose Fibres, The Textile Institute, 2001.