PROSPECTS FOR A CHLORO-ALKALI INDUSTRY
BRIGHTEN IN THE ECOWAS
Ing. Dr. Francis Acquah
Institute of Industrial Research, CSIR, Accra, Ghana
Published in The Ghana Engineer, March 1998
Reprinted with GhIE permission by the African Technology Forum

ABSTRACT

Comparison of the process technologies for the electrolysis of brine showed superior economic characteristics of the membrane-cell process over the diaphragm or mercury cell process. The membrane cell process is recommended for the chloro-alkali plant. The plant has a potential to integrate with the manufacture of basic chemicals, petrochemicals and other products in West Africa. The proposed natural gas supply from Nigeria to Ghana brightens the prospects for an ECOWAS (Economic Community of West African States) regional integrated chemical complex plant. Ghana has a potential solar salt production base for siting a regional plant to produce caustic soda, chlorine and related products.

INTRODUCTION

The industrial development strategy pursued in the period 1960-1980 led to establishment of various industries which were import dependent for basic raw materials. Those industries were vulnerable to foreign exchange scarcity. As a result most of the industries operated at low capacity, particularly the chemical and allied process industries. The process industries need essentially linkages with local production capacities for the sustainable and appreciable level of plant capacity utilisation. Caustic soda and chlorine are major raw materials used by most process industries. However, there is no production of these basic chemicals in the ECOWAS and that retards development of the process industries in the region. This article seeks to peer at the prospects for the establishment of the chloro-alkali industry in Ghana by assessing the demands for caustic soda, chlorine and related products, the raw material base for electrolysis of brine as well as available process technologies.

THE CHLORO-ALKALI PLANT

The largely import substitution based chemical industry in Ghana produces paints, varnishes, drugs, soaps, deter­gents, cosmetics, rubber and plastics as well as insecticides, mosquito coils, candle, glue, industrial starch, etc. The industry is heavily import dependent for raw materials except for local inputs namely palm oil for soaps and natural rubber for rubber products. As a result of import constraints, the sector is characterised by low capacity utilisation with only 17% of firms operating between 69-79% installed capacity and 33% firms operating under 20% installed capacity. The sector is hard-hit by the recent trade liberalisation, having to compete with imported products both in quality and price.

In general the chemical industry is characterised by the use of complex process technologies, sophisticated marketing strategies and inter-industry competition. For the sector to remain competitive, it requires linkages with local produc­tion of basic chemicals. The chloro-alkali industry pro­vides the necessary linkage. The major requirement of the existing chemical and allied industries is the intermediate product - caustic soda.  The largest consumers of caustic soda are soaps and detergents factories, plastics, paints and pharmaceutical industries. Large quantities of caustic soda are used in the textiles, paper and metallurgical industries and in the production of synthetic fibres. For the period 1992-1996, Ghana imported about 36,395 tonnes of caustic soda costing $16,979,566 (Table 1). The high demand for caustic soda occupied the attention of in­vestors for the past three decades, however, investment in the chloro-alkali industry was constrained by lack of local market for chlorine, a major product of the chloro-alkali plant.

Table 1 Caustic Soda Imports [1]

(* total of solid caustic soda and soda lye or liquid soda.)

In the industrialised countries, it is largely the demand for chlorine for the synthesis of various organo-chloro compounds which has stimulated growth and expansion of the chloro-alkali industry.

Figure 1 shows the various industrial uses of chlorine. The other product of electrolysis of brine is hydrogen. It is used in the synthesis of ammonia, hydrogenation of oils production of margarine, production of hydrochloric acid and in the petroleum industry. The use of hydrogen depends on the capacity of the chloro-alkali plant. For example, a 40,000 ton/year ammonium factory could be integrated with a 300,000 ton chlorine per year plant.

Technology Assessment

Most investment proposals encounter the problem of choice of process technology for the electrolysis. This article compares the key parameters of the various tech­nologies applied in the chloro-alkali industry. Three different process technologies are applied in the electrolytic production of caustic soda and chlorine. The technologies are characterised by the type of electrolytic cell used, namely, the diaphragm cell, mercury cell and the membrane cell. In all the processes, saturated brine 300-315 g/L is fed into the electrolytic cells.

In the diaphragm cell process, the anolyte is separated from the catholoyte by asbestos diaphragm to prevent contact between chlorine produced at the anode and the caustic soda and hydrogen at the cathode. in the membrane cell process, cation exchange membranes are used to separate the anode and cathode compartment of the elec­trolytic cell. At the anode, chlorine ions discharge with evolution of chlorine gas. At the cathode, hydrogen is evolved and caustic soda is formed. The cation exchange is impermeable to the chloride ions and therefore the caustic soda produced contains only trace levels of sodium chloride. In mercury cell process, the liquid mercury cath­ode forms amalgam with sodium. The amalgam is re­moved from the cell and reacted with water in denuders to give caustic soda and hydrogen. Chlorine is liberated at the anode.

Factors which were considered in the choice of process technology include flexibility of scaling capacity utilisation and environmental impact. Other factors considered were cell materials, power consumption and current effi­ciency. Comparison of the key parameters of the three processes are shown in Table 2.

Table 2. Characteristics of the various process technologies.

Consumption of cell materials

The various cells materials are specified in Table 2. The mercury cell does not use a diaphragm or membrane. The ion exchange membranes have longer life (over one year) than asbestos diaphragms. The consumption of asbestos diaphragm is about 800g/ton NaOH. In the mercury cell, mercury cathode is consumed at the rate of 0.15-0.2 kg per ton of caustic soda. Both diaphragm and mercury cells use graphite electrodes, which are consumed at a rate of about 8 kg/ton NaOH for diaphragm cell and 4 kg/ton NaOH for the mercury cell. The consumption of purified brine, 1.6 ton/ton NaOH is about the same for all three processes.

Electric Energy Consumption

The consumption of electric energy is an important param­eter of the electrolytic process. It is determined by the equation:

                W = (1/K) * (E/n) * 100                          (1)

where
W = consumption of electric energy (direct current) per Kg of product, Kw-h/Kg.
K = electrochemical equivalent of the product, g/a-hr
E = cell voltage, volt
n = current efficiency, %.

The cell voltage, E which determines the energy efficiency of electrolysis, is the sum of the decomposition voltage of NaCl, E₀, anode and cathode overvoltage, voltage drops at the electrolyte interphase E_el, at the anode, E_a, at the cathode, E_c and at the electrical contacts, E_{el contacts}:

E = E₀ + del(phi_a) + del(phi_c) + del(E_el) + del(E_a) + del (E_c) + del(E_{el contacts})                (2)

The theoretical decomposition voltage of sodium chloride E₀, 2.3 volts is the same for the diaphragm and membrane cells. The mercury cell his a higher decomposition voltage, 3.17 volts and therefore from equation 1 the energy requirement is more than that of the diaphragm or membrane cells. Over voltage of chlorine and hydrogen evolution at 80⁰C in saturated brine on the electrodes used by various cells are lower on the titanium lined steel, platinised titanium or metal-metal oxide electrodes (del(phi_a)=26 mV; del(phi_c)=45 mV for current  density- 1000 a/m²) than on carbon and steel electrodes (del(phi_a)=251 mV; del(phi_c)=180 mV for current density - 1000 a/m2). The cell voltage for membrane cell is therefore lower which accounts for the lower power consumption according to equation 1.

Quality of caustic soda

The greatest advantage of the mercury cell is the superior quality of caustic soda produced, 50% solution with trace levels of sodium chloride, 0.002-0.003%. The amalgam caustic is widely used in the rayon industry which requires high purity caustic soda without chlorides. In contrast, the diaphragm cell produces weak 10% caustic soda solution containing 15% sodium chloride. The weak solution is concentrated to 50% solution with 1% sodium chloride in a triple effect evaporation. The membrane cell produces higher concentration, 30-35% caustic soda containing trace levels of 0.001-0.002% sodium chloride. The caus­tic soda is evaporated to 50% solution but at a lower cost of steam usage.

Environmental Impact

Mercury cells have the greatest disadvantage posed by hazardous vapours of mercury at cell temperature, 70-80⁰C. Production floor area for the mercury cell process is about twice the area for the diaphragm cell process and even  larger than for the membrane cell process. Ade­quate ventilation is required to ensure that mercury vapours do not exceed 0.1 g/day/l 000 Kg of product in the working environment [2]. The electrolytic processes re­quire control of chlorine emission to the atmosphere and water bodies. Sources of pollution include the vent gases from chlorine liquefaction, storage and haulage. In the United Kingdom, Health and safety at work etc., Act 1974 permits, with a suitable chimney height, 0.023g/m³ chlo­rine discharge. Total residual chlorine discharge into navi­gable waters in USA is maximum 0.0032 Kg/ 1000 Kg of product for mercury cell process and 0.0l3 Kg/1000 Kg of product for diaphragm or membrane cell process, [2].

Choice of process technology

The membrane cell process, compared with diaphragm and the mercury cell processes, has superior economic characteris­tics. The membrane cell process operates in modules. It therefore provides for easy reduction or expansion of capacity to the demand for caustic soda and chlorine. It has low power consumption as a result of low voltage drops at the electrodes and membranes, closer electrode spacing, lower chlorine and hydrogen over voltages. The membrane cell process produces high purity caustic soda. The cell design does not use hazardous materials such as mercury or asbestos. Operating in modules and combining superior economic characteristics, the membrane cell process is considered most appro­priate choice for the electrolysis of brine.

PROSPECTS FOR CHLORO-ALKALI INDUSTRY IN GHANA

Solar salt production in Ghana is concentrated in the Central and Greater Accra Regions where climatic condi­tions, humidity and amount of rainfall are most favourable for salt winning. The total installed capacity of solar evaporated salt is about 1.2 million metric tonnes per annum. Table 3 shows the production of common salt in Ghana for the period 1993-1996.

Table 3. Total Production of Common Salt in Ghana, [1].

In 1994 and 1995, Ghana exported 72,151.45 tonnes and 69,350.55 tonnes of common salt respectively [3]. In the ECOWAS, not many countries have the potential for salt winning. Senegal and Ghana have the largest capacity for salt production in the region. Countries in the region including Mali, Nigeria, Burkina Faso, Cote d'Ivoire, Togo and Niger imported over 1 million tonnes of salt in the past decade. Imports of common salt by Cote d'Ivoire for the period 1982-1985 averaged 45,000 tonnes [4]. Ghana has unique position to exploit the opportunities. Export trade in common salt could combine with the production of caustic soda, chlorine, hydrochloric acid, bleaching powder and other chemicals to meet the demand for basic chemicals in the ECOWAS countries. Foreign trade statistics for Africa [5] show that between 1982 and 1991, ECOWAS countries imported $16.8 billion of chemicals and related products for the chemical and allied process industries.

In Ghana, the soap industries are the major importers of caustic soda. Table 4 shows local production and imports of soap for the period 1992-1996.

Table 4. Local Production and Imports of Soap, tonnes [1]

The steady increase of soap production and decline in imports of soap are very significant. The use of local palm oil in place of imported tallow and improved assess to enterprise funds for working capital and plant rehabilita­tion have enhanced the growth of soap manufacturing in Ghana. The major constraint in production is the imports of caustic soda. For the period 1992-1996, about $17 million of caustic soda was imported by the process indus­tries. Imports of hydrochloric acid have also increased significantly. In 1981 the import of hydrochloric acid was only 109 tonnes. In 1996 the country imported 2,194.74 tons of hydrochloric acid costing over one million dollars (table 5). The gold mining boom and future gold refineries will create more opportunities for basic chemicals produc­tion in Ghana.

Table 5. Imports of hydrochloric acid [1].

Another product which is based on chlorine is bleaching powder. It is used extensively in the textiles and paper industries, water treatment, laundries and general sanita­tion. Ghana Water and Sewerage Corporation imports over 300 tons chlorine gas and 800 tons of bleaching powder annually.

The forgoing import data on Ghana depict a demand volume too small for a chloro-alkali plant. There is the need for linkages with the prospective demands in the ECOWAS. The membrane cell process could however facilitate operation of a small plant with design capacity based on the current demand for chlorine and chlorine based products.

In the ECOWAS, Nigeria imported 112,443 tonnes of caustic soda costing $55,172 million in the period 1984-1987. The caustic soda imports of Senegal were also significant, on the average $4.2 million annually. Cote d'Ivoire, for instance, spent over $1.2 billion on imports of various chemicals and related products during the period 1986-1989 representing about 8% of ECOWAS imports. Thus, the process industries in the region have considerable capacity for the utilization of basic chemicals.

A large potential exists for the utilization of chlorine in the manufacture of ethylene dichloride, vinyl chloride, polyvinyl chloride, (PVC), hexachloro benzene, dichloro benzene, phenol, etc. in the ECOWAS. For example, Ghana has 40 plastic industries with a total installed capacity of 26,000 tonnes. The plastic industries import annually on the average 15,000 tonnes of resins for the manufacture of various thermoplastic products (plastic water tanks, plastic crates, PVC pipes, polyethylene bags, etc.) [6]. The plastic industry has grown significantly in the past decade with increased demand for plastic packaging materials particu­larly for the food and beverage industries. Production of thermoplastics promises to be the main consumer of chlorine of the chloro-alkali plant. A chloro-alkali plant, capac­ity 120,000 tonnes caustic soda/year is estimated for the ECOWAS region.

CONCLUSION

The cation-exchange membrane cell process offers lower production cost and flexibility in adjusting plant capacity according to demand for caustic soda or chlorine. This article underscores the potential opportunities for invest­ments in integrating the chloro-alkali and petroleum indus­tries. The ventures will boost the economies of ECOWAS through increased industrial capacity utilisation, diversifi­cation of production, acquisition of new industrial capaci­ties and employment generation. The supply of natural gas from Nigeria to Ghana gives brighter prospects for an integrated chemical complex plant. Ghana's natural gas deposits could also further enhance the development of the chemical industry. High demand for basic chemicals in the ECOWAS justifies reappraisal of feasibility studies con­ducted in the past two decades for the establishment of a chloro-alkali and related chemical industries in Ghana.

REFERENCES

1.  Ministry of Trade and Industry, statistical data, 1997

2.  Compendium of environmental guidelines and stan­dards for industrial discharges., W.H.O., Geneva, 1983.

3.  Ghana Export Performance, 1995 Non-traditional Products. Published by General Information Divi­sion, Ghana Export Promotion Council

4.  International Trade Statistics Yearbook, United Na­tions Publication. ST/ESA/SerG/37, 1990 & 1992.

5.  Foreign Trade Statistics for Africa, United Nations, E/ECA/STAT/SerC/l 1 & 12, 1992 & 1993

6.  Acquah F., Mensah, B., Ampofo, K., Obiri, H., Amoah, J., Kwaa, E. & Quashie, A. "Pre-feasibility study on the establishment of a plastic waste recycling plant in Ghana.", Technical Report, IRI, CSIR, October, 1996.

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