condition the ore and to control phosphate present in the ore. The slurry is pumped into pretreatment tanks where it is heated and held at atmospheric pressure to facilitate desilication of silica present in the ore by the caustic. The products of the desilication process pass out into the mud of the ore. The desilication also helps to reduce the buildup of scales in the pipes and tanks.
Fig. 2.5 A generalizedflowchart for the Bayer process
2.6 Bauxite Processing 63
After desilication, the slurry is pumped into the digester to extract the alumi- nates. The digestion is achieved by addition of steam and caustic soda to the slurry.
Conditions of digestion for the monohydrate bauxite (Diasporic) are different from conditions required for the digestion of the trihydrate bauxite (Gibbsitic). The digestion of the monohydrate bauxite occurs at higher temperatures and pressures of 250 °C and 3500 kPa, respectively. The trihydrate bauxite is usually digested at a temperature of 180 °C and a pressure of 500 kPa. Alumina in the bauxite forms a concentrated sodium aluminate solution leaving undissolved impurities, principally inert iron and titanium oxides, and silica compounds. The digester reactions occur as follows:
2NaOH + Al2O3.3H2O 2NaAlO2 + 4H2O
Caustic soda Gibbsite Sodium Water
aluminate
ð2:2Þ
2NaOH + Al2O3.H2O 2NaAlO2 + 2H2O
Caustic soda Diaspore Sodium Water
aluminate
ð2:3Þ
The digester vessels are sized to optimum holding time to extract a very high percentage of the alumina in the bauxite. About 30% of the bauxite mass remains in suspension after digestion. This is in the form of a thin red mud slurry of silicates and oxides of iron and titanium suspension. To separate the sodium aluminate formed from the rest of the slurry, the mixture is sent to a clarification unit. The unit is made up of three distinct steps. The first is a settler tank in which the liquor containing the aluminate is separated. Flocculants, for example, starch, are usually added to the settler feed stream to improve the rate of mud settling. The aluminate liquor overflows the tank and isfirstfiltered before sending to the precipitator. The residual mud is sent to washers where freshwater is added to recover the soda and aluminate content of the mud. To remove carbonate (Na2CO3), slaked lime is added to the dilute caustic liquor during the washing process. The carbonate forms by the reaction of caustic with compounds in bauxite and also from the atmosphere. The reaction reduces the ability of the liquor to dissolve alumina. The caustic soda is regenerated when lime reacts with the carbonate. The reaction produces calcium chloride, which, been insoluble, precipitates out and is removed with the waste mud. The reaction of lime and sodium carbonate proceeds as follows:
Na2CO3 + Ca(OH)2 CaCO3 + 2NaOH
Sodium Line Calcium Caustic soda
carbonate carbonate
ð2:4Þ
The liquor from the settler tank contains traces offine mud and is filtered in Kelly-type constant pressure filters using polypropylene filter cloth. Slaked lime slurry is used to produce afilter cake. Mud particles in the liquor are held onto the filter leaves for removal and treatment in the mud washer. With all solids removed,
64 2 The Extractive Metallurgy Industry
the pregnant liquor leaving thefilter area contains alumina in clear supersaturated solution. The alumina dissolved in the liquor is recovered by precipitation of crystals. The alumina precipitates as a trihydrate according to the following reac- tion:
2NaAlO2 + 4H2O Al2O3.3H2O + 2NaOH
Sodium Water Alumina Caustic soda
aluminate trihydrate
ð2:5Þ
The precipitation is achieved by charging and cooling the pregnant liquor in precipitation tanks. The process is facilitated by seeding the liquor with fine crystalline trihydrate. When the tanks are agitated, the alumina trihydrate crystal- lizes out of the solution in varied crystal sizes. The entry temperature of the liquor into the tank and the temperature gradient across the tanks, the seed rate and caustic concentration are manipulated to control the size of crystals formed. The formed crystals arefiltered from the liquor. The liquor is regenerated in evaporators and returned to the digester. The crystals are classified, and the finer crystals are returned to the precipitation tanks to serve as seed for the precipitation of incoming liquor. The coarser crystals are finally calcined to drive out unbound and bound moisture to form the alumina product. Alumina forms from alumina hydroxide according to the following reaction.
Al2O3.3H2O Al2O3 + 3H2O
Alumina trihydrate Alumina Water ð2:6Þ
Further Readings
Adams, M. D. (Ed.). (2005). Advances in gold ore processing (Vol. 15; 1st ed.). Elsevier Science.
Adams, M. D. (Ed.). (2016). Gold ore processing: Project development and operations (Vol. 1; 2nd ed.). Elsevier Science.
Administrativefiat of 1999 (letter no.AB.85/156/01), Government of Ghana.
Bermúdez-Lugo, O. (2016). The mineral industry of Ghana. U.S. Geological survey minerals yearbook—2013. USGS, 22.3 p.
Dunne, R. C., Kawatra, S. K., & Young, C. A. (Eds.). (2019). SME mineral processing &
extractive metallurgy handbook, vol. 1 & 2. Society for Mining, Metallurgy and Exploration (SME), USA.
Fuerstenau, M. C., & Han, K. N. (Eds.). (2003). Principles of mineral processing. Society for Mining, Metallurgy and Exploration, Inc. (SME), USA.
Gupta, A., & Yan, D. (2016). Mineral processing design and operations: An introduction (2nd ed.).
Elsevier.
Jain, R. (2015). Environmental impact of mining and mineral processing: Management, monitoring, and auditing strategies(1st ed.). Butterworth-Heinemann.
Marker, B. R., Petterson, M. G., McEvoy, F., & Stephenson, M. H. (Eds.). (2005). Sustainable mineral operations in the developing world. Geological Society of London (GSL).
Minerals Commission. (2007). Statistical overview of the mineral industry (2006 report).
Ramana Murty, V. V. (2014).Operational handbook of mineral processing(2nd ed.). Denette & Co.
2.6 Bauxite Processing 65
Ramkrishna Rao, G. S. (2014). Mineral processing techniques basics and related issues. Zorba Rao, D. V. S. (2017).Textbook on mineral processing. Scientific Publisher.
Republic of Ghana—The 1992 Fourth Republican Constitution of Ghana. Section 267(6).
Wills, B. A., & Napier Munn, T. J. (2006).Mineral processing technology: An introduction to the practical aspects of ore treatment and mineral recovery (In SI/Metric Units)(7th ed.). Elsevier Science & Technology Books.
66 2 The Extractive Metallurgy Industry
3
Textile and Fabric Manufacture
Abstract
Textiles have many uses in the household, industry and transportation industry.
They are used for making clothes, bags, carpeting, towels, upholstered furnishings, beds,filtration, conveyors, vehicle tyres, car seat covers, sails and many more. Fibers are classified into two groups, namely, naturalfibers that are harvested from plants (e.g. cotton, linen) or animals (e.g. wool, silk) or minerals (asbestos), and man-madefibers synthesized by chemical processes (e.g. chitin, viscose, glassfibers, ceramicfibers). Different methods are required to process natural and synthetic fibers into a yarn that could further be used to produce textiles. Unlike natural fibers, man-made fibers are made from polymers of natural origin produced from processing of natural materials (e.g. wood pulp), and those of synthetic origin obtained by organic polymerization reactions.
Extrusion or spinning process is used to convert the polymers tofibers. The two major routes of producing fabrics are knitting and weaving. This chapter covers in detail the raw materials for the production offiber, the different types offiber and the methods and technology used for the manufacture of textiles fromfiber.
The textile industry poses major risks to the environment. The treatment protocols required to mitigate the release of chemicals, dust particles, metals, etc.
into the environment have been considered in this chapter. An overview of the textile industry in Ghana is also presented.