Journal of Cleaner Production 32 (2012) 149e156
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Journal of Cleaner Production
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Cleaner production of soapstone in the Ouro Preto region of Brazil: a case study
M.L.M. Rodrigues, R.M.F. Lima*
Department of Mining Engineering, Federal University of Ouro Preto, Ouro Preto, MG, Brazil article info abstract
Article history: Soapstone is a metamorphic rock that is used for ornamental and construction purposes. Rock recovery Received 1 October 2011 during the exploitation process is low. Quarries in the state of Minas Gerais in Brazil sell small and low- Received in revised form quality blocks to artisans, who produce pans and souvenirs. This activity constitutes the main economic 23 March 2012 activity in several places within the Ouro Preto region, such as Santa Rita. Soapstone artisan workshops Accepted 25 March 2012 produce a large quantity of powder (approximately 10e15% rock recovery), which is discarded carelessly, Available online 3 April 2012 often causing environmental problems. The objective of this study was to characterise and purify soapstone powder from the workshop of an artisan who works exclusively with rocks from Bandeiras- Keywords: fi Soapstone residues Santa Rita de Ouro Preto in order to identify potential applications for the residues and puri ed prod- Talc ucts. First, the mineralogical composition was determined by X-ray diffractometry, the size distribution Industrial mineral was determined by sieving and the chemical composition of the residue was determined by inductively Residue characterisation coupled plasmaeoptical emission spectroscopy (ICP-OES). Next, purification tests were performed by Residue recovery magnetic separation for fraction sizes larger than 74 mm. Fraction sizes smaller than 74 mm were purified Flotation by flotation and leaching with hydrochloric acid. This research shows that it is possible to recover almost Magnetic separation all residues of both small and large size distributions for use as insecticide filler. In addition, it is possible Leaching process to recover 100% and 96% of fraction sizes larger and smaller than 74 mm, respectively, for use in the paper industry (wallpaper and packaging). For more valuable applications, it is necessary to leach the purified material from flotation using hydrochloric acid. In addition to decreasing the environmental impact of soapstone powder discharge, it is possible for soapstone artisans to sell this powder for a profit, whether it is purified or not. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction cosmetics, pharmaceutics, insecticide, explosives, soap, candles, textiles and cereal polishing (rice, corn and barley). For each Soapstone is a metamorphic rock that comes from primitive application, the talc must have certain properties and characteris- sedimentary limestone rocks or from basic/ultrabasic igneous rocks tics in terms of size distribution, morphology, mineralogy, purity, by hydrothermal and metasomatic alteration of magnesium sili- and other factors (Roe, 1975; Pontes and Almeida, 2008; cate. The rock texture can be powder, massive or compact and Pogrebenkove et al., 1997 and Nkoumbou et al., 2008). Soapstone lamellar (Lara Filho, 1997). To be classified as soapstone, the rock is often used as building stone and dimension stone. Fig. 1 proportion of the mineral talc in the rock must be above 35%, and presents the main Brazilian deposits of talc, including soapstone, either the proportion of the minerals chlorite, quartz, calcite, which occurs in the states of Minas Gerais (MG), Goiás (GO) and dolomite, magnesite, tremolite, amphibole and hematite must be Bahia (BA). above 25% (Kuzvart, 1984; Pinheiro, 1973). Talc and soapstone are In the Ouro Preto region of Brazil, located in Minas Gerais State, often used synonymously (Skinner et al., 1997). there are several soapstone quarries that exploit this rock as Talc (Mg3(SiO4)10)(OH)2) is a magnesium silicate hydrate made dimension stone. These places have rich deposits of talc that are up of 31.7% MgO, 63.5% SiO2 and 4.8% H2O(Skinner et al., 1997). It is extracted as by-products and sold to ceramic industries. The rough a filler-type industrial mineral that is used in many products, blocks are exported to European and North American countries, including paint, paper, ceramics, plastic, roofing products, rubber, where they are cut into building stones. Currently, the annual production of soapstone in Bandeiras at Ouro Preto region is 4800 tons of soapstone and 2400 tons of talc. * Corresponding author. Departamento de Engenharia de Minas, Universidade During the exploitation of dimension stones, such as soapstone, Federal de Ouro Preto, Campus Universitário, Morro do Cruzeiro, S/N, CEP: 35400- 000, Ouro Preto, MG, Brasil. Tel.: þ55 31 3559 1590; fax: þ55 31 3559 1593. only 30% of the rock is recovered. The remaining 70% is composed E-mail address: [email protected] (R.M.F. Lima). of small blocks that are used in artisan workshops, where pans and
0959-6526/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2012.03.028 150 M.L.M. Rodrigues, R.M.F. Lima / Journal of Cleaner Production 32 (2012) 149e156 souvenirs are produced. In the production of pans, for example, the 2007). The morphology of the particles of purified talc was lamellar, recovery of rock is approximately 10 wt% (Santos, 2009), which as shown in Fig. 3. means that the global recovery of soapstone exploited in quarries is This paper presents the characteristics and the main results of only 40 wt%. Approximately 60 wt% of the rock is discharged as the purification tests carried out on the powder residues from an residue. Fig. 2 shows the main activities performed in a typical artisan workshop in Ouro Preto (Fig. 2(G)), the objective being to soapstone quarry in Santa Rita de Ouro Preto and the products that decrease the residues produced in this activity in order to find are sold to local artisans. Details of the production processes for a classical application of this industrial mineral, as applications souvenirs and pans are presented. A saw is applied to cut the rock suggested by Driussi and Jansz (2006). and after a wood lathe is used to make and polish the produced objects. The large quantity of soapstone powder discharged from 2. Materials and methods artisan workshops is also shown. Lima et al. (2009) performed a technological characterisation of Representative soapstone residues were collected in the work- soapstone samples (Fig. 2(D)) used in artisan workshops from five shop of an artisan who works exclusively with rocks originating different soapstone quarries in the Bandeiras region of Santa Rita e from the Bandeiras region in Santa Rita de Ouro Preto. First, the Ouro Preto. The minerals were identified using optical microscopy, powder residue was homogenised and quartered to obtain X-ray diffraction and scanning Electron Microscopy with an subsamples to determine the size distribution of the particles, as Energy Dispersive System (SEM/EDS). The following minerals well as to perform mineralogical and chemical characterisation. were found: silicates [talc (Mg3(Si4O10)(OH)2), chlinoclore ((Mg, The size distribution of the material was determined by wet Fe)(Si,Al)4O10(OH)8, phlogopite (KMg3(Si3Al)O10(F,OH)2), tremolite/ sieving, and the fraction size 74 mm was determined by laser þþ actinolite (Ca2Mg5(Si8O22)(OH)2/Ca2(Mg,Fe )5Si8O22(OH)2) and diffraction (Cilas 1064). Then, chemical analyses of size fractions orthoclase (K(AlSi3O8)], carbonates [magnesite (MgCO3) and dolo- greater and lower than 74 mm were carried out using an Inductively mite (CaMg(CO3)2)], sulphides [pyrite (FeS2), arsenopyrite (FeAsS) Coupled Plasma e Optical Emission Spectrometer (Spectro model and pentlandite ((Fe,Ni)9S8)], oxides [magnetite(Fe3O4)/hematite Ciros/CCD), except for SiO2 content, which was determined by the (Fe2O3) and ilmenite (FeTiO3)]. The apparent porosities of the difference between 100% and the sum of all chemical element/ soapstone rock samples were between 0.3 and 0.4%, and the compound contents plus the loss on ignition (LOI). apparent water absorption was approximately 0.11%. The Rockwell The mineral composition of soapstone powder in both fraction hardness of the samples was of a high range (73e84). This size groups was performed using the X-ray diffraction (XRD) total variability was attributed to the mineralogical heterogeneity on powder method. The radiation used was Cu Ka. The goniometer the faces analysed. The uniaxial compressive strength of the rock velocity was 1.2 /min, the counting time was of 58 min and data samples was between 20 and 25 MPa, and rupture occurred on were collected from 2 to 70 . A Rigaku model 3550 diffractometer the first or second cycles of the thermal tests. was used. Mineral identification was performed using the pattern The density of soapstone powder collected from the artisan diffraction tool of the JADE 7.0 software package. workshop in the Bandeiras region varied from 2.73 to 2.83 The purification of soapstone powder was realised using (Rodrigues, 2010). The value of oil absorption of the purified magnetic separation (Carpco Inc. model WHIMS 3X4L e serial 210- product was 28.5 (g/100 g). This value is within the specification for 97) and froth flotation (Cimaq cell). Leaching tests were performed pigments and extenders (Rodrigues and Lima, 2011; Casanova et al., with the concentrate of an optimised flotation test using hydro- chloric acid (HCl). In the magnetic concentration (fraction size above 74 mm), the influence of the variable magnetic field intensity (3000; 4300 and 4900 G) on the mass recovery and whiteness of purified talc was studied. These experiments were replicated for each condition. The experimental procedure for the wet magnetic concentration tests consisted of the following steps. First, the magnetic field intensity was fixed to the desired value. Then, with the water tap open (flow rate of 670 mL min 1), a 100 g sample of dry soapstone powder was fed into the equipment. When it was observed that no particles were dropping into the non-magnetic bucket product, a new bucket was substituted in. Then, the equipment was turned off and the magnetic product was collected in the new bucket. Finally, both the magnetic and non-magnetic products were filtered, dried and weighed to calculate mass recovery. Finally, the products (magnetic and non-magnetic) were sampled to obtain subsamples for the determination of whiteness, CIE (L*a*b*) indices and chemical composition. Purification of the soapstone powder (fraction size e 74 mm) was performed by flotation. Minitab 15 software was used to create and analyse the results of a fractional planning design (1/8) of eight factors (variables) on variable responses (mass recovery and whiteness). For each variable, two levels (minimum and maximum) were tested, as presented in Table 1. This analysis was performed on a total of 32 flotation tests. The following experimental procedure was used for the flotation tests. A mass of soapstone powder ( 74 mm) and the water quan- Fig. 1. Location of the talc deposits, including soapstone, in Brazil (CE e Ceará; PI e Piauí; BA e Bahia; GO e Goiás; MG e Minas Gerais; SP e São Paulo; PR e Paraná; RS e tities necessary to obtain the pulp density desired (10 or 20%) were Rio Grande do Sul) (modified from Lara Filho, 1997). put into a 2 L cell cube. The cell speed was then adjusted (800 or M.L.M. Rodrigues, R.M.F. Lima / Journal of Cleaner Production 32 (2012) 149e156 151
Fig. 2. Typical operations in a soapstone quarry, the commercial products and the souvenir production process in artisan workshops in the Ouro Preto region: (A) Detail of the exploitation operation in a soapstone quarry of Santa Rita de Ouro Preto, (B) A rough block of soapstone, (C) Transport of the rough block to be exported, (D) Small pieces of soapstone to be used in artisan workshops, (E) Saw applied to cut the objects produced in artisan workshops, (F) An artisan performing his work and (G) Soapstone powder.