Download Synthesis of copper oxy-chloride and iron oxide pigments using

 Synthesis of copper oxy‐chloride and iron oxide pigments using leachate from the bioleaching of a copper sulphide minerals flotation concentrate Carlos de Souza, Luis Sobral, Débora de Oliveira, Paula do Nascimento and Gabriel Peixoto Centre for Mineral Technology, Brazil ABSTRACT The hydrometallurgical approach is an appealing one when extracting copper from weathered ore and silicates, as well as sulphide mineral flotation concentrates and even low‐grade sulphide ores and residues of flotation process that are difficult to concentrate by conventional physical‐chemical methods. The hydrometallurgical approach consists of leaching the ground ore with the appropriate acids using, in some cases, high pressures in autoclaves or micro‐organisms for digesting the sulphide minerals in atmospheric pressure, even from more refractory crystalline structures such as chalcopyrite (CuFeS2). This bench scale experimental work is aimed at synthesizing copper oxy‐chloride (3Cu(OH)2*CuCl2) and iron oxide‐bearing pigments such as goethite (FeOOH) out of leachates from the bio‐assisted digestion of a copper sulphide minerals concentrate, composed, largely, of bornite (Cu5FeS4) and chalcopyrite (CuFeS2). The production of copper oxy‐chloride is justified since, apart from being intensively used in the chemical industry, it may also be employed as an active principle in copper‐bearing fungicide formulations, which are widely used in a variety of crops in leaf application. In the case of iron oxide pigments, these may be used in the production of ceramics and paint formulations due to their high surface coverage power, as well as for the adsorption of heavy metals from industrial effluents. – 1 – INTRODUCTION The goethite (α‐FeOOH) is one of the most important iron oxides in soils. It is found in a wide variety of climate and hydrological conditions. In lateritic soils, due to the relative accumulation of iron and aluminum, as well as in waste generated from changes of surface mineralized base metals sulphides, such as iron oxides, goethite (α‐FeOOH) appears as one of the most abundant mineral phases [1]. The goethite is also found in the aquatic environment, being a common constituent of the suspended and settled ones, joining, in a complex way, with other clay minerals, organic matter and even microorganisms [2]. Other elements present on the surface but in insufficient concentrations to form their own minerals, can associate with goethite through surface adsorption and/or incorporate in its structure [3]. The sorption of heavy metals by goethite is strongly influenced by their hydrolytic properties. Metals such as manganese, lead, nickel, cobalt and copper with high affinity for hydroxyl ions in the solution, also have high affinity for hydroxyl groups of goethite. With the increasing interest in iron oxides of a few nanometres to a variety of use, efforts are being made to find new methods for goethite synthesis and other forms of iron oxides in shape and size desired. The hydrolysis of Fe3+ ions, at room temperatures, produces a Fe3+ amorphous hydroxide. This amorphous compound, like all other amorphous precipitates, is thermodynamically unstable and can be gradually transformed into more crystalline structures. The time, temperature, and the pH, in particular, are the main parameters that drive the rate of transformation to crystalline phases [4]. The copper oxy‐chloride is a light‐green colour salt used in the manufacture of catalysts in organic chemistry, such as chlorination and/or oxidation, fungicides formulations, herbicides and insecticides, as well as other copper compounds. The use of pesticides, arguably, has contributed, a great deal, to increased agricultural production worldwide. However, the indiscriminate use of these compounds can lead to high concentrations of copper in the soil. Therefore, there are adverse effects on soil biota and plants [5]. The copper migration through contaminated soils poses a risk to groundwater quality. This is especially true for the acid soils, cultivated soils and soils affected by intense erosion and low sorption capacity [6]. Among many products used in the agriculture industry using copper salts on their formulations, alone or in combination, as active ingredient, such as copper sulphate (CuSO4), Cu‐oxy‐chloride (3Cu(OH)2*CuCl2), copper hydroxide (Cu(OH)2), cuprous oxide (Cu2O), copper hydroxy‐sulphate (CuSO4*3Cu(OH)2) and Bordeaux mixture (CuSO4+Ca(OH)2) [7]. In this scenario, the oxy‐chloride has an overwhelming preference due to its wide use in controlling the anthracnose, blight, rust, mildew and other diseases of diverse cultures among which cotton, potatoes, coffee, citrus, beans, tomatoes and mainly vegetables and fruits [8]. METHODOLOGY Bioleaching tests The chemical analysis, accomplished by atomic absorption spectrometry and x‐ray diffraction, revealed the contents of the copper flotation concentrate under study, as being: 32.2% Cu, 17.9% Fe and 22.0% S. The sample was composed by chalcopyrite, bornite, pyroxene, dolomite, magnetite, – 2 – apatite, talc, chlorite, biotite, and hornblende. The particle size distribution is showed on Table 1. Chalcopyrite (CuFeS2) and bornite (Cu5FeS4) was the only significant copper carrier, with respectively 70% and 30% of contained copper. Table 1 Copper concentrate particle size distribution Diameter Interval (mesh) Size (mm) Fraction weight retained (%) 150 0.106 10.7 200 0.074 23.0 325 0.043 27.6 < 325 < 0.043 38.7 A quartz mineral sample was used as support rock for being coated with flotation concentrate. In this case, such mineral was sent to a jaw crusher and further dry classified in Keason sieves in particle size range between +3 mm and – 6 mm. The copper and iron‐bearing leachate was obtained from a column bioleaching test work, with 4 metres high and 0.54 meters in diameter column made of glass fibber. The column was loaded, with 650 kg of quartz coated with 67.1 kg of flotation concentrate as a slurry made out of mixture of flotation concentrate and microbial consortia (mesophile microorganisms, moderate thermophiles and extreme thermophiles) in a microorganism density of 106 cells/g of flotation concentrate, being such mineral bed further irrigated at a flow rate of 22.1 L. m‐2.h‐1 , being such test run for 85 days. The solution used for irrigating the column content was made of sulphuric acid solution at a pH 1.8 containing mesophiles, moderate and extreme thermophiles microorganisms in charge of oxidizing iron and/or sulphur, besides sources the nutrients, such as nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg). The upward air blowing system assured the oxygen and carbon (CO2) supply to the reaction system. Additionally, an electric heating system ensured a better temperature control inside the column and leaching solution tank, as shown in the schematic version of Figure 1. The three consortia of sulphur and/or iron oxidising microorganisms were cultivated at different temperatures: mesophiles (at 30°C), moderate thermopiles (at 50°C) and extreme thermophiles (at 68°C), using MKM culture medium (Modified Kelly Medium). The pH was monitored/adjusted in the range of 1.2 to 1.8, which was done with 5M H2SO4 solution. As energy sources, 25 g/L of FeSO4*7H2O , 2.5 g/L of Sº, 5 g/L of pulverized pyrite and 10 g/L of chalcopyrite concentrate were used. For the cultivation of extreme thermophile microorganisms an additional 0.2 g/L of yeast extract was added. – 3 – Figure 1 Outline of the column bioleaching reaction system Experimental procedure Iron oxide pigment particles were synthesized using a leachate from the bio‐assisted leaching of copper flotation concentrate. This liquor contained 18.0 g/L of copper and 3.18 g/L of total iron, where 58.35% were in the form of Fe3+. The reaction system used for accomplishing the synthesis of iron oxide based pigment (FeOOH) is comprised, as shown in Figure 2, of a jacketed reactor with usable capacity of 4 litres, made out of borosilicate glass with temperature control and vertical mechanical variable stirring, and solution aeration device. The reactor is loaded with 2.6 litres of leaching solution, which is equivalent to two thirds of its capacity, followed by a slow addition of hydrogen peroxide (H2O2) as an oxidizing agent, equation 1, in order to obtain a Fe3+/Fe2+ ratio of 0.9, blowing 5 L/min of air and mechanically stirred at 1200 rpm. Then the solution pH was raised using 1 mol/L sodium hydroxide (NaOH) up to 3.5 so as to produce the hydrolyzed material, keeping the stirring and air supply, at 50°C for 24 hours. The solids obtained were filtered, washed with deionised water and dried in an oven. 2 Fe 2+ + H 2O2 + 2 H + → 2 Fe3+ + 2 H 2O – 4 – (1) Figure 2 Schematic version of the 3 L stirred reactor for goethite precipitation The filtrate pH was raised to 6.5 by adding 1 mol/L sodium hydroxide solution (NaOH), in the same stirred reactor, where copper hydroxide was precipitated [Cu(OH)2], and further separated from the aqueous phase containing, essentially, sodium sulphate (Na2SO4), according to equation 2. CuSO4 + 2 NaOH → Na2 SO4 + Cu(OH ) 2 (2) The copper hydroxide precipitate [Cu(OH) 2], after being washed out, was re‐solubilised using hydrochloric acid, and the solution pH was further raised up to 7.0, using NaOH solution, to precipitate the copper oxy‐chloride [3CuCl2.Cu(OH)2] (equations 3 and 4). Cu(OH ) 2 + 2 HCl → CuCl2 + 2 H 2O (3) 4CuCl2 + +2 NaOH → 3CuCl2 .Cu(OH ) 2 + 2 NaCl (4) Characterization techniques The x‐ray diffractogram (XRD) of the solid products were generated on a Bruker D4 Endeavor, on the following operating conditions: Co Kα radiation (35 mA kV/40); goniometer speed of 0.02 ° 2θ per step with time Count one second per step and collected 40 to 80° 2 θ. The qualitative interpretations of the spectra were done by comparison with standards contained in the database PDF02 (ICDD, 2006) Bruker Diffrac Plus software. Particle size analyses were performed in a Sedigraph 5100 equipment with software Micromeritic V1.02. The dispersion was prepared using 60 cm3 of sodium hexametaphosphate 0.05% w/v, and – 5 – then homogenized for 40 minutes on a magnetic stirrer and followed for 4 minutes in ultrasound (amplitude 30). The analytical conditions used were for high speed analysis. Analytical measurements The pH and redox potential (Eh) measurements were done directly in the reaction system with the micro‐processed Analion pHmetro AN2000 using a combined glass electrode and platinum electrode (vs. Ag°/AgCl), respectively. The iron ionic species concentrations (Fe3+ and Fe2+) were carried out by colorimetric method [9]. This method is based on the complexation reaction of ferrous ion (Fe2+) by orthophenanthroline (FenH+) with formation of an orange coloured complex (Fe(Fen)32+). The absorbance reading was accomplished at a wavelength of 510 nm (where it is observed the maximum absorption of the complex formed) using the Lamatte Smart Spectrum spectrophotometer. Copper concentration was analysed by atomic absorption spectrometry. RESULTS AND DISCUSSION Figure 3 shows the concentration of iron ionic species and copper over time. From the initial concentrations of species of Fe3+ and Fe2+ as being, respectively, 1.85 g/L and 1.32 g/L, an increase of the [Fe3+] / [Fe2+] ratio to 0.92 was observed after the first hour due to the addition of hydrogen peroxide (H2O2), oxidizing agent that is independent of the reaction system salinity, unlike the use of oxygen, which in this study was provided by air‐blowing into the solution. The addition of hydrogen peroxide has a direct influence on the redox potential of the reaction system due to the oxidation of ferrous ions to ferric ones, which can be evaluated by using the Nernst equation (equation 5), which relates the standard potential of a given oxidation‐
reduction reaction with changes in the concentrations of those ionic species involved. This equation can be written as follows: Figure 3 Concentration of copper and iron species in solution over time – 6 – (5) For this reason, the Eh values can be used to define whether the experimental conditions are suitable for keeping high the ferric ions concentration, as shown in the of thermodynamic equilibrium diagram for the Fe‐H2O system. In Figure 4, it is observed that the stability region of ferric ions is very limited, and dependent on high values of Eh in combination with low pH values [10]. Figure 4 Thermodynamic equilibrium diagram (Eh‐pH diagram) for the Fe‐H2O system at 25 ºC. HSC Software 7.0 (Outotec) [11] It is also observed a decrease of the copper concentration during the experiment, varying initially from 18.0 g/L to 17.6 g/L after 24h of test. This copper concentration reduction may suggest an isomorphous substitution between copper and iron, where, according to the literature, there is the incorporation of elements such as Cu, Pb, Cd, Co, Ni and Zn resulting in changes in the goethite crystal structure [12,13], on its crystals morphology and also on its solubility and the ability to incorporate new elements; however, the imbalance caused is compensated, stoichiometrically, with increasing water content, by hydroxyl ions insertion [2]. Regarding the precipitation of copper oxy‐chloride, the average copper content in the salt produced in this study was of 50.7%. However, in the market one can find different sort of copper content oxy‐chloride, varying from 30 to 50%, which, in fact, such content will depend upon the pH range used for neutralizing the previously produced copper chloride solution. X‐ray diffraction (XRD) analysis The X‐ray diffraction is the instrumental technique used to identify mineralogical species of goethite. The XRD pattern of the ‐37 μm goethite sample, commercialised by the leading national producer, is shown in Figure 4. After analysing the XRD pattern of Figure 4, the peaks are characteristic of goethite with high crystallinity. On the other hand, the analysis of the XRD pattern for the ‐37 μm fractions of the precipitate obtained out of the bioleaching of pyrite concentrate (Figure 5 and 6), show characteristic peaks of goethite of low crystallinity, as well as characteristic peaks of natro‐jarosite [NaFe3(SO4)2(OH)6], which means that the produced goethite was not enough washed. Their presence indicates a possible insufficient washing of the precipitate, since the – 7 – formation of hydrosulphate can be explained by the use of sodium hydroxide (NaOH) for raising the pH of the reaction system. Figure 5 X‐ray diffraction of commercial pigment Figure 6 X‐ray diffraction of synthetic pigment aged for 24h at 50ºC – 8 – Particle size analysis The results of particle size distribution of ‐37 μm fraction were obtained by sedigraphie. The size distribution curves, illustrated in Figure 7, indicate that 94% of the pigment particles, which means the synthesized iron oxide pigment, have particle size below 20 μm, below 62% of 5 mm and 42% are below 2 μm. While the commercialised pigment, by the leader industry in the Brazilian market, 98% of the particles has size below 20 μm, 94% below 5 μm and 92% below 2 μm. The technique uses the Sedigraph equipment based on Stokes’ law and determines the particle size by sedimentation. The particle size used as the standard for commercial use in the ink industry is 80% below 2 μm. Figure 7 Particle size distribution of synthetic and commercial pigment (fraction –37 μm) obtained by sedimentation technique CONCLUSIONS The iron oxide pigment obtained out of the leachate from the bioleaching of a copper sulphides flotation concentrate, consisting mainly of chalcopyrite (CuFeS2) and bornite (Cu5FeS4), is considered as a potentially attractive way for recovering iron an iron oxide pigment without using organic reagents, as those used in solvent extraction processes, despite of high selectivity for copper, also present considerable flammability and toxicity risks. The pigment obtained in this test work, in the above mentioned experimental conditions, did not show high crystallinity, compared with the one commercialised in the Brazilian market. Observing the particle size of the synthesized sample, it is possible to conclude that the synthesized pigment showed particle size distribution of 94% below 20 μm that is, with these characteristics is possible the use such pigment in the ceramic industry, in the building sector as part of a mixture for producing special bricks and cement or even as an adsorbent material. The copper oxy‐chloride, a widely used agriculture pesticide with broad spectrum for controlling of fungal diseases in several crops, mainly horticulture and fruit production, may be obtained easily from the remaining solution of the iron‐base pigment precipitation process by raising the pH of the copper chloride solution using sodium hydroxide. As previously mentioned, the copper content of – 9 – that salt produced in this study was of 50.7%, which is, to some extent, within the range of those commercialised in the market (from 30 to 50%). REFERENCES Cornell, R.M. & Schwertmann, U. (1996). The iron oxides: structure, properties, reactions, occurrence and uses. VCH Publishers Inc., New York, USA, 573 p. [1] Netto, M. Silvania. (2001). Caracterização cristaloquímica da incorporação de íons cobre (II) em goetita (α‐FeOOH) sintética, PhD. Thesis, Unicamp. 117 p. [2] Schwertmann, U. & Taylor, R.M. (1989). Iron oxides. In: Dixon, J.B. and Weed, S.B. (eds.) Minerals in soil environments (2nd ed.) Soil Sci. Soc. Am. Book Series no 1, Madison, WI, pp. 379‐438. [3] Mohapatra M., Rout K. & Anand S. (2009) Synthesis of Mg(II) doped goethite and its cation sorption behavior. Journal of Hazardous Materials 171, pp. 417‐423. [4] Michaud A.M., Bravin M.N., Galleguillos M. & Hinsinger P. (2007). Copper uptake and phytotoxicity as assessed in situ for durum wheat (Triticum turgidum durum L.) cultivated in Cu‐contaminated, former vineyard soils, Plant Soil 298, pp. 99‐111. [5] Novoa‐Muñoz, J.C., Queijeiro J.M.G., Blanco‐Ward D., Alvarez‐Olleros C., Martinez‐Cortizas A. & Garcia‐
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