KINETIC INVESTIGATIONS OF OXIDATIVE ROASTING AND AFTERWARDS LEACHING OF COPPER-LEAD MATTE

In this paper, results of copper – lead matte investigations are presented. Investigated copper-lead matte is intermediate product of lead production in TREPCA-Zvecan. In the first part of the paper characterization of starting material is presented, consisting of: chemical composition analysis (XRQ), sceaning electron microscopy (SEM) and diffractometry (XRD). Thermal properties of matte investigated were determined using differential thermal analysis (DTA) at characteristic temperatures. Using results of induced analysis, mechanism of matte oxidation process was determined. In second part of the paper kinetic parameters describing oxidative roasting and afterwards leaching in sulfuric acid of copper-lead mate are presented.


Introduction
Besides metallurgical slags, sulphide mattes present the other, also # Corresponding author: dminic65@ptt.yuimportant material obtained as the result of primary and secondary smelting.Mattes can be defined as high-temperature solid solutions of metal sulphides that are mach denser then the slags [2].In the smelting operations mattes are used for collection of basic noble metals (smelting of Cu and Ni) or for collecting of sulphur and the impurities in sulphide phase (smelting of Pb) [2].In lead metallurgy, only mattes rich in copper can be recycled for copper utilization in the operation of secondary materials smelting [3].Otherwise, mattes are regarded as waste material and are discarded.From ecological aspect mattes obtained in lead smelting are defined as ″toxic waste″ No.10 04 00 according to European catalog of waste (comity decision No 94/3EC) [4].
Knowledge of metal distribution and phase compositions of metallic phases in mattes is of special interest for determination of possibility of their further processing or safe disposal.In that sense information about mechanism and kinetics parameters describing process of mattes utilization treatments are important.
Great percent of lead and copper in, lead production intermediate, copperlead matte leads to the fact that number of authors [5][6][7][8][9][10] investigated problems concerning valorization of the valuables from that material.This is why number of procedures describing copper-lead mattes processing are present.Most of those are pyrometallurgical procedures [6].Small extend of papers are concerned with hydrometallurgical treatment of mattes [7][8][9].Heaving in mind that hydrometallurgical procedure is maybe unjustly ignored, in this paper leaching of copper-lead mattes, after oxidative roasting, was investigated.Copper-lead mate is mixture of sulfides [11], mostly PbS and Cu 2 S. Berg [12] in his monograph did publish results of the Pb-Cu-S system investigations.Structure of lead sulfides was subject of interest for Hansen [13], while phase diagram of the system PbS-Cu 2 S was presented by Èižakov [14].Investigations concerning sulfide melts were presented by Voghan [15].

Experimental
Characterizations of copper-lead mate were done using sceaning electron microscopy (SEM) with EDEX-9100 analyses and PAX software package, with resolution of 1 nm (30kV), excitation voltage of 0,2-30kV, magnification of 500000 X and with the secondary electron detector.Mineralogical investigations were done at the atomic powder diffract-meter APD SYSTEM PW 1710 "PHILIPS", under following conditions: 2θ range (4-90 O ), velocity 0,05 O /s, Cu anti-cathode (Cukα = 1,54178Å) with 30 mA current and voltage of 40 kV.Chemical composition of starting sample was determined using Xray quantum-meter (ARL 86480) while Cu content in leaching solution was determined using standard analytical methods.
DTA investigations were performed at the apparatus DTA STA 409 (Netzsch, Germany).With sample mass 30 mg and heat rate 25 O /min.Roasting of the investigated matte was done in electric-resistant furnace while leaching was conducted in three-neck tank with condenser, mechanical stirrer and thermometer.Leaching temperature was controlled using ultrathermostat.Liquid to solid ratio, during leaching, was 5:1 with sulfuric acid concentration 0,4 M.

Characterization of the starting sample
For experimental investigations presented in this paper samples of copperlead matte were used, chemical composition of the samples are presented in Table 1.Where rest presents series of elements: Si, Ca, Se, Hg, Sn, Te, Mn, Sr, Pd, Mo and Cs.
If observing results presented in Table 1, it is obvious that amount of valuable elements, copper and lead in this case, is very high.In further experimental work copper-lead matte presented as sample 1 was used.
Results of X-Ray analyses of starting sample are given in Fig. 1.There are three main crystal phases identified: Cu 2 S (chalcocite), PbS (galena) and small amount of CuSO 4 (Chalcocyanite).

Fig.1. RDA results for the starting sample of copper-lead matte
Results of the SEM analysis of the starting sample are presented in the Fig. 2. Results of semi-quantitative analyses are given in Table 2.According to the results presented in Table 2, it is obvious that lead and copper are segregated in the separate phases.Black colored phases are enriched in copper while light ones in lead.Sulfur is present in both with amount higher in lead enriched phase.

Thermal analysis
The results of DTA analysis of the copper-lead matte sample are presented in Fig. 3.
At the DTA curve, Fig. 3, two exothermal peaks are present in temperature interval 500-550 O C and one in 620-680 O C. Further temperature rise leads to appearance of endothermal behavior in temperature interval 700-740 O C, after which new exothermal peak appear in interval 800-840 O C.
For the products of oxidative roasting of mattes at the temperatures 550, 650 and 900 O C, X-Ray analysis were done with purpose to describe thermal behavior of copper-lead matte.Results are presented in Fig. 4.
Phase composition of the starting sample and samples oxidative roasted at 550, 650 and 900 0 C are given in Table 3.

Fig.2. Micro-structural purview of matte surface (enlargement x 625) Table 3. Phase composition of investigated copper-lead mattes roasted at different temperatures
Using the results of DTA analysis, (Fig. 3), combined with results of X-Ray analysis, Table 3, and thermodynamics analysis of the system Cu-Pb-S-O [16], (Fig. 5), it is possible to define mechanism of copper-lead mattes oxidative roasting.and less significant reaction: Further temperature rise leads to new exothermal peak in temperature interval 620-680 O C, which can be characterized with intensification of reactions (1-3) and appearance of reaction: Further temperature rise leads to appearance of the endothermal peak in temperature interval 700-740 O C which can be described by reaction of dissociation of copper sulphate formed: With further temperature rise following reaction is predominant: As well as the reactions: PbO + PbSO 4 = PbO*PbSO 4 (7) 6/7PbS + 3/2O 2 = 3/7PbOPbSO 4 + 3/7SO 2 (8) Parallel with reactions (6-8) in the temperature interval 800-840 O C, which is characterized with significant exothermal peak at DTA curve, following reaction is possible: Crystal phases of copper are not identified at the diffractogram for sample roasted at 900 O C. Reason for such an appearance is higher dissipation of Xrays at lead then on copper atoms.This is why Cu phases reflections are with low intensities even when their presence is not small.

Calculation of kinetic parameters for the process of oxidative roasting of copper-lead matte
Oxidative roasting was performed by introducing distinct amount of air in the furnace reaction space.Roasting temperatures were in the range of 400-700 O C.

Fig. 5. Results of Gibbs free energy dependence on temperature for the characteristic reactions describing oxidation in the Pb-Cu-S system
Analysis of kinetic parameters for the process of oxidative roasting of copper-lead matte was done using method defined by H.J.Sharp.Results presenting degree of desulfurization as the function of time at different roasting temperatures are given in Fig. 6.
According to the results present in Fig. 6 it is obvious that most desulfurization is obtained at 700 O C during 30 min and maximum value is 46%.This means that reduced half time of reaction cannot be determined since degree of desulfurization did not reach 50%.Because of that linearization of experimental data was performed used nine different kinetic equations proposed by H.J.Sharp, Table 4. Chosen kinetic equation is D4 because it linearize experimental results the best.
Using equation D3, linearization of experimental data was performed.From the inclination of linearized isotherms, rate constants were determined and Arrhenius diagram, Figure 7, constructed.

Fig. 6. Degree of desulfurization of copper-lead matte as the function of time and roasting temperatures
Table 4. Kinetic equation proposed by H.J.Sharp [1] According to the Arrhaenijus diagram, activation energy of the process is calculated and its value is 17 kJ/mol.Kinetics parameters of the process are presented in Table 5.

Table 5. Kinetic parameters for the process of copper-lead matte oxidative roast
Function Equation Process that define rate of reaction Randomly formation of new phase nucleus, one nucleus per one particle Randomly formation of new phase nucleus, Avrami equation (I)  According to Fig. 8, it is obvious that most utilization of copper is obtained by leaching of the matte roasted at 700 O C. Newer the less, utilization of copper from the mattes roasted in temperature interval 500-700 O C is not much different but is substantially higher then for the matte roasted at 400 O C. From that reason, in further text, results are given for the process of leaching of the matte roasted at 500 O C. Main reason for that is because of energy saving, at industrial scale, if matte is roasted at 500 O C instead at 700 O C.
In Fig. 9, results presenting utilization of copper during process of leaching as the function of time and leaching temperatures are given.Mattes were, roasted prior leaching, at 500 O C.

Fig. 9. Copper utilization as the function of time and leaching temperature
According to Fig. 9, it is obvious that largest copper utilization is achieved at temperature of 100 O C and its value is 82,6%.
Using the results given in Fig. 9, according to the Sharp method of reduced half time of reaction, adequate kinetic equation for linearization of experimental data was chosen; as given in Fig. 10, where: α -degree of reaction, τ/τ 0,5 -reduced half time of reaction.

Fig. 10. Choice of kinetic equation to be used for experimental results linearization
From Fig. 10, it is obvious that equation F1, Table 4, should be chosen for further experimental data linearisation.Linearisation of experimental results, using that equation is presented at Fig. 11 and Arrhenius diagram for the process of roasted matte leaching is presented in Fig. 12.  6.

Fig. 8 .
Fig.8.Utilization of copper during process of leaching as the function of time and temperature of copper-lead matte roasting

Table 2 .
Semi-quantitative analyses of compositions presented in Fig.2