Introduction
It is known the fact, that the copper sulfides as the first treatment option, obey the natural flotation and then as concentrates are send to the smelter where metallic copper is obtained separated from the slag through a high temperature treatment and the sulfur is expelled as sulfur dioxide.
It is also known that as time passes, the copper grades have decreased substantially while the impurities grades are increasing steadily. Impurities containing copper concentrates have already been described [1] containing mainly Cd, Zn, Tl, As, Sb, Bi, Pb, Te and others.
Arsenic is an impurity that is associated with copper minerals forming enargite , tennantite, tetrahedrite, and that due to the pyrometallurgical treatment either in conversion or roasting furnaces, it goes into the atmosphere together with the sulfur dioxide. A great quantity of literature can be found to eliminate arsenic or try to separate arsenic from copper . Most of these researches are carried out through hydrometallurgical procedures and have as their main objective the dissolution of arsenic minerals . Thus we can find studies on arsenic leaching from concentrates where one of the early investigation appears between 60–95 °C, in acid medium and Fe3+ [2], other authors used chlorine/ chloride mixture [3], leaching with sodium hypochlorite in oxidizing media and the arsenic is dissolved like arseniate [4], slow ammoniacal leaching works [5], complete leaching of enargite over 170 °C in acidic sulfate and chloride media [6], at low temperatures of 130–160 °C incomplete reaction by elemental sulfur that prevents the enargite dissolution [7], being investigations at 220 °C and 100PSI in reactor with sulfuric acid deliver a complete dissolution [8], others studies between 160–200 °C adding pyrite to force a galvanic effect [9] and lately the leaching between 140–200 °C, with sulfuric acid and oxygen adding ferrous sulphate [10].
On the other hand, in the subject of bioxidation, efforts have been made to treat sulfide minerals with leptospirillum [11], minerals and concentrates with Fe3+ and sulfolobus [12] and enargite treated with mesophiles and thermophiles at high temperatures [13].
There are also reviews of treatment of concentrates with enargite under pressure [14] as recent trends in the processes of enargite concentrates [15]. This latter one, separates the processes of copper and arsenic leaching , with those of selective dissolution of arsenic .
However, the dissolution of minerals or concentrates with arsenic does not seem to be the solution since, as we have seen, there is a lot of literature on this subject and very few practical applications, being the pyrometallurgical procedures the ones that are mostly used, being the roast the one that occupies a place important at present, despite the emanations into the atmosphere.
The hydrothermal reaction process [16] allows selectively to pass the impurities to solution and concentrate the solid phase in copper which means a separation easily through a solid-liquid filtration .
There is not much literature that studies this problem. Previous work by the authors [17] demonstrated the separation of 30% of arsenic using this technique. Tennantite and tetrahedrite have not been fully studied, possibly requiring extreme conditions for their transformation .
Experimental
Materials
A sample of “enargite ” from Huencavelica Peru, was used for the experiments. This is a enargite sample of quite purity (~96% enargite , ~4% tennantite), ground and sieved to minus 15 µm. This sample was used in an experiment whose objective was to characterize the product of the total conversion as well set the overall stoichiometry of the reaction.
The sample was characterized by X Ray Diffraction, analyzed chemically and was also used in the kinetic experiments.
Autoclave Experiments
Experiments were performed in a Teflon-coated stainless steel PARR-4563 stirring reactor with a programmable heating controller. The stirring speed was 800 min−1 in all experiments. The system was raised to the working temperature at a rate of ~5 °C/min, then maintained at this temperature for a fixed time (nominal time), until it was cooled to the ambient temperature , again at a rate of ~5 °C/min. After this treatment, the pH and the solution volume were measured. Copper and arsenic in the final solution were determined by A.A.S. Residual solids were characterized by XRD, reflected-light microscopy and SEM /EDS .
The nature of the reaction of enargite with copper ions was investigated in the range 140–300 °C. 1 g of 15 < μm enargite particles was treated with 100 cm3 of slightly acidified (H2SO4, pH 1.2) copper sulfate solutions of known concentrations. Under these conditions, there is sufficient Cu2+ excess to assume practically constant copper concentration, pH and solution volume.
Experiment of Complete Reaction
Sample: 1.00 g
Temperature : 300 °C
Time: 1 h
Initial volume of solution: 100 mL
Initial Cu2+ concentration: 10 gpL (in sulphate of copper form).
Initial pH: 1.2
Initial ORP: 434 mV (vs. Ag/AgCl)
Final pH: 0.4
Final ORP: 368 mV(vs. Ag/AgCl)
The Kinetics of the Process Was Studied in the Range of 140–300 °C
For study of kinetic 0.400 g of the sample 1 for each experience, was used and 0.200 L of 10 gpL of Cu2+, using a pH = 1,26 at initial time. Solutions after experiment were analyzed for arsenic , copper by AAS, and solid phases were analyzed for As and Cu by AAS. The solid products were also analyzed for XRD and SEM /EDS .
Results and Discussion
X Ray Diffraction Study
XRD patterns of original sample
Chemical Analysis Study
Composition of sample and theoretical enargite
Element | Sample | Cu3AsS4 |
|---|---|---|
Cu | 44.7 | 48.41 |
As | 16.0 | 19.02 |
Sb | 1.8 | – |
S | 31.9 | 32.57 |
Hydrothermal Treatment Results in Stoichiometry Experiment
XRD patterns of product from complete reaction of enargite , compared with original enargite
The results of experiment indicates 3.5 mol of Cu consumption from solution by mol of arsenic reactioned, that is approximately by mol of enargite reactioned. Also indicate the appearance of 0.79 mol of sulphate in solution by mol of enargite reactioned. The relation Sb/As in solution is 0.09, near to relation of 0.07 contained in initial enargite . This indicate that Sb from enargite passes also into solution besides the As.
Kinetics Experiments
Arsenic conversion
140 °C | 190 °C | ||
|---|---|---|---|
Time (min) | Conversion (%) | Time (min) | Conversion (%) |
42 | 8.1 | 50 | n/a |
72 | 11.9 | 78 | 32.5 |
100 | 13.5 | 112 | 36.9 |
250 °C | 300 °C | ||
Time (min) | Conversion (%) | Time (min) | Conversion (%) |
82 | 55.1 | 88 | 78.6 |
94 | 57.8 | 108 | 86.4 |
122 | 63.0 | 146 | 91.9 |
Kinetics of arsenic extraction
Temperature Effect
F(α) v/s time
F(a) is plotted to facilitate parameter estimation in an non explicit equation for conversion v/s time
Arrhenius plot
The obtained activation energy in the interval 140°–300 °C was 51 kJ/mol, a value that through of an electrochemical model , can explains the hydrothermal conversion of the enargite [18].
Conclusions
According the stoichiometry experiment, Enargite reacted completely after 80 min. A solid product was obtained and characterized by XRD, and the main product was chalcocite (M) and some traces of djurleite and chalcocite (Q).
Experimental stoichiometry of the reaction is consistent with the Cu2S formation and the step of As passing to As(III).
DRX study confirm that tennantite it is not apreciable attacked.
From fitted data of the kinetics experiment and Arrhenius equation the calculated activation energy (Ea) was 51 kJ/mol.
According to the hydrothermal transformations studied, the reactivity of different phases found in the copper concentrates was bornite > chalcopyrite > covellite > sphalerite > pyrite > enargite > tennantite.
Acknowledgements
The authors wish to thank to the support of the “Serveis Cientifico-Technics of the Universitat de Barcelona” and Mrs. E. Vilalta in the characterization studies is also gratefully acknowledged.