Treatment of Sulfide Minerals by Oxidative Leaching

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Treatment of Sulfide Minerals by Oxidative Leaching with Ozone a

Francisco Raúl Carrillo Pedroza , María de Jesús Soria Aguilar b

a

c

, Teresa Pecina Treviño , Antonia Martínez Luévanos & Marco Sánchez Castillo

d

a

Facultad de Metalurgia, Universidad Autónoma de Coahuila, Monclova, Coahuila, Mexico b

Escuela Superior de Ingeniería, Universidad Autónoma de Coahuila, Nueva Rosita, Coahuila, Mexico c

Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, Saltillo, Coahuila, Mexico d

Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, San Luis Potosí, San Luis Potosí, Mexico Available online: 04 Jul 2011

To cite this article: Francisco Raúl Carrillo Pedroza, María de Jesús Soria Aguilar, Teresa Pecina Treviño, Antonia Martínez Luévanos & Marco Sánchez Castillo (2012): Treatment of Sulfide Minerals by Oxidative Leaching with Ozone, Mineral Processing and Extractive Metallurgy Review: An International Journal, 33:4, 269-279 To link to this article: http://dx.doi.org/10.1080/08827508.2011.584093

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Mineral Processing & Extractive Metall. Rev., 33: 269–279, 2012 Copyright # Taylor & Francis Group, LLC ISSN: 0882-7508 print=1547-7401 online DOI: 10.1080/08827508.2011.584093


TREATMENT OF SULFIDE MINERALS BY OXIDATIVE LEACHING WITH OZONE Francisco Rau´l Carrillo Pedroza1, Marıá de Jesu´s Soria Aguilar1, Teresa Pecina Trevi~no2, Antonia Martıńez Lue´vanos3, and Marco Sańchez Castillo4 1

Facultad de Metalurgia, Universidad Autońoma de Coahuila, Monclova, Coahuila, Mexico 2 Escuela Superior de Ingenierıá, Universidad Autońoma de Coahuila, Nueva Rosita, Coahuila, Mexico 3 Facultad de Ciencias Quı´micas, Universidad Autońoma de Coahuila, Saltillo, Coahuila, Mexico 4 Facultad de Ciencias Quı´micas, Universidad Autońoma de San Luis Potosı´, San Luis Potosı´, San Luis Potosı´, Mexico Sulfide minerals are one of the most important sources of metal values. On the other hand, some sulfides such as pyrite are considered as one of the major impurities in solid fossil fuels such as coal. Oxidation of these sulfides has been one route to obtain base metals and to remove sulfur by leaching. This work summarizes the results obtained regarding the use of ozone in the oxidation of sulfide ores containing gold, silver, and copper. Results show that the extraction of gold and silver is increased by at least 15%, with lower cyanide consumption; the extraction of copper increased by 16% and in less time; and in the case of coal, sulfur is removed above 70%. These results indicate that oxidation treatment with ozone could be a promising clean technology to optimize ore processing, which is difficult to treat by current industrial methods. Keywords: coal, copper, gold, hydrometallurgy, ozone, silver, sulfide

INTRODUCTION Valuable metals are recovering worldwide relevance due to the development of a whole new range of potential applications in electronics, environmental catalysis, material science, and biomedicine, among other fields with significant impact on daily life activities. To date, sulfide minerals (i.e., pyrite, FeS2, chalcopyrite, and CuFeS2) are one of the most important sources of value metals, such as gold, silver, copper, zinc, etc. Due to the strong sulfur binding to these minerals, metals are usually extracted by chemical oxidation. Aqueous oxidation can be conducted under elevated temperature and Address correspondence to Francisco Rau´l Carrillo Pedroza, Facultad de Metalurgia, Universidad Autońoma de Coahuila, Carr. 57. km 5, Monclova, Coahuila, Mexico, C. P. 25710. E-mail: frrcarrillo@ yahoo.com.mx 269


270

F. R. C. PEDROZA ET AL.

pressure, and also at ambient conditions, which makes it environmentally and economically attractive (Deng 1992). For this reason, studies to optimize aqueous oxidation and to explore more efficient oxidants have been made. However, in mining industry (especially in precious metals extraction), the use of ozone as an oxidant has not been discussed in detail, although lab-scale experiments indicate that ozone may be an alternative to overcome economic and ecological disadvantages of existing aqueous extraction process. Ozone has a very high oxidation potential (2.07 V) compared with hydrogen peroxide (1.77 V) and chlorine (1.4 V), making it advantageous to use in several applications (Hill and Rice 1997; Antwerp and Lincoln 1987). Importantly, ozone can create favorable conditions to oxidize sulfide minerals in aqueous media. According to the Eh–pH diagrams shown in Figures 1 and 2, the sulfide species, such as pyrite or pyrrhotite (Figure 1) and chalcopyrite (Figure 2), can be oxidized to sulfate in the presence of an oxidant such as ozone, in a pH range from 2 to 14; the oxidized products could be solids or solutions. At very acidic conditions (i.e., pH < 2), it is possible to dissolve metals as Fe and Cu ions. In this context, oxidative leaching with ozone is relevant in copper-iron sulfide and gold- and silver-containing sulfides. Moreover, oxidative leaching of coal-containing iron sulfide might also have a positive impact on coal cleaning prior to its use in energy-related applications. In this article, we show the beneficial effect of using ozone in processes of environmental and commercial importance, and outline the role of ozone layer in process optimization. The practical significance of the study cases is briefly discussed next. Sulfide copper minerals, such as chalcopyrite (CuFeS2), are the most abundant copper-bearing minerals and represent approximately 70% of the world’s known copper reserves (Davenport et al. 2002). Chalcopyrite is also the most stable of copper minerals due to its structural configuration (face-centered tetragonal lattice) and, consequently, the most refractory for aqueous extraction processing. For decades, several processes have been developed to leach copper from chalcopyrite ores

Figure 1 Eh-pH diagram for S-Fe system under standard conditions at 25 C (color figure available online).


TREATMENT OF SULFIDE MINERALS BY LEACHING

271

Figure 2 Eh-pH diagram for Cu-S-Fe system under standard conditions at 25 C.

and concentrates, and conditions are well established. Small isolated deposits and mixed- and low-grade ores also need to be exploited. However, there is an increasing interest in optimizing the aqueous extraction process for copper production due to the negative environmental impact caused by the chemical reagents used (Peacey, Guo, and Robles 2003; Shijie 2005). Although leaching of copper ores is carried out in diluted sulfuric acid medium, ferric sulfate as oxidant (Antonijevic and Bogdanovic 2004; Ukasik and Havlik 2005) and low-cost reagents, different approaches have been suggested to increase chalcopyrite rate dissolution. The most common is to increase process temperature, but this implies higher energy requirements. Another suggested alternative is the use of strong oxidants such as ozone (Havlik et al. 1999), hydrogen peroxide (Antonijevic, Jankovic, and Dimitrijevic 2004), and manganese nodules (Havlik et al. 2005). In this work, we report the combined effect of O3 and ferric ions (Feþ3) on the acid leaching of a low-grade chalcopyrite ores. Cyanidation is the most aqueous leaching process used to extract gold and silver. However, it has some disadvantages when precious metals are encapsulated in matrixes of iron sulfide minerals, such as arsenopyrite and pyrite (Shoemaker 1990). In this case, the minerals receive an oxidation pretreatment (as oxidation roasting, chemical oxidation under pressure, or biological oxidation) to facilitate gold and silver extraction by a cyanide solution (Weir and Berezowsky 1986; Burbank, Choi, and Pribrey 1990; Chen and Reddy 1990). An alternative to these methods is the use of ozone, which increases the oxidation potential and the oxygen content of solution during cyanidation (Haque 1992; Roca, Cruells, and Vi~ nals 2000; Salinas et al. 2004; Elorza-Rodrı´guez et al. 2006; Carrillo et al. 2007). In this work, we add evidence of the beneficial use of ozone in the pretreatment of sulfide ores in cyanidation process. Another process is ozone application for iron sulfide oxidation in coal, one of the most important fossil fuels used for energy production. However, due to its nature, coal requires a cleaning stage based on physical methods before its use to meet


272


air pollution regulations (Apenzaller 2006), but organic sulfur and syngenetic pyrite are removed with low efficiency to the required level (Ozbayoglu 1998). Syngenetic pyrite–-one of the two forms of iron sulfide–-is found as a very fine and highly disseminated mineral in coal, which complicates its separation by conventional cleaning process (Baruah et al. 2006). Several studies have been conducted to explore pyrite dissolution by oxidants in aqueous media. Oxidizing agents such as oxygen, hydrogen peroxide, ferric sulfate, ferric chloride, potassium permanganate, and perchloric and nitric acids have been used to oxidize pyrite (Elliot 1978; Kawatra and Eisele 2001; Borah 2006; Carrillo et al. 2009). In this context, this article reports the use of ozone to remove pyritic sulfur of different coal samples.

EXPERIMENTAL PART Copper Low-grade chalcopyrite ore (0.5 Cu wt%) samples were ground and classified according to particle size (1200 mm to þ750 mm). For the experiments, 300 g of mineral and 1 L solution, having different ferric ion concentration [Feþ3] and sulfuric acid concentration [H2SO4], were placed in a glass reactor. Experimental tests were carried out at constant temperature (20 C) and under mechanical agitation (600 rpm) for 60 min. At the beginning of the experiment, an oxygen–ozone mixture (Ozone Ecological Equipments) was injected through an aerator (pore size of 2 mm) installed at the bottom of reactor. Ozone mass flow rate was 0.5 or 1 g=h, and it was determined by iodometric method. Experimental conditions are summarized in Table 1. In all tests, aliquots of the reaction mixture were taken at different time intervals to determine copper content in solution by Atomic Absorption Spectrometry (PerkinElmer, Optima 3000XL).

Gold and Silver Tests were conducted with three samples of pyrite containing gold and silver, with a size distribution of 75%, i.e., 75 mm. Chemical analysis, mineralogy, and liberation degree for each sample are shown in Table 2. The detailed process for experimental tests was previously reported (Carrillo et al. 2007). Experiment included a pretreatment (before cyanidation) with ozone (1.38 gO3=h) directly in mineral slurry (25% solids with a natural pH of 6) during 20 min, using a column as a contacting

Table 1. Experimental condition used in the dissolution of copper from chalcopyrite Test 1 1a 2 3 4 5

[Fe3þ] (M)

[H2SO4] (M)

O3 (g=h)

Cu Dissolved (%)

0 0 0.5 0 0.5 0.5

0.1 0.5 0.25 0.25 0.1 0.5

0 0 0 1 0.5 1

1.12 1.15 10.45 11.24 16.84 16.04


273

Table 2. Pyrite samples, containing gold and silver, used in the dissolution of precious metals by cyanidation with and without ozone pretreatment Sample 1


Initial contents, g=tone Mineralogy

Sample 2

Sample 3

Au

Ag

Au

Ag

Au

Ag

4.5

435

6.7

326

0.3

34

Au and Ag native Au native included or Au native micro-included and Au-Ag (as electrum) associated with pyrite and in pyrite; Ag as argentite, included in pyrite and quartz. Ag included in pyrite, mainly associated in quartz. sphalerite, and galena Liberation degree At 75 m, Au, 80%; At 75 m, Au, 70%; At 75 m, Au, 60%; Ag 70%. Ag 75%. Ag 75%. 48 h cyanidation

Pretreatment without ozone: Metal dissolution (%) 84.69 71.51 76.25 71.24

70.21

16.67

48 h cyanidation

Pretreatment with ozone: Metal dissolution (%) 91.86 81.99 97.39 83.33

61.70

75.00

reactor. Subsequently, the solid sample was treated for 48 h under conventional cyanidation conditions. Efficiency of ozone pretreatment in precious metals dissolution was determined by cyanide consumption analysis (silver nitrate method) and also by measuring gold and silver concentration in solution (fire assay and atomic absorption, respectively). Coal Coal samples with different pyrite content were ground and classified at 175 to þ104 mm particle size. Experiments were carried out in a column-type reactor with 500 mL of a 0.3 M H2SO4 solution at room temperature (20 C). Solid content was fixed to 15% w=w. An air–ozone mixture was injected through an aerator (pore size of 2 mm) installed at the bottom of the reactor. An ozone concentration of 0.33 g=h (as determined by iodometric method) was used in the tests. Total dissolved Fe was analyzed by spectroscopy (GENESYS 20 spectrophotometer) and atomic absorption spectroscopy (PERKIN ELMER, Optima 3000XL). Total sulfur in coal was chemically analyzed by sulfur determination (LECO CS600). Pyrite was measured by Fe and S balance (Fe as oxide or sulfate was not found in the analysis). RESULTS AND ANALYSIS Dissolution of Copper From Chalcopyrite Figure 3 shows dissolved copper profiles (given in %) at conditions reported in Table 1. For test 1, without any oxidant (no Fe3þ and also no ozone), copper dissolution was insignificant at two different acid concentrations. On the other hand, test 2 (conducted with no O3 in the system) showed the beneficial effect of Fe3þ. In agreement with the literature, it can be postulated that for this experimental condition,


274


Figure 3 Dissolved copper from chalcopyrite leaching, at different condition tests.

copper extraction occurred through the following reaction (Havlik and Skrobian 1990; Habashi 1999; Peacey, Guo, and Robles 2003): CuFeS2 þ 4Feþ3 ¼ Cuþ1 þ 5Feþ2 þ 2S :

ð1Þ

It should be noted that the amount of copper extracted in this condition might be limited by the formation of the sulfur layer indicated in Equation (1); this condition drastically limits reagents diffusion and, in consequence, the overall efficiency of the process. In a similar way, test 3 (conducted with no Fe3þ in the system) showed the beneficial effect of ozone. In this case, it can be postulated that copper extraction took place through the following reaction (Havlik and Skrobian 1990; Habashi 1999; Peacey, Guo, and Robles 2003): 3CuFeS2 þ 8O3 ! 3CuSO4 þ 3FeSO4 :

ð2Þ

In this condition, the reaction seems to be dependent on ozone mass flow rate into the system. Figure 3 shows the significant improvement in the amount of extracted copper when both oxidants, Fe3þ and O3, were conjunctly used (tests 4 and 5). A comparison of experiments 1 and 4 indicates that the percentage of extracted copper increased from 1.12% to 16.84% when both oxidants were used. The beneficial effect of ozone addition is shown when comparing experiments 2 and 4. It increased the amount of copper extracted from 10.45% to 16.84%. Thus, oxidant condition required for copper extraction process was notoriously enhanced by ozone. According to Equation (1), a significant amount of ferrous ion (Feþ2) is formed. The continuous addition of ozone into an acid solution favors the oxidation of Feþ2 to Feþ3, according to Equation (3): 6Feþ2 þ O3 þ 6Hþ ¼ 6Feþ3 þ 3H2 O:

ð3Þ



275

The occurrence of reaction (3) in preferential acidic and oxidant conditions is enhanced and favors reaction (1), consequently copper dissolution. Therefore, a possible mechanism for chalcopyrite dissolution in the presence of Feþ3 and ozone is that Feþ3 ions quickly react with the mineral surface to produce copper ions, Feþ2 ions, and also an elemental sulfur surface layer. Feþ2 ions must diffuse through this layer to keep the dissolution process. In addition, ozone must diffuse from the gas bulk to the solution, and then to the interface of chalcopyrite particles to react with chalcopyrite and Feþ2 ions. This last step [Equation (3)] might be faster, leading to the formation of more Feþ3 in solution. As a result, the increased concentration of Feþ3 ions promotes copper dissolution process [Equation (1)]. Importantly, experiment 5 shows that an increase in [H2SO4] and ozone mass flow rate had no beneficial impact on the amount of extracted copper (from 16.84% in test 4 to 16.04% in test 5). Thus, it can be postulated that an excess of ozone in an acidic medium might lead to an excess of Feþ3 ions. In this condition, the dissolution might be close to equilibrium and slower the overall rate of copper extraction. This condition might be particularly relevant for iron-rich minerals and high-grade chalcopyrite concentrates or suspensions with high chalcopyrite content.

Dissolution of Au and Ag Table 2 shows gold and silver composition of samples tested in this study, and Figure 4 displays the amount of metal recovered from the cyanidation process. It is evident that a 20-min ozone pretreatment increased the dissolution of gold in cyanidation, particularly for the sample with the highest gold composition. For samples 1 and 2, dissolution value increased from 85% and 76% (without ozone pretreatment) to 92% and 97% (with ozone pretreatment), respectively. In sample 3, the dissolution of gold was not affected by ozone pretreatment but the sample showed a low content

Figure 4 Dissolution of precious metals during cyanidation treatment, with and without ozone pretreatment.


276


of gold, may be due to chemical analysis precision. However, it is interesting to note that it is still possible to recover gold from sample 3 by the cyanidation method. Figure 4 also shows the dissolved percentage of silver during the cyanidation process. When no ozone pretreatment was made, the amounts of dissolved silver were 71%, 71%, and 16%. However, when a 20-min ozone pretreatment was carried out, the amount of silver dissolved increased to 82%, 83%, and 75%, respectively. The increase in samples 1 and 2 was about 15%, but sample 3 showed a very significant increment in the extracted silver. The mineralogy of this metal could explain the difference: the sample was probably in the form of argentite, a silver sulfur that is not extracted during the cyanidation process. Previous results suggested that ozone introduced in the slurry chemically reacts with pyrite’s sulfur, increasing the oxidation potential of the slurry. In a previous work, we have shown that ozone treatment leads to partial oxidation of sulfide minerals and to sulfate ion formation, specifically in the oxidation of sulfur (Carrillo et al. 2007). Improvement in gold and silver recovery from ore with ozone pretreatment indicated that reaction intermediate products promote the conditions for cyanide diffusion to the precious metals in the subsequent cyanidation process. Sulfur Removal From Coal A previous work conducted with a high sulfur (2.95%) coal sample demonstrated that sulfur can be decreased by ozone and ozone-hydrogen peroxide treatments (Carrillo at al. 2009). Table 3 shows composition of samples used in the present work, differentiating content of total sulfur and pyrite sulfur forms. In addition, amounts of sulfur in coal (given in %), before and after the ozone treatment, are shown in Figure 5. It is pointed out that two set of samples were used: three samples (1, 2, and 3) of ‘‘all-in-one’’ coal (without water-washing conventional cleaning process) and two samples (4 and 5) of washed coal (coal cleaned by washing and ready to use in any industrial process). Importantly, Figure 5 indicates that pyrite can be removed in all the studied cases. In ‘‘all-in-one’’ coal samples, pyrite is present in epigenetic and syngenetic forms. In the epigenetic form, pyrite is coarse and partially liberated from coal grain; an important fraction of this pyrite is easily removed in washing process. Therefore, in samples 1–3, a higher fraction of the epigenetic form required either a higher ozone concentration or a higher treatment time to oxidize pyrite. In samples 4 and 5, cleaned by washing, the liberated pyrite (possibly epigenetic form) was partially removed. Therefore, the no-liberated fraction of the pyrite Table 3. Coal samples used in sulfur removal with ozone

Coal sample 1 2 3 4 5

Initial total sulfur (%)

Initial pyrite composition (%)

Total sulfur after O3 treatment (%)

Total sulfur removed (%)

Pyritic sulfur removed (%)

1.81 2.74 3.36 1.24 1.05

0.95 1.88 2.5 0.38 0.19

1.34 1.27 2.33 0.95 0.86

25.97 53.65 30.65 23.39 18.10

49.47 78.19 41.20 76.32 78.95



277

Figure 5 Sulfur removal in coal samples using ozone as oxidant in aqueous media.

(syngenetic and epigenetic forms) reacts with ozone, achieving a 76%–78% of pyrite removal in 60-min treatment. The rest of the sulfur present in coal is organic sulfur. Results indicate that a significant decrease of pyrite sulfur in coal could be achieved using an ozone treatment, and also that a cleaner coal could be obtained if a water-washing of the sample is conducted prior to ozone treatment. It could be postulated that oxidation of pyrite in samples containing gold and silver, and oxidation of pyrite found in coal, takes place by two basic steps. First, the dissolution of pyrite to Feþ2 ions is carried out through the formation of an iron-deficient or a sulfur-rich layer, rather than elemental sulfur. Second, a further oxidation of this layer takes place, forming sulfides of lower iron content which are eventually converted to elemental sulfur. In severe oxidizing conditions, elemental sulfur could be oxidized to oxy-sulfuric species. This general mechanism is described in the following reactions: FeS2 ¼ Fe2þ þ 2S þ 2e :

ð4Þ

Elemental sulfur is stable at low pH and redox potential, and could be oxidized to sulfate by ozone at higher potentials, as obtained by Equation (3), according to the following equation: FeS2 þ 8H2 O ¼ Fe3þ þ 2SO4 2 þ 16Hþ þ 15e :

ð5Þ

Then, oxidation of pyritic sulfur by ozone occurs by means of the following equations: FeS2 þ O3 þ H2 O þ 2O2 ¼ FeSO4 þ H2 SO4 ;

ð6Þ

FeS2 þ 7=3O3 þ H2 O ¼ FeSO4 þ H2 SO4 :

ð7Þ


278


These reactions suggest that ozone is a strong oxidant to remove the pyrite and also that pyrite dissolution is enhanced by acidic medium. The previous analysis shows the beneficial effect of ozone in processes that have important implications in extractive metallurgy of copper, gold, and silver, and mineral processing of coal. The fundamental understanding of ozone oxidation processes is useful to optimize process conditions. An excess in the use of ozone can be harmful in process efficiency and economically. This process should be evaluated in conditions resembling advanced oxidation process where more oxidant species are generated. Several technologies might be useful for this purpose (O3=H2O2, O3=UV), including the possibility to generate a control amount of ozone, in situ, by using cold plasma technology in solution.

CONCLUSIONS Oxidation of different sulfide minerals using ozone was confirmed in the present study. Results show that the treatment with ozone increases sulfur oxidation in the different cases. Sulfur is the element that, in copper, gold, and silver cases, inhibits metal dissolution present in minerals. On the other hand, the removal of sulfur is greatly needed in the case of coal, due to emissions of SO2 gas during coal fire combustion. Thus, the use of ozone can be a promising auxiliary agent in the actual process of obtaining metals and coal with the following advantages: (1) decreasing operational costs of low-grade chalcopyrite leaching, (2) increasing gold and silver recovery in cyanidation, and (3) decreasing the sulfur-containing coal used in energy generation and iron and steelmaking.

ACKNOWLEDGMENTS The authors thank CGEPI-UAdeC and CONACYT for financial support (Project 67039 and Project 2000405010).

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