Desulphurisation

Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.

Introduction

Desulphurisation offers an interesting integrated approach to tailings management and control of acid mine drainage (AMD) (e.g. Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003, Bois et al. 2004, Hesketh et al. 2010). In desulphurisation, acid forming sulphide mineral fraction is either partly or fully separated from the tailings by froth flotation before the final deposition into the mine waste area (Kongolo et al. 2004, Kauppila et al. 2011).

Description of technology

Separation of sulphide minerals reduces a volume of potentially acid generating high sulphide content tailings to be managed when performed at the end of primary process treatment circuit (Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003, Bois et al. 2004, Hesketh et al. 2010). Desulphurisation can be performed in the mineral processing plant as a part of the process (European commission 2009). In the process, acid forming sulphide mineral fraction is either partly or fully separated from the tailings by froth flotation before the final deposition into the mine waste area (Kongolo et al. 2004, Kauppila et al. 2011). Separation generates a small volume of sulphide-rich concentrate and a large stream of tailings with a low sulphur content (Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003, Bois et al. 2004, Hesketh et al. 2010) (Figure 1). These two streams can be handled in different ways. As low sulphur content tailings are rather non-reactive, they do not require as comprehensive decommissioning measures and may be deposited to large-volume repositories or alternatively used as a cover material. Sulphide rich concentrate can be deposited for example under water in the tailings pond or as paste backfill, or covered with desulphurised tailings (Benzaazoua & Kongolo 2003, Sjoberg Dobchuck et al. 2003, Bois et al. 2004, INAP 2009) if it can be confirmed that this procedure will cause no harm to water quality either during the operational phase or after the closure in the long term (INAP 2009). An alternative use for sulphide rich tailings is to use them as a source of sulphuric acid in other mining operations (Hesketh et al. 2010). In the literature, the simultaneous use of paste backfill technology with desulphurisation is often mentioned and may be worth considering (Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003, Bois et al. 2004).

Figure 1. The principle of desulphurisation (Adapted from Bois et al. 2004).

A non-selective froth flotation is the most common and technically feasible way to perform the removal of sulphides (Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003, Bois et al. 2004). Three types of reagents are needed in flotation: collectors, frothing agents and pH modifiers (Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003). So called xanthate-based collectors have an ability to collect sulphide minerals in general, and are the most common reagents used in non-selective flotation. Amyl type xanthates are mainly used due to their good collection capability, although the use of other types of collectors may also be successful. However, there are some specific reasons that may result in inhibition of pyrite flotation and should be taken into account. (Benzaazoua et al. 2000, Benzaazoua & Kongolo 2003, Kongolo et al. 2004) It is noteworthy that the flotation process may also release other chemical elements or usable minerals from the tailings. Also the need for water treatment may diminish. (Kauppila et al. 2011)

The means of performing desulphurisation vary case-specifically and depend on the ore type of the mine as well as previous metallurgical processes used in the mineral extraction. For example, a separate flotation circuit or additional flotation cells can be used. If the aim is to use depyritized tailings as cover material, their potential not to form acid drainage has to be confirmed (INAP 2009) for example with acid base accounting (ABA) test. Also kinetic testing or leach testing may provide useful information if more detailed analyses of their suitability are needed (MEND 2001).

Bois et al. (2004) have presented two alternative approaches to perform desulphurisation:

1.     Complete desulphurization:

In complete desulphurisation all tailings are desulphurised by froth flotation. As a result of the separation, an acid generating high sulphur fraction with a reduced volume, and a high volume of potentially non-acid generating low sulphur fraction are formed. Low sulphur non-acid generating tailings do not represent a long term liability, which is the most important advantage of the method. (Bois et al. 2004)

2.     Partial desulphurisation:

Partial desulphurisation means that the tailings fraction is desulphurised only during a few years period before the mine closure. Non-acid generating tailings can be used as an inert dry cover material to cover acid generating tailings. The layer of 1 to 2 meters of desulphurised material acts as an elevated water table and keeps sulphide rich tailings saturated. As the tailings remain saturated also an oxygen barrier will form, thus limiting oxygen diffusion even further. (Bois et al. 2004)

Case studies are presented by:

Sjoberg Dobchuck et al (2003): A case study at the Detour Lake Mine in Ontario, Canada where a 1-1.5 m thick engineered cover constructed of desulphurised tailings was used to cover sulphide bearing tailings.

INAP (2009): Strathcona Tailings, Ontario, Canada, the use of desulphurised tailings cover

Technique is to planned to be applied at the Aitik Cu mine in Sweden during the last years of ore processing to produce low sulphide tailings to be used at the covering of the high sulphide tailings.

Appropriate applications

The method is suitable for tailings. The suitability is strongly dependent on the tailings characteristics and has to be analysed case-specifically. The flotation process has to be able to remove sulphide minerals to an extent that desulphurised tailings are not able to form acid drainage. (INAP 2009) According to INAP (2009) the method is widely demonstrated to be effective in temperate and mid-latitude climates.

Performance

Different studies have proven the technical feasibility of the method. Also the cost aspects of desulphurisation are widely studied. For example, the studies of Benzaazoua et al. (2000), Benzaazoua & Kongolo (2003), Bois et al. (2004) and MEND (2001) have proven the method to be economically comparable to other existing techniques used for tailings management.

The use of desulphurisation can reduce reclamation costs due to the reduced transportation and material costs (E.g. Benzaazoua et al. 2000, MEND 2001, Benzaazoua & Kongolo 2003, Kongolo et al. 2004). Although the costs of desulphurisation may be high itself, low sulphur tailings can be potentially used as a cover material, which reduces the costs compared to the conventional deposition methods where materials are often transported from elsewhere. (INAP 2009) According to European Commission (2009), the viability of the method depends on the amount of sulphide minerals that needs to be removed, because too high sulphide content generates negative cost impacts.

The costs of desulphurisation can be divided into capital costs and operating costs. According to the study of Benzaazoua et al. (2000), the operating costs for three mine sites studied varied between CAD$ 0.27 and CAD$ 0.55, and the estimated capital costs were around one million Canadian dollars in each. Benzaazoua & Kongolo (2003) estimated the operating costs of desulphurization to be CAD$ 0.35 /t for the two tailings studied. The capital costs were also around one million Canadian dollars in this study.

Bois et al. (2004) performed a comparative cost analysis for four different tailings disposal option. The results of the study are presented in Table 1. According to their results, an underwater disposal is the best option economically if the site topography and base are appropriate. However, the difference in costs between underwater disposal and partial desulphurisation was only a minor, especially if NP (neutralisation potential) of the tailings is low. A partial desulphurisation can generate cost savings if the tailings pond is located in flat topography site and a soft base (normally the costs for dam construction are high in these kinds of cases). The operation costs for partial desulphurization are lower because only a part of the tailings is treated. The size of the capital investment is the same as in complete desulphurisation (a flotation circuit at the end of the process circuit). Complete desulphurisation of tailings is economically viable if the construction of low permeability tailings dams becomes expensive. (Bois et al 2004)

Table 1. Results of the cost study analysis (in $ / ton of tailings). Note: The results are valid only for new operations. (Bois et al. 2004, modified)

References

Benzaazoua, M. & Kongolo, M. 2003. Physico-chemical properties of tailing slurries during environmental desulphurization by froth flotation. International journal of mineral processing 69, 221-234.

Benzaazoua, M., Bussière B., Kongolo, M., McLaughlin, J. & Marion, P. 2000. Environmental desulphurization of four Canadian mine tailings using froth flotation. International journal of mineral processing 60, 57-74.

Bois, D., Poirier, P., Benzaazoua, M., Buissière, B. & Kongolo, M. 2004. A Feasibility Study on the Use of Desulphurized Tailings to Control Acid Mine Drainage. Proceedings 2004 – 36^th Annual Meeting of the Canadian Mineral Processors.

European Commission 2009. Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities, January 2009. Directorate-General JRC Joint Research Centre. Sustainability in Industry, Energy and Transport. European IPPC Bureau.

Hesketh, A.H., Broadhurst, J.L. & Harrison, S.T.L. 2010. Mitigating the generation of acid mine drainage from copper sulfide tailings impoundments in perpetuity: A case study for an integrated management strategy. Minerals Engineering 23, 225-229.

INAP 2009. The GARD Guide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com

Kauppila, P., Räisänen, M.L. & Myllyoja, S. 2011. Best Environmental Practices in Metal Ore Mining. Finnish Environment 29 en/2011.

Kongolo, M., Benzaazoua, M., de Donato, P., Drouet, B. & Barrès O. 2004. The comparison between amine thioacetate and amyl xanthate collector performances for pyrite flotation and its application to tailings desulphurization. Minerals Engineering 17, 505-515.

MEND 2001. MEND Manual, Volume 4. Prevention and Control. MEND Report 5.4.2d. February 2001. Editors: Tremblay, G.A. & Hogan, C.M. Mine Environment Neutral Drainage Program (MEND).

Sjoberg Dobchuk, B., Wilson, G.W. & Aubertin, M. 2003. Evaluation of a Single-Layer Desulfurised Tailings Cover. In: Proceedings of 6th International Conference Acid Rock Drainage (ICARD), July 14-17, Cairns, QLD, Australia, AusIMM.