Chemical passivation using microencapsulation

Päivi M. Kauppila, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, Finland, e-mail: paivi.kauppila(at)gtk.fi

Introduction

Generation of acid mine drainage due to the oxidation of sulphide minerals is one of the key environmental problems related to mine wastes. Once the oxidation induced acid production starts, it may continue for hundreds of years. A possible way to control and reduce the acid production potential is to remove those components that promote oxidation: i.e. oxygen and ferric iron, water, bacteria or the sulphide minerals (ITCR 2010).

Passivation methods aim at preventing the oxidation of reactive mineral surfaces by isolating the reactive or acid-generating rock or mineral surface in such a way, that access of water and oxidizing agents (i.e. O₂, Fe³⁺) into the surfaces is restricted. According to Evangelou (2001) this could be the only way to control the long-term pyrite oxidation. By creating a chemically inert and protective surface using passivation compounds or materials, the leaching of oxidation products from the surface is also limited (Eger and Mitchell 2007, INAP 2009, ITCR 2010). Passivation methods are regarded as promising proactive, source-inhibition technologies opposite to the conventional end-of-pipe techniques, such as collection and treatment of effluents, covering or disposing waste underwater (Eger and Mitchell 2007, Sahoo et al. 2013).

Various passivation methods applying plastics, polymers, cementation or a variety of inorganic and organic chemicals and compounds as a passivation agent have been trialled (ITCR 2010, Sahoo et al. 2013). Microencapsulation typically refers to the sealing techniques using inorganic chemicals (Evangelou 2001, Eger and Mitchell 2007) and is the focus of this evaluation. However, despite the studies in the laboratory and in the field (e.g. Evangelou 2001, Eger & Mitchell 2007, Mauric & Lottermoser 2011), the technology is quite recent and experiences of the long-term performance of these methods are only scarce and the methods haven’t still been used in practice (INAP 2009).

Method description

In the microencapsulation, the reactive or acid-generating rock or mineral surface is sealed or coated with a passivation chemical to restrict the access of water and oxygen into the surface – and to prevent the oxidation of the surface. Formation of chemically inert and protective surface using passivation chemical subsequently limits the leaching of oxidation products from the surface (INAP 2009, ITCR 2010).

Sealing layer can be created with a variety of inorganic chemicals, such as phosphate, silica or permanganate (ITCR 2010), which are sprayed on the surface as a solution. A ferric coating is precipitated as a result of the chemical addition on the surface of the reactive material to prevent the transport of oxidants into the surface. The precipitation of the coating consumes ferric iron (Fe³⁺) so that there is no Fe³⁺ left to become an oxidant (Evangelou 2001, ITCR 2010). In the method presented by Evangelou (2001), H₂O₂ was used as an additional oxidant chemical to increase the pyrite oxidation and the amount of released Fe³⁺ to facilitate the precipitation of iron phosphate and iron silicate coating. Table 1 presents short descriptions of a choice of microencapsulation methods.

Table 1. Microencapsulation methods experimented to prevent sulphide oxidation and production of AMD.

Microencapsulation techniques have only been studied in laboratory experiments or in small scale field trials and there are apparently no case studies where these technologies would have been adapted in a full scale (e.g. INAP 2009, ITCR 2010).

Appropriate applications

Passivation methods are potential techniques to be used in the prevention or minimization of AMD from pit wall surfaces, underground mine workings and waste rock dumps. They are typically easy-to-use spray-on techniques in which chemicals are applied as a solution or slurry to treat the reactive surfaces. With these techniques the generation of AMD can be prevented at the source, thus reducing the need for effluent treatment both during operation and mine closure. (ITCR 2010).

Coatings created with microencapsulation are expected to be very stable and therefore to provide a long-term solution for the prevention of AMD. However, data of the long-term performance of these techniques is still lacking. In addition, microencapsulation can treat only those surfaces that are reachable with the chemical solution. This makes the success of the technique uncertain for example in large existing waste rock piles. Microencapsulation may also increase the amounts of some unwanted substances in the mine water or mine waste effluents as a result of the use of various chemicals. (ITCR 2010) For example, increased levels of phosphate, arsenic, and sulphate have been detected in the rinse water of phosphate treatment, and silica treatment applying lime has resulted in an increase of the effluent pH (Nordwick et al. 2006, Eger and Mitchell 2007, ITCR 2010). Finally, the trials have mostly been carried out using chemical grade analytical reagents, which can make the technique expensive in large scale (Mauric & Lottermoser 2011).

Major advantages and disadvantages of the microencapsulation techniques are summarized in Table 2.

Table 2. Advantages and disadvantages of the microencapsulation techniques (McCloskey et al. 2005, ITCR 2010)

Performance

Performance of microencapsulation techniques has been tested in numerous laboratory studies and in a few field-scale experiments. Laboratory tests have shown that coatings created using phosphate, silica and permanganate based compounds area able to prevent or delay the onset of the sulphide oxidation in short term, i.e. for some days or for some weeks (e.g. Huang & Evangelou 1994, Eger & Mitchell 2007, Sahoo et al. 2013). However, the silica coating succeeded to prevent AMD for even 6 years in a laboratory experiment in which crushed unoxidized waste rock was tested in a humidity cell (Eger & Mitchell 2007). According to Sahoo et al. (2013) silica coating is indeed one of the most promising microencapsulation techniques due to the high availability, and thus low cost, of silica, maintenance free usage in the field (low requirements for monitoring and process adjustment), and negligible side effects (no leaching of environmentally harmful compounds).

There are only a few field studies evaluating the performance of the microencapsulation techniques and thus more data would be needed on their long-term effectiveness. However, for example U.S. Environmental Protection Agency (EPA) (McCloskey et al. 2005, Nordwick et al. 2006) and Mauric & Lottermoser (2011) have trialled phosphate and silicate based compounds in a field scale to prevent AMD. Within these experiments, permanganate treatment has performed best in reducing leaching of metals and sulphate from the waste materials and pit walls (McCloskey et al. 2005, Nordwick et al. 2006). According to Nordwick et al. (2006) permanganate method is cost effective compared to the Ecobond and KEECO methods and is also expected to preserve well with time. Overall, the performance of the microencapsulation techniques has been worse in the field scale than in the laboratory tests in which conditions are easier to control (Nordwick et al. 2006). In the field scale, problems have arrived e.g. from leaching of unexpected compounds (SO₄, As, TDS), formation of preferential flow paths in the waste piles, and from too low dosage of the passivation chemical (Norwick et al. 2006, Mauric & Lottermoser 2011). Apparently, occasional retreatment of e.g. the pit walls with the passivation solution would be needed to ensure permanent coating on the surfaces. Summary of the performance of some of the microencapsulation techniques tested in field scale is presented in Table 3.

Costs of the microencapsulation techniques vary largely depending on the chemicals used in the technique and the need to repeat the treatment (USEPA 2006). U.S. EPA has calculated some theoretical costs for the microencapsulation methods tested in the field experiments (McCloskey et al. 2005). Based on these calculations costs for Ecobond were 8.50 USD/m³ of waste rock and for UNR permanganate 10.50 UDS and for KEECO silicate 33 UDS/m³ of waste rock, respectively, when treating some 380 000 m³ of waste rock (Nordwick et al. 2006). For pit walls the treatment cost was 3.65 USD/ft² for UNR magnesium oxide and UNR permanganate treatment and 7.63 USD/ft² for Ecobond treatment (McCloskey et al. 2005).

Table 3. Summary of the performance of a choice of encapsulation techniques in the field tests.