Environmental geosciences Predictive methods for assessment of the chemical composition of drainage from sulfide mine waste

Edelev A.V.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia

EdelevAV@ipgg.nsc.ru

The development of sulfide deposits causes the formation of acid mine drainage (AMD), and this is one of the most significant environmental problems caused by mining activities. The ore concentration produces large amounts of waste (overburden, tailings etc.) that contain sulfide minerals. Oxidation of iron sulfides by air oxygen in the presence of moisture leads to the formation of acidic metal-laden effluents. The overall reaction of acid formation in the oxidation of pyrite (FeS2), one of the most common sulphide minerals can be presented as follows:

4FeS₂ + 15O₂ + 14H₂O => 4Fe(OH)_3(solid) + 16H⁺ + 8SO₄^2- (1)

Oxidation of sulfides is a long-term source of hydrogen cations. But there is a short-term and quick source of H+. The minerals precipitate during evaporation of acidic, metal- and sulfate-laden water within mine-waste materials and store (for potential subsequent release) acid generated by iron sulfide oxidation. The minerals can produce acid as a result of dissolution and hydrolysis (2). As usual, they are sulfate salts of iron and aluminum (coquimbite ((Fe,Al)(SO₄)₃∙9H₂O), copiapite (MgFe₄(OH)₂[SO₄]₆∙20H₂O), jarosite (KFe₃(SO₄)₂(OH)₆) et al.).

(Fe,Al)(SO₄)₃∙9H₂O => Al(OH)_3(Solid) + 3SO₄^2- + 6H⁺ + 3H₂O + Fe(OH)_3(solid) (2)

The most common minerals that can effectively neutralize the acidic water are carbonates (calcite CaCO₃, magnesite MgCO₃, dolomite Ca_0.5Mg_0.5CO₃ et al.). The most effective of them is calcite. The scheme reaction for carbonate with acid, which is produced in the reactions 1 and 2, is presented in reaction 3.

2H⁺ + XCO₃ =>X²⁺ + H₂O + CO_2(aq), where X=Ca²⁺, Mg²⁺ (3)

At the same time with monitoring of sulphide-containing mine waste, the great importance has the studies concerning further interaction between naturally occurring waters and sulphide-containing mine wastes as well as the development of methods for prediction of acid mine drainage. The level of potential hazard of mine waste for environment causes the choice of nature protection measures in the manipulation with wastes. Studies on particular objects promotes both obtaining a more complete information concerning them, and using the established relationships for other similar systems.

The aim of present work is establishing interrelation between experimentally obtained physicochemical analytical results concerning the substance waste (paste pH, acid producing and acid neutralizing potentials) and the chemical composition of drainage waters (total ion concentration of heavy metals such as iron, zinc, copper, cadmium, nickel, lead).

The analysis of the substance of sulphide waste products was carried out for the samples of overburden rocks of the Veduga gold deposit (Krasnoyarsk Kray); the samples of overburden rocks of the Taseevo gold deposit (Trans-Baikal Kray); the samples of clinkers of the Belovo Zinc Plant; the samples of tails of the Komsomolsk Gold Recovery Plant; the samples of tails of the Salair Ore-Dressing and processing Enterprise (Kemerovo Oblast). The objects differ from each other in the content of sulphide and carbonate minerals, in the age of wastes, in the technological process of ore-dressing and, as a consequence, in the chemical composition of wastewaters and the solutions of tailings ponds.

In the work various methods were used for the investigation of rock acid production. For all the samples paste pH values, acid producing and acid neutralizing potentials (AP and NP, respectively) were determined.

On the basis of data concerning the acidity of paste it could be estimated the ability of the substance to produce acid as well as to neutralize acid in the course of interaction with water. "Paste" is meant to be a highly concentrated suspension consisting of fine-dispersed sample and distilled water. Preparing the pastes and measuring pH values were carried out in concordance with a technique described in [1].

The analysis of the acid-base ratio used for the estimation of the potential drainage includes the determination of AP and NP and their comparison among themselves.

The acid producing potential (AP) represents the estimation of the maximal acid generation level in the sulphide waste. It is supposed, that the process of oxidation occurs according to reaction (1). The value of AP is determined from the content of sulphide sulphur. The AP value can be expressed as the amount of CaCO₃ (in kg/t) necessary for H⁺ neutralization (It is supposed the neutralization process occurs according to reaction (3)).

The acid neutralizing potential (NP) allows one to estimate the ability of rocks to work as neutralizing barriers. The value of NP is determined from the content of carbonate. The value of NP is determined by the amount of CaCO₃ (in kg/t); this value always exceeds or corresponds to the content of calcite in substance.

Dynamic experiments adequately simulating the interaction of rock with water flows were carried out with the samples of the Veduga deposit overburden rock. Peroxide experiments were carried out with the samples of the Taseevo deposit overburden rock. The conditions of the experiments were aimed at the acceleration of sulphide oxidation in order to extrapolate the process for long term.

Different mineral composition, the technologies of extraction processes for components required, conditions for storing the substance of waste piles determine different ratio values between AP and NP and paste pH values. There is an interrelation observed between AP, NP, paste pH value and the total content of metal ions in drainage waters.

When the acidity level of the paste corresponds to subalkaline medium, the total content of metal ions in wastewaters is rather low (less than 1 mg/L). A low paste pH value indicates that the total content of metal ions in drainage wastewaters is rather high (more than 100 mg/L).

The results of dynamic and peroxide experiments allow to make the following conclusions. The value of ratio NP and AP correlates well with the total content of metal ions in the solution at the end of dynamic experiments and in the solution of peroxide experiments.

The predictive estimate of the total content of metals (Zn, Cu, Cd, Fe, Ni, Pb) in drainage wastewaters resulting from ore-processing industry waste for the near future (days, weeks) can be performed basing on the paste medium pH value, whereas a long-term prediction (months, years) could be made basing on NP and AP ratio.

References:

1. Sobek, A.A., Schuller, W.A., Freeman, J.R. Smith, R.M. (1978) Field and Laboratory Methods Applicable to Overburden and Minesoils, U.S. EPA 600/2-78-054, 203 pp.