Modeling Approach - Flowchart for an In-situ Acid Mine Drainage Neutralization

Transcription

1 Modeling Approach - Flowchart for an In-situ Acid Mine Drainage Neutralization Batshiku Basika and Robert Huberts Abstract Mining disturbs geologic formations that took millions of years to form; once disruption has taken place, a variety of problems may arise, from physical hazards to pollution of water and soil. The main goal of this paper is to demonstrate the technical feasibility of ferric and ferrous ions in-situ removal from Acid Mine Drainage and to design suitable flowcharts that can be used to model in-situ neutralization. The principle is based on lime neutralization without agitation that consists of diffusion and neutralization of hydrogen ions within the solution, which will increase the ph and leads to diffusion and precipitation of ferric and ferrous ions. Experimental tests were conducted with raw Acid Mine Drainage solution from Randfontein Mine (Johannesburg, South Africa) and the method shows that it is possible to achieve about 90-98% efficiency for ferric and ferrous ions in-situ removal from Acid Mine Drainage. Keywords Acid Mine Drainage (AMD), Flowchart, Modelling, Neutralization. A I. INTRODUCTION MD is considered as one of the largest environmental problems facing the mining industry today [1]. It can occur as a natural process whereby sulphuric acid is produced when sulphides in rocks are exposed to air and water. When the environment becomes severely acidic the result is usually a drastic decline in biodiversity because most kinds of life are adapted to live in environments that are neutral or slightly acidic in nature (ph between 5.5 and 7.5) [2]. That is why AMD should not be released into the open environment. In some cases, drainage occurs that is not acidic; this is possible where the geology is rich in carbonates so that effluent will become closer neutral because carbonate in the overburden neutralizes acid before the drainage emanates, the alkaline material may be sufficient to reduce the occurrence of oxidation from or to neutralize the acid from pyrite. Higher alkalinities also help to control bacteria and restrict solubility of ferric iron, which are both known to accelerate acid generation [3]. Using this theory, our study focuses on cases where the mine voids are not yet flooding with water, by Batshiku Basika. Department of Chemical Engineering Technology, University of Johannesburg, Beit Street Doornfontein Campus, PO Box Doornfontein 2080 Johannesburg, South Africa (corresponding author. Tel ; d_basika@yahoo.fr). Robert Huberts. Department of Chemical Engineering Technology, University of Johannesburg, Beit Street Doornfontein Campus, PO Box Doornfontein 2080 Johannesburg, South Africa ( roberth@uj.ac.za). placing bags of ground limestone in sections of the mine, soon to be abandoned, or even as mine tunnels are dug. Once the mine starts flooding, the AMD is neutralized in-situ; water ph is increased about 8. At that ph, most toxic metals become insoluble and precipitate. And, because there is no fresh production of AMD due to the lack of oxygen, the water rising the surface will be neutralized and offer a much better quality. The provision of limestone could potentially be treated as an operating cost, and in this way the cost of AMD prevention is not delayed until after the end of the mining project. II. BACKGROUND A. Acid Mine Drainage generation The acid generation process consists of three phases: Initiation; Propagation; and Termination. The initiation phase can begin as soon as pyritic materials are exposed to an oxidizing environment; however, the acid load generated is relatively small. In the propagation phase, acid production increases rapidly. In the termination phase, acid production gradually declines [4]. The basic reactions to be considered are: In the initial step, pyrite reacts with oxygen and water to produce ferrous ions, sulphate and acidity. 2FeS 2 + 7O 2 + 2H 2 O = 2Fe SO H + (1) Pyrite + Oxygen + Water = Ferrous ion + Sulphate + Acidity The second step involves the conversion of ferrous ion to ferric ion, which occurs when sufficient oxygen is dissolved in the water or when the water is exposed to sufficient atmospheric oxygen [4, 5]. 4Fe 2+ + O 2 + 4H + = 4Fe H 2 O (2) Ferrous ion + Oxygen + Acidity = Ferric ion + Water The third step involves the hydrolysis of ferric ion with water to form the solid ferric hydroxide and the release of additional acidity [4, 5]. 4 Fe H 2 O = 4Fe(OH) H + (3) Ferric ion + Water = Ferric Hydroxide (yellow boy) + Acidity The fourth step involves the oxidation of additional pyrite or other metals by ferric ion. FeS Fe H 2 O = 15Fe 2+ +2SO H + (4) Pyrite + Ferric ion + Water = Ferrous ion + Sulphate + Acidity 48

2 B. Mechanism of Acid Mine Drainage neutralization The neutralization process of AMD can be described as the replacement of the undesirable cation components (H +, Fe 2+, Fe 3+, Al 3+, etc.) by a more acceptable cation which is Ca 2+ in the limestone neutralization process [6]. The Overall reaction of metal-acid wastewater neutralization process by limestone can be written as follows: CaCO 3 + 2H + = Ca 2+ + H 2 O + CO 2 (5) Equation (5) represents the condition where dissolution of calcite produces "alkalinity" in excess of "acidity" production and raises the ph above 6.4. The implication of reaction (6) below is that acidity produced from 1 mole of FeS 2 (64 grams of S) can be neutralized by 2 moles of CaCO 3 (200 g), or 1 g S to g CaCO 3. FeS 2 + 2CaCO O H 2 O = Fe(OH) 3 + 2SO Ca CO 2 (6) The acid-base accounting (ABA) evaluates the balance between acid generation processes and acid neutralizing processes. The values arising from the ABA are referred to as the maximum potential acidity (MPA) and acid neutralizing capacity (ANC). They are only used to predict the potential of mine to produce acid [7]. Several researchers have arrived independently at the conclusion that equal quantities of ANC and MPA (computed by multiplying the total sulphur in percent by a factor of 3.125) do not prevent AMD. The new method presumes 4 moles of CaCO 3 are required to neutralize the MPA produced by the oxidation of 1 mole of FeS 2. Therefore, the multiplication factor for computing MPA from the overburden sulphur concentration, in weight percent, should be increased from to 6.25 as considering in the stoichiometry of the following overall reaction [7]: FeS 2 + 4CaCO O H 2 O = Fe(OH) 3 + 2SO Ca HCO 3- (7) C. Mass diffusion and mathematical model Mass transfer is mass transit as the result of a species concentration difference in a mixture. Mass diffusion always occurs in the direction of decrease concentration until there is no net transport of species across the imaginary plane [8]. Fick s first law of diffusion represented by the equation below can be used to model the diffusion of elements within the AMD solution because it is the more fundamental, straight forward way to model diffusion processes. (8) the sign (-) means that the diffusion is taking place from the high concentration to the low concentration. N is the rate of diffusion in mol/m 2 s. D is the diffusion coefficient in m 2 /s.. ΔC is the concentration difference between two sections at a given time, in mol/m 3. ΔX is the film thickness in m. For small values of ΔC and ΔX after a short time ΔT: (9) A is the membrane surface in m 2. The difference in the number of moles diffusing between two sections will be: (10) ΔC 1 is the concentration difference between the current and the previous section in the direction of diffusion. ΔC 2 is the concentration difference between the next section and the current section. Δn 1 is the number of atoms diffusing in the previous diffusion. Δn 2 is the number of atoms diffusing in the current diffusion. The change of concentration after a given time will be calculated by: From equation (11) we can generalize: C n is the molar concentration in the current section. C n-1 is the molar concentration in the previous section. III. EXPERIMENTAL PROCEDURE (11) (12) The neutralization process of AMD in the lab was done for 2 months, where poly vinyl chloride (PVC) pipes were toped up with raw AMD. A total of three different samples were prepared with different pipe lengths (sample1: 0,5m, sample2: 1m and sample3:2m). Figure 1 below shows the lab setup of the experiment. PVC cap Raw AMD solution Permeable membrane PVC cap PVC pipe (50 mm diameter) Limestone Fig. 1 Illustration of the lab setup of AMD neutralization process The total sulphur in the sample was determined using an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The result helped to determine the quantity of limestone to be used for each pipe for the neutralization process, using equation (7). Concentrations of ferrous and ferric ions were obtained using the potassium dichromate titration method and the ph was monitored using a ph meter. 49

3 IV. RESULTS AND DISCUSSIONS A. Lab experiments - ph Fig.2 below shows the relationship between ph and time. It was observed that ph for sample1 increased rapidly in the first week and then reached a plateau of about 8.2. After that it remained more or less stable. The ph for samples 2 and 3 have both increased slowly but steadily over the two months and reached respectively maximum values of 7.29 and These results can be explained by the impact of the column length. The reaction of sulphuric acid with limestone is controlled by hydrogen diffusion in the ph range of One explanation is that the hydrogen ion, despite its small size, is diffusing faster in that ph range and it is not strongly hindered by the accumulation of Ca 2+, CO 2, CO 3 2- and HCO 3-. The only species that could slow down the reaction is the presence of iron and aluminum ions [6]. The hindering of H + by iron ions is less effective in sample1 than samples 2 and 3 probably due to the difference in the concentration of iron ions create during the neutralization process. Fig. 3: variation of [Fe 3+ ] with time - Ferrous ions Fig.9 shows the relationship between the concentration of Fe 2+ and time. It was observed that concentration of Fe 2+ for sample 1 decreased rapidly in the first two weeks due to the rapid increase of ph during the same period. Concentrations of Fe 2+ for samples 2 and 3 have both decreased slowly during the first 5 weeks. The decrease of Fe 2+ concentration when ph is below 6 can be explained the same way as for Fe 3+ as explained for fig.3. Oxidation of Fe 2+ to Fe 3+ in presence of atmospheric oxygen can also be one of the reasons of the decrease of Fe 2+ when ph is still below 6; Fe 2+ will oxidize to Fe 3+ and then Fe 3+ will precipitate when the ph reached the value of 3. Fig. 2 variation of ph with time - Ferric ions Fig.3below shows the relationship between the concentration of Fe 3+ and time. Concentration of Fe 3+ for sample 1 decreased rapidly in the first week. This can be due to the rapid increase of ph that leaded to the precipitation of Fe 3+ when ph raised the value of 3. For samples 2 and 3, concentrations of Fe 3+ have both decreased quite at the same rate over the two months; this is due to the same rate of ph increase for both samples. The decrease in Fe 3+ concentration when the ph is still below 3 can be due to the diffusion process; the ph at the bottom of the pipe (interphase solutionlimestone) will quickly increase to the maximum value because of being in direct contact with limestone; this will then lead to the neutralization of Fe 3+ at the interphase and that will then create a difference in Fe 3+ concentration between the interphase and the previous section of the pipe and favors the diffusion process of Fe 3+. Fig. 4: variation of [Fe 2+ ] with time B. Designed flowcharts of modeling approach The modeling approach developed for H +, Fe 3+ and Fe 2+ diffusion and precipitation are represented in different flowcharts below: 50

4 A nx [H + ] nx =10-3 (enough ferric can precipitate to maintain a ph of 3) (A nx )>=3? [H + ] nx =A nx B 2 (no ferric or ferrous precipitate) B 2 > 0? (A nx )>=6? (either no ferric is left or not enough is present to maintain the a ph of 3) C 2 C 2 > 0? (enough ferrous can precipitate to maintain a ph of 6) [H + ] nx =10-6 (ph will be between 3 and 6) [H + ] nx = A nx +3[Fe 3+ ] (n-1) x [H + ] nx = A nx +2[Fe 2+ ] (n-1) x 3[Fe 3+ ] (n-1) x is only added once as the ph rises above 3. It then becomes zero. 2[Fe 2+ ] (n-1) x is only added once as the PH rises above 6. It then becomes zero Fig.5: Flow chart of modelling approach for H + A nx & B 1 (A nx )<3? [Fe 3+ ] nx =B 1 (ph will be >= to 3) (no precipitation of Fe 3+ only diffusion) B 2 B 2 > 0? [Fe 3+ ] nx =B 2 (there is no Fe 3+ left to diffuse or precipitate) [Fe 3+ ] nx =0 (both diffusion and precipitation of Fe 3+ are taking place) Fig.6: Flowchart of modelling approach for Fe 3+ 51

5 A nx & C 1 (A nx )>6? (ph will be >= to 6) (no precipitation of Fe 2+ only diffusion) C 2 C 2 > 0? (there is no Fe 2+ left to diffuse or precipitate) [Fe 2+ ] nx =0 (both diffusion and precipitation of Fe 2+ are taking place) [Fe 2+ ] nx =C 1 [Fe 2+ ] nx =C 2 Fig.7: Flow chart of modelling approach for Fe 2+ A nx = [H + ] (n-1)x (D + H )*(ΔT/ΔX 2 )*(([H + ] (n-1)x - [H + ] (n-1)(x-1) ) ([H + ] (n-1)(x+1) - [H+] (n-1)x )) (12) A nx is the concentration of H + at a given time B 1 = [Fe 3+ ] (n-1)x (D 3+ Fe )*(ΔT/ΔX 2 )*(([Fe 3+ ] (n-1)x - [Fe 3+ ] (n-1)(x-1) ) ([Fe 3+ ] (n-1)(x+1) - [Fe 3+ ] (n-1)x )) (13) B 1 is the concentration of Fe 3+ for a given time considering only the diffusion of Fe 3+ B 2 = [Fe 3+ ] (n-1)x - (D 3+ Fe )*(ΔT/ΔX 2 )*(([Fe 3+ ] (n-1)x - [Fe 3+ ] (n-1)(x-1) ) ([Fe 3+ ] (n-1)(x+1) - [Fe 3+ ] (n-1)x )) (1/3*( A nx )) (14) B 2 is the concentration of Fe 3+ for a given time considering the diffusion and the precipitation of Fe 3+ C 1 = [Fe 2+ ] (n-1)x (D 2+ Fe )*(ΔT/ΔX 2 )*(([Fe 2+ ] (n-1)x - [Fe 2+ ] (n-1)(x-1) ) ([Fe 2+ ] (n-1)(x+1) - [Fe 2+ ] (n-1)x )) (15) C 1 is the concentration of Fe 2+ for a given time considering only the diffusion of Fe 2+ C 2 = [Fe 2+ ] (n-1)x (D 2+ Fe )*(ΔT/ΔX 2 )*(([Fe 2+ ] (n-1)x - [Fe 2+ ] (n-1)(x-1) ([Fe 2+ ] (n-1)(x+1) - [Fe 2+ ] (n-1)x )) (1/2*( A nx ) (16) C 2 is the concentration of Fe 2+ for a given time considering the diffusion and the precipitation of Fe 2+ n= Time; x= Length; D= Diffusion coefficient CONCLUSION The method used has proved that AMD can be neutralized in-situ and the water rising to the surface will be neutralized and offer a much better quality. The modelling approach dev eloped can help for modeling, using software like Matlab, to predict the ph, ferric and ferrous concentration at any given time and depth. ACKNOWLEDGMENT The authors gratefully acknowledge the Project, Energy and Environment Technology Station of the University of Johannesburg. REFERENCES [1] Fernandes, H.M.; Franklin, M.R. Acid mine drainage as an important mechanism of natural radiation enhancement in mining areas, Instituto de radioprotecao e dosimetria, Rio de Janeiro (Brazil), [2] Stephen McGinness, Treatment of acid mine drainage, Science and environment section, House of Commons library, London. Research paper 99/10. 2nd, pp. 9-11, February [3] J. Skousen & all, A handbook of technologies for avoidance and remediation of Acid Mine Drainage, The national Mine Land Reclamation Center, West Virginia university, Morgantown, West Virginia, June pp [4] Christine Costello, Acid mine drainage- Innovative treatment technologies, National network of environment management studies fellow. For U.S. Environmental protection agency office of solid waste and emergency response. Technology Innovation Office Washington, DC. October pp. [5] Aljoe W.W. and J.W. Hawkins, Hydrologic Characterization and In- Situ Neutralization of Acidic Mine Pools in Abandoned Underground Coal Mines, in Proceedings Second International Conference on the Abatement of Acidic Drainage, September 16-18, 1991, Montreal, Canada, Volume 1, pp. [6] Paul Barton and Vatanatham Terdthai, Kinetic of limestone neutralization of acid water, Environmental science and technology, Volume 10, Number 3, 1976, 10(3), pp. [7] K.L. Ford, Passive treatment systems for acid mine drainage, U.S. Bureau of Land Management Papers. Paper 19. National Science and Technology Center, USA, April [8] Bergman, T.L., Dewitt, D.P., Incropera, F.P &Lavine, A.S. (2007): Fundamentals of Heat and Mass Transfer. United States of America, John Wiley&Sons,2007, pp880 52