Towards a minimum impact copper concentrator: A sustainability assessment

Transcription

1 Towards a minimum impact copper concentrator: A sustainability assessment Tuukka Kotiranta, Susanna Horn, Kaj Jansson (*), and Markus A. Reuter Outotec Oy, Finland ABSTRACT The aim of this study has been to improve the understanding the sustainability of a minimum impact copper concentrator during estimated 15 years long mine lifetime As background information from earlier studies of minimum impact Cu concentrator operational risk results has been used. These operational risk was focused on decreasing water stress, environmental hazards, and possible dam break-up. In this paper, the main focus has been set on conceptual study on simulating and calculating economics of four different tailings management facility methods and their impact on the site water balance as well as the sustainability assessment. The selected tailings treatment flowsheets that form the basis of this study are (i) Case 1: conventional (unthickened), (ii) Case 2: thickened, (iii) Case 3: paste and (iv) Case 4: filtered tailings options. All mass balance calculations where performed using Outotec s HSC Chemistry simulation tool, that provided the basis for the cost calculation. The costs were estimated based on Finnish cost basis with an assurance of ±30% As location for the concentrator a flat mine site in temperate climate zone was selected. This showed that the filtered option decreases the water footprint significantly i.e. from 26.7 to 5.8 kg blue water/kg concentrate produced (or 0.51 to 0.11 kg blue water/kg ore feed) as well as improving the water quality and toxicity indicators. These simulation results were also used to perform an environmental assessment using the GaBi software. There was no significant carbon footprint and acidification potential variation between the four cases kg CO 2 -eq/ca. kg blue water/kg concentrate or ca kg CO 2 -eq/ca. kg blue water/kg ore (when rounded). Keywords: Water management, minimum impact concentrator, tailings management, process simulation, LCA! 1!

2 INTRODUCTION Minerals processing concentrators are increasingly facing large challenges with the fresh water availability and quality, as well as new environmental limitations both from the old traditional tailings management facilities as well as the volume and quality of the discharge. The trend of lower grades in mineral deposits is also enlarging tailings management facilities size and the water consumption. In addition climate change is having an effect on water supply (IPCC, 2014). Water recycling, management in mineral industry has been studied earlier ( Joe et al ) as well as energy and water savings in tailings handling (Sellgren 1984). Sustainable disposal of mining waste has been studied by Franks Studies with full process including tailings and water management and their impact on the operational risk, economics and water savings has been discussed by Jansson et al The Water Footprint Network ( has since 2008 provided a leadership in the water footprinting numerous societal activities. Various studies can be downloaded from this site as well as various references sourced via this portal to understand the impact of human activities on the use of blue, green and grey water ranging from food production to the impact of energy carriers (Hoekstra et al., 2011, Mekonnen and Hoekstra, 2012, Dallemand, JF and Gerbens-Leenes, PW, 2013). While there are various studies on the analysis both economic and environmental of potable water systems (Fagan et al. 2011, Loubet, et al. 2014, Nair, 2014) few studies link not only energy and water but also material and thus water quality. Van Schaik et al applied a very large system optimization model that linked industry and farming water effluent south of Rotterdam and linked it to an industrial water treatment plant in Brabant as well as metallurgical processing of residues to maximize resource efficiency. This studies follows a similar approach as Van Schaik et al. (2010) in that simulation basis was used to map compounds and total flows with the objective then to perform an environmental analysis to determine the optimal economic solution but also considering the environmental impact (Reuter et al. 2015). A comprehensive sustainability indicator framework has been recently developed that combines numerous impacts into one simple result, while comparing it to a benchmark (Rönnlund et al. 2015). Water, energy and materials are thence combined and evaluated at the same time. In summary, the objective of this study is to clarify the difference between the whole mineral process operational cost during its lifetime with different layouts and tailings treatment. The focus was set on environmentally friendly solutions, with minimum fresh water usage, effluent volumes and processes without a wet tailings dam. This paper develops further the concepts presented by Jansson et al. (2014). METHODOLOGY The basic water and material balances were calculated for four different mineral process schemes with Case 1: Conventional tailings management, Case 2: Thickened tailings management, Case 3: Paste tailings management and Case 4: Filtered tailings management following the approach discussed by Reuter et al. (2015). In all mass balance calculation the fresh water intake was under focus to estimate the amount needed in the process and thus estimate the fresh (blue) water footprint ( as well as estimated effluent volumes from each system. The climate conditions for the selected site were gathered from The soil seepage capacity was estimated from data collected from mines nearby the imaginary location for this minerals processing site. All data where put into the Outotec HSC simulation tool to calculate the average flows as shown in Figure 1. The simulation tool is a steady state simulator and it calculates the situation where all material put to the process also comes out from the process and does not take into account the normal process variation. These flows where then used as a basis of the cost 2!!

3 calculations discussed by Jansson et al 2014, and also to produce the data used in the environmental footprint software GaBi ( (see Reuter et al. (2015) for details). MR BAT,%Flow%Sheets%&%Recycling%System%Maximizing% Resource%Efficiency% %Benchmarks $US%/%t%Product%(CAPEX%&%OPEX) Recyclability%Index%(based%on%system%simulation%of%whole%cycle) Energy:(GJ(&(MWh(/(t(Product((source(specific) Exergy:(GJ(&(MWh(/(t kg(co 2(/(t(Product( kg(so x(/(t(product g(no x((/(t(product m 3 (Water(/(t(Product!(including!ions!in!solution) kg(residue(/(t(product((including!composition) kg(fugitive(emissions(/(t(product kg(particulate(emissions(/(t(product Etc. Environmental%Indicators%based%on%BAT Driving%Benchmarks%of%Industry ReCiPe%(and%similar)% %Endpoint%estimation Global(Warming(Potential((GWP) Acidification(Potential((AP) Eutrification(Potential((EP) Human(Toxicity(Potential((HTP) Ozone(Layer(Depletion(Potential((ODP) Photochemical(Ozone(Creation(Potential((POCP) Aquatic(Ecotoxicity(Potential((AETP) Abiotic(Depletion((ADP) Water(footprint((Green,(Blue,(Grey) Etc... Figure 1: A summary of the methodology: Left the HSC Simulation ( of the water system for the four cases and right the environmental footprint calculation produced from the simulation results by GaBi ( (Reuter et al., 2015). Project scope As calculation basin an imaginary 20Mt/a capacity copper concentrator (porphyry Cu) was set up with an estimated 15 years life time. As a turnkey cost basis we used Finnish cost calculations and the estimated calculation level is around ±30% as net present costs. The Concentrator plant is equipped with 1 SAG mill, 2x Ball mills, 2 lines with 8 flotation cells each, 1 regrinding HIG mill, one concentrate thickener as well as 2 PF concentrate filters with a storage system. For the site conditions the concentrator was placed on flat area in temperate climate conditions. Fresh water intake was determined to be about 10 km away from the concentrator with a 25 m static head. The tailings area was also located 10 km away from the concentrator with 25 m static head. A study limitation is that no mine water, freezing conditions, dust control (wind), acid mine drainage (AMD) generated water or earth quakes were taken into account. For operational costs a fresh water price of /m 3 was used, electrical price 100 /MWh and labour cost 2960 /month. In summary, the project scope is the following: Project scope Total site costs of conventional, thickened, paste and filtered tailings (concentrator + tailings mgmt (variable) + water mgmt (variable)) Finnish cost basis Costs are based on net present costs no financing costs included! 3!

4 Accuracy ±30% Concentrator Cu (porphyry copper) Capacity 20 Mt/a (2500 t/h) 15 years mine lifetime Fresh water distance 10km, static head 25m Tailings distance 10km, static head 25m Site conditions Located in temperate climate Flat mine site No mine water, freezing, dust control or earth quakes events are taken into account Optimizing the operational costs The fundamental idea of the project was to investigate possibilities to decrease the whole operational costs of the minerals processing during its expected 15 year life time. As an illustration, Figure 2 shows estimated full costs of three alternative process layouts. It should be noticed that this figure is simply an illustration of the target setting for the project. Creating the risk matrix For the estimation of the risks allocated to the studied mineral processes, a simple risk matrix, with risk value in versus potential risk level was set up with the basic operational risks today at any typical mineral process. Figure 3 depicts various scenarios for three solution types and shows their respective risk value and level. In Figure 3 the yellow color indicates conventional tailings management, brown for paste and blue for filtered tailings. As seen from this evaluation, the highest operational risks are allocated to conventional tailings management, and lowest to the filtered tailings. The thickened tailings method was left out due to the clarity issues of figure, and because it posses very similar risk than Conventional method. Figure 2: Illustration of minimizing the total operational costs over the lifetime of a mineral processing operation. 4!!

5 l e v %le k is R h ig H m iu d e M w o L x M >B Risk%value Conventional%tailings%mgmt Paste%mgmt Filtered%mgmt 1. Dam5wall5breakage5(upstream) 2. Pollution5of5both5surface5and5ground5 water5through5seepage 3. Socio5political5acceptance55 4. Image5risk5for5whole5group 5. Water5shortage5during5dry5months 6. Risk5of55total5operation Figure 3: Simple risk matrix for estimation of operational risks associated with different tailings process. Risks related to the tailings dam wall and its constructions In order to estimate the costs related to tailings dam construction for conventional, thickened and paste following dam designs construction were estimated. It should be pointed out that the estimation was made for a completely flat surface (the high end price). It will be of benefit if there are bowl shaped earth structures available, which would lower the estimation towards zero. A naturally formed lake or bowl is the cheapest option but typically it is not part of options due to strict environmental legislation. For this study the following could be roughly estimated for the most common tailings dam types: Upstream: It has the highest risk associated to dam wall breaking with lowest initial costs, making it the most popular one. Downstream: This option has the lowest risk of dam wall breaking, thus the price is the highest due to the biggest amount of material needed for its construction. Centerline as the first modification of these previous to get a more stable construction than with the upstream model, but with lower investments than the downstream model. Modified Centerline as one more modification to get a stable construction with lesser investments than for the centerline. For the calculations in this study the Downstream model was selected to reflect the environmentally sound and sustainable option. Risks related to the climate conditions and seepages from tailings facilities Tailings facility seepage is considered as a water consumer. As said before, the concentrator was located in the temperate zone where summers are relatively hot and dry and autumns spring are wet with lots of rain. The rainfall, evaporation, water locked in to the tailings as well as the type of the soil under the tailings facility were used as an input for the calculations. Two different graphs could be taken out showing the average sum of these. Figure 4 shows the estimated seepage amount of these 4 different tailings management facilities, where the highest seepage is related to conventional and thickened ones due to wet physical appearance. As paste and filtered tailings have their water in sand as locked in by capillary forces, then the only accountable and partly recoverable seepage should come during heavy rain fall.! 5!

6 On the basis of Figure 4 data, the climate impact on average fresh water demand could be estimated. It can be seen that highest need of water is for the traditional and thickened tailings management processes. On the other hand, the filtered tailings squeezed out most of the water from the tailings and therefore has the highest capability for reusing and conserving the water m 3 /h *.During.heavy.rain.conditions Conventional. (Case.1) Thickened. (Case.2) (a)...(b) Paste... (Case.3)* Filtered... (Case.4)* Figure 4: Risk related to seepages from tailings management facilities and impact of evaporation and seepage during summer months (a) seasonal variation and (b) estimated total tailings seepage for a 20 Mt/a copper concentrator with known soil filterability. RESULTS AND DISCUSSION As the basis for the full operational costs calculation water and material balances had to be fixed and estimated. Four different estimation where made for conventional-, thickened-, paste- and filtered tailings. It should be noted that rainfall, evaporation and seepages are not shown in these simplified process calculations, due to reading clarity reasons (see Jansson et al. 2014). Case 1: The simulation results with Conventional tailings management effects on fresh water usage Figure 5 provides a specific simulation for the conventional option as shown. 6!!

7 0.51(m 3 /t(solids Ore$feed 2500(t/h Raw$Water 1646(m 3 /h Grinding(Area Required(fresh water 1286(m 3 /h 3015(m 3 /h PW(to(Flotation 60(wt>% Process(Water (PW) Process 3376(m 3 /h 35(wt>% 4574(m 3 /h 30(wt>% Fines(Removal 108(m 3 /h OF Concentrate( Thickener UF 60(wt>% 3130(m3/h Water(from(tailings(area Talings(Area Concentrate( Filter MR 70(wt>% Water(lock>up(to(tailings 48(t/h Concentrate Figure 5: Simplified water balance calculations for conventional tailings management. It should be noticed that the tailings dam gives the biggest source of variation to the process, both by rainfall, evaporation and filtration and of course lock-up. Using the simulation for sensitivity analysis one can estimate that the average need for fresh water is around m 3 /t-ore or 1400 m 3 /h with a typical 15% e.g. addition to the calculation. Thus one has to recognize that during the dry season, the fresh water need is doubled, figure 4a. Operational risks in the conventional tailings are related to the dam risks, seepages, high water usages, negative/positive water balances that are drivers for socio political risks. Case 2: The simulation results with thickened tailings management effects on fresh water usage Figure 6 shows the impact of thickened tailings. A sensitivity analysis with the simulation model showed that the thickened tailings method did not decrease the fresh water usage more than ~200m 3 /h to a level of 1200m 3 /h. This means an average fresh water usage of m 3 /t ore. From the risk management point of view same risks remains as with conventional tailings management.! 7!

8 0.44(m 3 /t(solids Ore$feed 2500(t/h Raw$Water 1646(m 3 /h PW(to(Grinding Grinding(Area Required(fresh water 1105(m 3 /h 3015(m 3 /h PW(to(Flotation 60(wtD% Process(Water (PW) OF Tailings( Thickener 35(wtD% Process 3557(m 3 /h 2558(m 3 /h UF 30(wtD% Fines(Removal 107(m 3 /h 55(wtD% OF Concentrate( Thickener UF 60(wtD% 830(m 3 /h Water(from(tailings(area Talings(Area Concentrate( Filter MR 70(wtD% Water(lockDup(to(tailings 48(t/h Concentrate Figure 6: Simplified water balance calculations for thickened tailings management. Case 3: The simulation results with paste tailings management effects on fresh water usage As the paste thickener underflow is estimated to be near 70% and the sand lock-in-capability to 30% then a very small or no effluent treatment is needed. The fresh water usage is still in the range of m 3 /h, and the average need around 1100m 3 /h including the +15% needed for the process capacity. Figure 7 shows the effect of a thickened paste option. Figure 7: Water balance calculations for paste thickening. From risk management many of the same risk as with conventional and thickened exist, however maybe not as high. If a backfilling option do exist then the operational risk are very low except of the water related issues. 8!!

9 Case 4: The simulation results with filtered tailings management effects on fresh water usage This example shows that the big opportunity for lowering the required fresh water amount comes from the removal of wet tailings. In addition this approach also minimizes seepage issues and risk through contaminating ground water, evaporation losses are minimized, nor does rainfall impact the dry stacking option in a same ways as in the previous three cases. Thus there will be a lower operational risks than the other options. Under optimal conditions the fresh water usage is estimated to be around m 3 /t ore and the fresh water flow around 310m 3 /h including the +15% as before. 0.11(m 3 /t(solids Ore$feed 2500(t/h Raw$Water 1647(m 3 /h PW(to(Grinding Grinding(Area Required(fresh water 282(m 3 /h 3015(m 3 /h PW(to(Flotation 60(wtA% Process(Water (PW) OF Tailings( Thickener 35(wtA% Process 4380(m 3 /h 4240(m 3 /h UF 55(wtA% 30(wtA% Fines(Removal 108(m 3 /h Tailings(Filter Tailings(Cake 88(wtA% OF Concentrate( Thickener UF 60(wtA% 32(m 3 /h Water(from(tailings(area Talings(Area Concentrate( Filter MR 88(wtA% Water(lockAup(to(tailings 48(t/h Concentrate Figure 8: Simplified water balance calculations for filtered tailings. The disadvantage of the filtered tailings options comes with the new and more equipment and technology needed. Thus a significant result of this study shows that the water (blue) footprint is significantly lower for Case 4 (the filtering option) as shown by Figure 9, while not impacting significantly on the carbon footprint, that will become evident from the next section. 30 kg#fresh#(blue)#water/kg#concentrate## Tailings-pond- (Case-1) Thickenedtailings-(Case-2) Paste-thickening- (Case-3) Filter (Case-4) Figure 9: A comparison of the fresh water footprint of the four cases based on kg output concentrate.! 9!

10 Environmental impact Figure 10 shows the data that was applied and the flows included in the analysis. The data have been derived from the simulation models that have been used to generate Figure 5 to Figure 8. Figure 10: The data used for the Case 4 Filter option to perform the GaBi analysis. Figure 11 depicts that there is little difference in the global warming and acidification potentials between the four options. However, it is clearly evident that Case 4, due to its less water usage, has a significant lower water footprint and also lower impact on water quality as shown by the eutrophication potential and eco toxicity, noting that the toxicity and eutrophication originate through the various in- and outflows through the system boundaries(e.g. power, diesel, lime etc.). Figure 11: The environmental impact analysis for each of the four cases, showing global warming potential (GWP), acidification potential (AP), eutrophication potential (EP-water), Eco toxicity, Water depletion and Resource Depletion (ADP) per kilogram of concentrate. 10!!

11 Summary of results The estimated calculation shows that the lowest total operational risk is achieved with the filtered tailings. If the paste backfill processes option exist,then it will have the lowest cost. It should be noted that these calculations fresh water price of /m 3 were applied. If water price is higher, then the difference of the filtered tailings to the other ones is also increasing. It should be clearly evident that that the usage of either paste or filtered tailings technology decreases the effluent generation to nearly zero levels. CONCLUSION Today new environmental challenges are emerging to the minerals processing sectors. Risk related to poor fresh water quality and scarcity, wet tailings dam and new effluent limitations are growing worldwide. All of these are setting new guidelines towards minimum impact concentrators, where the concentrator plant is carved out from the surroundings instead being an part of the it, with minimal effects on it Rich mineral deposits are diminishing, and large, low grade deposits are dominating the Greenfield mine projects. From this work it can be concluded that: Water management future is in more closed water loops and also smaller volumes to overcome higher operational risks. For conventional & thickened tailings management the selected tailings management facilities have a huge impact on the total operational risk. Paste thickening is a very good alternative with low operational risk if a backfill option does exist, but it does not solve the water management issues. Filtered tailings give in this study the lowest operational risk, with closest neutral water balance of these four studied. Large, low grade deposits have challenges in fresh water intake, and ultimate level of recycling of process water becomes standard design of new Greenfield plants.the filtering option (Case 4) shows a significant lower water footprint and impact, while the carbon footprint of all cases are more or less similar. REFERENCES Dallemand, J.F, Gerbens-Leenes, PW eds. (2013): Bioenergy and Water, European Commission, Joint Research Centre, Institute for Energy & Transport, 289p. ISBN Fagan JE, Reuter MA, Langford KJ (2010): Dynamic performance metrics of integrated urban water systems Resource Conservation Recycling, 54: GaBi 6, Software and System Databases for Life Cycle Engineering, Stuttgart-Echterdingen, ( ). Franks, D, Boger, D, Côte, C, and Mulligan, D, (2011), Sustainable development principles for the disposal of mining and mineral processing wastes, Resource Policy 36(2): GaBi 6, Software and System Databases for Life Cycle Engineering, Stuttgart-Echterdingen, ( ). Hoekstra, AY, Chapagain, AK, Aldaya, MM, Mekonnen, MM (2011): The Water Footprint Assessment Manual Setting the Global Standard, Earthscan Ltd, London, UK, 203p. ISBN: ! 11!

12 HSC and HSC Sim 7.1&8, Thermochemical and process simulation, Outotec Research Center, ( ). IPCC, Climate Change 2014 Synthesis Report Summary for Policymakers, Fifth Assessment Report (AR5). 32p. Jansson, K, Kauppi, J, Kotiranta, T, (2014): Towards minimum impact Cu concentrator A conceptual study, Materia: 4, Loubet, P, Roux, P, Loiseau, E, Bellon-Maurel, V (2104): Life cycle assessments of urban water systems: A comparative analysis of selected peer-reviewed literature, Water Research, 67(12): Jansson Kauppi, J, Kotiranta, T, (2014): Towards minimum impact Cu concentrator A conceptual study; 2014 IMPC SUSTAINABILITY SYMPOSIUM, Santiago de Chile Joe, E. (1984):Water and solution recycling practice in the Canadian mineral industry, Conference on Mineral Processing and Extractive Metallurgy (IMM and CSM), Kunming, China. Joe, E. and Pickett, D: (1974), Water reuse in Canadian ore-concentration plants-present staus, problems and progress, Minerals and the environment, Proceedings of an international symphosium, The institution of Mining and Metallurgy, London. Mekonnen, MM, Hoekstra AY (2012): A Global Assessment of the Water Footprint of Farm Animal Products, Ecosystems, 15: Nair, S, George, B, Malano, HM, Arora, M, Nawarathn, B (2014): Water energy greenhouse gas nexus of urban water systems: Review of concepts, state-of-art and methods, Resources, Conservation and Recycling, 89(8): Reuter, MA, Van Schaik, A, Gediga, J (2015): Simulation-based design for resource efficiency of metal production and recycling systems, Cases: Copper production and recycling, ewaste (LED Lamps), Nickel pig iron, International Journal of Life Cycle Assessment, 20(5): Rönnlund, I, Reuter, MA, Horn, S, Aho, J, Päällysaho, M, Ylimäki, L, Pursula, T (2015): Sustainability indicator framework implemented in the metallurgical industry: Part 1-A comprehensive view and benchmark, International Journal of Life Cycle Assessment (submitted). Sellgren, A, (1984), Energy and water savings in handling of mine tailings slurries, The Ninth International Technical Conference on Slurry Transportation, USAation and Recycling, 89(8): Van Schaik, A, Reuter, MA, Van Stokkom, H, Jonk, J, Witter, V (2010): Management of the Web of Water and Web of Materials, Minerals Engineering, 23: Water Footprint Network ( ): Various reports and literature, 12!!