Integrating series and series-parallel circuit configurations for optimum plant performance

Transcription

1 Sidney J. Archer DRA Mineral Projects (Pty) Ltd, South Africa Angus Feather Cognis Ireland Ltd part of BASF, Ireland Integrating series and series-parallel circuit configurations for optimum plant performance ABSTRACT Heap leaching of low grade copper ore has been practiced extensively, predominately in dry regions, and is associated with a low capital outlay. These operations typically employ solvent extraction to produce LME Grade A electrowon copper metal. Certain operations in Africa s Copper Belt have considered the same leaching technique as opposed to the traditional mill and agitated leach operation; however they have to contend with extreme seasonal rainfall. During the wet season (October to March) up to 1600 mm/annum rainfall can be expected resulting in dilution of leach solution copper tenors. The annual evaporation rate is only 500 mm and thus excess volume needs to be processed and disposed of. This paper describes an investigation into one of the possible processing routes of optimising a conventional solvent-extraction circuit where the copper tenor fluctuates as a result of excessive dilution or varying copper grade. The study considers the potential savings due to minimal disruption of copper production as well as the simplicity of operation with limited equipment changes. A conventional solvent-extraction series circuit can be manipulated from a partial to a full seriesparallel circuit thereby compensating the fluctuating copper tenor and increased leach solution flowrate. The manipulation of the process flow allows for the same equipment to be employed with minimal changes to organic parameters. The study uses BASF Isocalc to compare four scenarios. 3 7 October Santiago, Chile. 1

2 INTRODUCTION Heap leaching of low grade copper ore has been practice extensively, predominately in dry regions, and is associated with a low capital outlay. These operations typically employ solvent extraction to produce LME Grade A electrowon copper metal. Certain operations in Africa s Copper Belt have considered the same circuit configuration as opposed to the traditional mill and agitated leach operation; however they have to contend with extreme seasonal changes. During the wet season (October to March) up to 1,600 mm/annum rainfall can be expected resulting in dilution of pregnant leach solution (PLS) copper tenors. The annual evaporation rate is only 500 mm and thus excess volume needs to be processed and disposed of. This paper describes an investigation into one of the possible processing routes of optimising a conventional solvent-extraction (SX) circuit where the copper tenor fluctuates as a result of both excessive dilution or varying copper grade to ensure constant copper production throughout the year. A conventional SX circuit can tolerate limited fluctuations in PLS conditions due to its design. Changes to SX to accommodate extra PLS flow or solution chemistry have been reported in the past [1,2]. However in cases where the PLS copper concentration decreases radically with increasing PLS flowrate the production personnel are either forced to reduce or stop copper production. To limit this impact the circuit is over designed based on the maximum expected process conditions while employing methods to reduce PLS variations. Over designing the circuit can be avoided by manipulating it from a conventional through to full series parallel circuit and back thereby compensating for seasonal fluctuations in Cu tenor and PLS flowrate. The manipulation of process flow allows for the same or smaller SX equipment to be employed with minimal changes to organic parameters. The study considers a heap leach operation designed to produce 50,000 tpa of LME Grade A copper from oxide ore in which seasonal changes impact on PLS parameters and shows that by manipulating SX process flows a constant copper production can be achieved. The concept is illustrated by compiling an annual mass balance over the entire operation and then using PLS composition and flowrates for four conditions to demonstrate the SX change from conventional to series parallel and back. MASS BALANCE MODEL The flow sheet selected for this investigation is shown in Figure 1 and is based on an on/off heap leach design for oxide ore comprising of primary and secondary heap leaches draining into PLS and intermediate leach solution (ILS) ponds respectively. The PLS is pumped to the SX circuit and the raffinate is collected in the raffinate pond. The leach liquor, acidified raffinate, is pumped onto the secondary heap leach which percolates through the heap and collected in the ILS pond. The ILS solution is then further acidified and pumped onto the primary heap leach to produce a PLS solution. The operation is semi-continuous with the solution flowing continuously counter current to the batch operated heaps. 3 7 October Santiago, Chile. 2

3 The sizing of the heaps was based on a run-of-mine (ROM) head grade of 1% copper with an overall leaching efficiency of 70%. The ore required for leaching is stacked 4m high into equal number of primary and secondary leach heaps with a total area of 324,000 m 2. The PLS, ILS and raffinate ponds were sized to give approximately 24 hours residence time in a 3.25 m deep ponds during the dry season resulting in an area of 6,400 m 2 per pond. The mass transfer of copper from solid to liquid is assumed not to be affected by increased solution volumes during the wet season since the acid addition is controlled to provide a PLS exiting the heap at a ph of 1.8. Solution flowrates pumped onto the respective heaps were calculated using a percolation rate of 6 L/m 2 /h which is lower than what is typically achieved in most heap leach operations and is intentionally done to allow for higher rates to be achieved during the wet season thereby avoiding ponding and runoff from the heaps [3,4]. Leach solution onto heaps is kept constant allowing the drainage to fluctuate with rainfall and evaporation rates. Figure 1 Block flow diagram of the heap leach operation Dynamic pond simulation allows for the ILS pond to overflow into the PLS pond during the wet season however the PLS pond fluctuates as the mass of Cu to the SX circuit is maintained to ensure constant copper production in the electrowinning (EW) circuit. The constant mass flowrate is achieved by increasing the flowrate when the Cu tenor decreases in the PLS and vice versa when the tenor increases. The fluctuation in tenor and flowrate was predicted using the annual rainfall and evaporation rates obtained from an operation in Lubumbashi in the Democratic Republic of the Congo (DRC). The monthly values are provided in Table 1 to which a probability density function was applied to calculate daily values. These values in conjunction with exposed surface areas for the heaps and ponds were used to calculate the fluctuating PLS and ILS solution flowrates and Cu tenors. The smoothened precipitation rates do not show extreme conditions when exceptionally high rainfall occurs. Figure 2 shows that the maximum rainfall of 24 mm per day is achieved however recorded values show this to be as high as 3 7 October Santiago, Chile. 3

4 65 mm per day for as much as three consecutive days. This extreme condition is also considered when discussing the SX operation. Table 1 Monthly rainfall and evaporation rates Month August September October November December January Rainfall (mm) Evaporation (mm) Month February March April May June July Rainfall (mm) Evaporation (mm) Figure 2 Daily rainfall and evaporation rates The daily PLS flowrate and Cu tenor predicted from the mass balance model over a period of one year is shown in Figure 3. The maximum PLS Cu tenor is reached during the dry season were as the required flowrate to achieve constant Cu production is at its minimum. The variation between seasons is as high as 42% for both PLS Cu concentration and flowrate. For extreme wet seasonal conditions this could be as high as 50% for Cu tenor and 100% for PLS flowrate. From these outputs four scenarios were selected to demonstrate the concept of changing the SX operation from a conventional circuit during the dry season through to a full series parallel circuit for the wet season and back again. These scenarios are tabulated in Table October Santiago, Chile. 4

5 Figure 3 PLS flowrate and Cu tenor Table 2 Seasonal SX solution composition Scenario Season Dry Start Wet Wet Extreme Wet Month June Nov Dec Dec Rainfall mm/day Max Evaporation mm/day PLS Flowrate m3/h 974 1,147 1,384 2,010 Cu Concentration g/l Raffinate Bleed Flowrate m3/h ,082 Table 2 also reports a bleed stream that needs to be catered for in the design as the overall water balance is a net positive. This stream is taken from the raffinate as it is low in copper. However the contained acid is lost, which means higher acid consumption. This stream also controls the build-up of impurities which in the case of many DRC ore bodies also contains valuable cobalt. This stream is usually processed to remove iron, aluminium and manganese by means of oxidation and precipitation with lime prior to producing a cobalt precipitate. This is either sold or processed further to produce highgrade cobalt cathode. SOLVENT EXTRACTION SX modelling was carried out on each seasons expected leach solution using BASF Isocalc software, with the aim of minimising the size of SX plant to recover at least 95% of the copper leached. Organic flow, and hence SX mixer settler and tank sizes are based on the 3 7 October Santiago, Chile. 5

6 assumption that maximum PLS flow (extreme wet season) can be treated in 2 PLS streams at an advance organic/aqueous (O/A) ratio of 1. This fixes the organic flow rate and a configuration of 3 extraction stages with 1 strip stage (3E, 1S) was found to be optimal. For the start of the wet season, the plant piping is changed from 3 extraction stages in series to a standard series-parallel arrangement. As the PLS flowrate is increased going to the wet season, the extra PLS is accommodated by increasing PLS flow to the parallel stage. PLS flow to the series stages stays unchanged. Extractant concentration is increased slightly to maintain recovery. At the time of heaviest rainfall both series and parallel PLS flows will be at the maximum. Table 3 summarises the operational conditions of the SX for each of the seasons. Table 3 Seasonal SX operating conditions Season PLS [Cu] g/l PLS Flow m3/h [LIX984N] % Cu Recovery (%) Dry Start Wet , Wet , Extreme Wet , The extractant concentration can also be varied seasonally to assist in the maintenance of the required copper transfer across SX. Extractant make up is often carried out on a weekly basis in operating SX plants, and this can be managed to achieve the desired concentration. In the event of PLS flows increasing further than expected due to short periods of very high rainfall, it would be possible to run all 3 extraction stages in a parallel configuration. This would require an extra PLS pump and raffinate piping to suit. The PLS flow to extraction stages 1 and 2 would then be run at the maximum design flow rate, and the balance would be fed to extraction stage 3. The standard series-parallel configuration was compared with the optimum seriesparallel configuration (OSP) [2]. With the PLS composition expected, the gains are below 1% in copper recovery. However, the piping required to convert to OSP is very low and can be done easily when needed. CONCLUSIONS SX can be reconfigured easily and at low cost to treat a wide range of PLS flow and tenor in order to maintain a constant copper production rate on a heap leach plant in a high rainfall region. This option requires additional piping, valves and pumps. For new operations, the SX equipment can be designed for dry conditions where the PLS flowrate is low thus reducing capital cost. To further increase copper recovery, the optimum series-parallel SX configuration can be easily implemented. 3 7 October Santiago, Chile. 6

7 This option avoids difficult water management issues such as solution storage and management as well as covering of ponds and heaps. Floating pond covers can be employed however this only accounts for a small exposed area when compared to that of the heaps. Covering on/off heaps is problematic as exposed areas are in the excess of 300,000 m 2. This option requires additional PLS clarification during the wet season to keep suspended solids to values normally reported during the dry season. Acid consumption will also be higher for this option as the ph post leach is maintained at a value of 1.8 during the wet season as well as larger raffinate bleed needs to be processed. Processing the raffinate bleed would require a larger downstream facility however these costs can be recovered where a secondary product such as cobalt is produced. REFERENCES Miller, G. M., Readett, D.J. & Hutchinson, P. (1979) Experience in Operating the Giralambone Copper SX-EW in changing chemical environments. Minerals Engineering, Elsevier Science Limited. Great Britain, vol. 10, no. 5 pp [1]. Rojas, E.V., Araya, G., Picardo, J.M. & Hein, H. (2005) Serie Paralelo Óptimo: Un Nuevo Concepto de Configuracion en Extraccion por Solventes. Proceedings of HydroCopper 2005, Santiago, Chile. [2]. Taylor, A. (2008) ALTA Heap Leaching & its Application to Copper, Uranium & Nickel Ores. Short Course, Johannesburg. [3]. Thiel, R., Smith, M.E. (2004) State of the Practice Review of Heap Leach Pad Design Issues. Geotextiles and Geomembranes, Volume 22, Issue 6, Pages [4]. 3 7 October Santiago, Chile. 7