Olympias Selective Flotation Plant Makes Lead-Silver, Zinc and Gold-Arsenopyrite-Pyrite Concentrates From 400 ktpa of Underground Ore

This report describes the phased processing strategy for the Olympias Project, covering rehabilitation of the existing concentrator, a new flash smelting-based gold recovery route, and associated hydrometallurgical and smelter facilities.

Report context

The Olympias Project technical report, dated 2011, describes a phased development approach for the processing of reclaimed tailings and underground ore. The project is planned to initially use a rehabilitated existing concentrator at Olympias to treat reclaimed tailings for 3 to 4 years, followed by ore from underground for a further 4 years. A new concentrator is scheduled to be built at the Stratoni mine site for year 9 of operations to accommodate increased ore production of 850,000 dry tonnes per annum (dtpa).

Processing route

Phased Development and Feed Sources

The project is split into two main phases. In the first phase, the rehabilitated existing concentrator at Olympias will treat approximately 2.4 million tonnes of reclaimed tailings grading 3.4 g/t gold at up to 720,000 dtpa. These tailings accumulated from historical operations due to inefficient recovery of gold-bearing pyrite/arsenopyrite concentrate. After the tailings are exhausted, ore mined from developed zones of the underground mine will feed the plant at 400,000 dtpa for a further 4 years.

In both phases, three concentrates will be produced: lead/silver concentrate, zinc concentrate, and gold-bearing pyrite/arsenopyrite concentrate. These will be trucked to Hellas Gold's Stratoni port facilities for bulk sea freight shipment or containerised and transported to Thessaloniki for export.

Existing Rehabilitated Concentrator

The Olympias mine and concentrator were commissioned in 1976 and operated until 1995, when depressed metal prices made operations uneconomic. The plant was shut down properly with the mills ground out. Complete mechanical and electrical overhaul is required, along with replacement of missing equipment including flotation cells, pumps, filters, electrical distribution, and a new control system.

The original design capacity was approximately 40 tph of run of mine ore (about 310,000 tpa), although the plant operated efficiently at higher throughputs on occasion. The fully refurbished plant, including a new process control system, will be able to treat 50 tph continuously. Rehabilitation work to buildings commenced in mid-2010, with walls repaired by sand blasting and shotcrete application, windows replaced, and a new roof mounted.

For tailings reprocessing, the reclaimed material will be mechanically reclaimed and re-pulped. The slurry will be pumped to a cyclone, with underflow passed through the ball mill for de-agglomeration before flotation. The cyclone overflow will go to conditioning tanks where reagents are added, then to flotation where gold-bearing minerals are floated in rougher, scavenger, and cleaner stages. The high-grade gold concentrate will be thickened and filtered for despatch.

When treating run of mine ore, the flotation circuit will be slightly modified to produce the three concentrates. Ore will be crushed to 80% passing 12 mm, then milled to approximately 80% passing 180 microns. Separation into the three concentrates is achieved through pH control and conventional reagent additions for depression and activation of the mineral species. The plant is being refurbished with mechanical flotation cells: 3 m³ units for roughing and scavenging and 1.5 m³ for cleaning.

Flotation Recoveries and Grades

The metallurgical performance of the plant when processing ore is expected to match previous results. Table 17-1 in the report summarises expected flotation recoveries: lead recovery of 91% with 91% silver recovery to a lead concentrate grading 63% Pb and approximately 1,800 g/t Ag; zinc recovery of 85% to a zinc concentrate grading 51% Zn; and gold recovery of 85% to a pyrite/arsenopyrite gold concentrate grading 22 g/t Au, 9% As, and 40% S. Lead and silver recoveries on resumption of operations may be below expectation due to oxidation of near-surface ore, and an allowance has been made for this in the business plan.

Tailings Management

Tailings from the concentrator will be directed to hydro-cyclones. The coarse fraction will go to a surface backfill plant or the new underground backfill plant at Olympias on the -210 m level, where it will be mixed with cement for placement underground. When mining activities proceed below the -250 m level, only the backfill plant on the -210 m level will be used. The fine fraction will be directed to the tailings management facility.

The fine fraction, about 20% of total tailings, will be filtered by a new filter press installed during rehabilitation, with the filter cake transported to Stratoni for disposal on the existing TMF. In the first years, coarse residue from the tailings phase will be used by local contractors as aggregate. In the medium term, coarse tailings will be used for underground backfill with no long-term surface disposal requirement.

Flash Smelting Process

A flash smelting-based process was developed by Outotec for treatment of Olympias gold pyrite/arsenopyrite concentrate. The process was based on Outotec's flash smelting technology, initially provided as a Preliminary Information Package in 2005 and advanced to pre-feasibility study in early 2006. Key changes from the original concept included co-feeding copper concentrate to improve gold recovery to the matte phase, replacing the ambient arsenic stabilisation circuit with an autoclave to form stable scorodite, and eliminating the precious plant so the final product is a gold-bearing copper concentrate derived from the flash smelter matte.

The flash smelting furnace off-gas exits through the uptake shaft and is cooled in a quench tower. Approximately 99% of incoming arsenic is contained in this gas flow. Gas cooling involves water spray evaporation, which pre-washes the gas and binds arsenic-containing dust. The water becomes acidic (pH approximately 1 to 2), and part of the dust dissolves in the wash water. Solids are separated from the quench tower circulation, and dissolved arsenic in the liquid circulation is controlled to avoid precipitation. A portion of the clarifier overflow is continuously bled from the system. Off-gas then flows to further cleaning in the gas cleaning section of the sulphuric acid plant. Outotec recommended simulation modelling to verify the process design for handling the large off-gas volumes.

The matte from the flash smelting furnace is granulated directly from granulation launders using a conventional water spray nozzle granulation system. The granulated copper/iron matte is wet ground in a ball mill, then leached with sulphuric acid at ambient pressure, 90°C, and pH of approximately 1.5 to leach out some iron. The solid phase undergoes solid/liquid separation, with underflow filtered. The gold-containing copper filtrate is the concentrate product expected to grade about 20% copper and 175 g/t gold.

The flash smelting furnace slag is tapped through slag tapping holes and launders to ladles, then cast to solid blocks. Slag granulation is available as an option but not costed. Slag will be stored as blocks or, if granulated, disposed of with plant tailings on the TMF, which has provision for slag storage. The slag is low in arsenic (approximately 0.1%) and sulphur (2.5%), with these elements locked in a glassy matrix, potentially suitable for construction material.

Sulphuric Acid Plant

The sulphuric acid plant follows the 3+1 double absorption process. Process gas from the flash smelting furnace is cooled and cleaned in the quench tower, then further cleaned in a gas cleaning section including a gas washing tower, two scrubbers, and primary and secondary wet electrostatic precipitators.

In the contact section, gas composition is adjusted to a maximum SO₂ content of 12.0% and minimum O₂:SO₂ ratio of 1.0 using dilution air. Water vapour is removed in a drying tower where gas flows counter-currently to concentrated sulphuric acid (96%). The process gas is heated to auto-ignition temperature by recovering heat from the exothermic conversion reaction, with successive catalyst passes and partial cooling between passes. The double absorption process increases conversion and reduces tail gas SO₂ emissions through a secondary SO₂ to SO₃ conversion and absorption process.

All acid produced in the circulating acid system is removed as 98.5% H₂SO₄ after the final absorbing tower and pumped through a product acid cooler to storage. Off-gas flows to the stack after final absorption. The extra heat evolved is mainly transferred into cooling water.

The flash smelting concept was tested by Outotec in Pori, with both pyrite/arsenopyrite smelting and hydrometallurgical treatment of iron matte and arsenic precipitation examined. Testwork confirmed the chemistry works and is in accordance with the submitted Environmental Impact Statement. However, all matte leaching and arsenic precipitation tests were batch tests; continuous hydrometallurgical test runs with recycles at bench pilot scale, along with additional pyrometallurgical tests, are required for more accurate plant design data.

Flash smelting for arsenopyrite/pyrite concentrate is a new application of the technology still in research and development. Outotec stated more continuous pilot-scale tests are needed. Modelling of the quench tower process is required to increase confidence in process design parameters. Testwork to date indicated gold recoveries of more than 90%, and a flash smelting furnace matte is reported to contain over 90% of the gold.

Scorodite Formation and Arsenic Removal

The process incorporates arsenic removal as scorodite. In the first step, arsenic is precipitated as ferric arsenate in an agitated reactor at 90°C and pH 1.8, with iron to arsenic ratio of 1.25. Oxygen gas is added to oxidise iron to Fe³⁺ and arsenic to As⁵⁺, with controlled additions of SO₂ gas and milk of lime for pH control. The oxidised product flows to solid/liquid separation, with clear thickener overflow returned to the quench tank. Solids are directed to an autoclave where pressure oxidation forms crystalline scorodite (FeAsO₄·2H₂O), binding arsenic in a form stable for long-term storage complying with the EU Mine Waste Directive. The autoclave operates at 160°C and modest pressure under acidic pH conditions.

The product from the autoclave is cooled, with solids filtered and directed to residue storage. Most overflow from the ferric arsenate thickener is recycled as quench water. Excess overflow and scorodite thickener overflow are collected, with a fraction recycled to the matte leach vessel but most flowing to a second arsenic and iron removal section. There, residual arsenic is precipitated as amorphous ferric arsenate (FeAsO₄) in a stirred reactor at ambient pressure, pH 4, with Fe-As ratio maintained near 4. Reactor solids undergo solid/liquid separation, with thickener underflow filtered and filtrate returned. Any extra iron not used in the arsenic reaction is precipitated as goethite/jarosite. Final neutralisation prevents accumulation of metallic impurities, and thickener overflow bleed purifies discharged water to below environmental impurity levels.

Tailings Disposal and Water Management

Tailings disposal is designed to minimise land take and water management requirements. Long-term stability tests and Toxicity Characteristic Leaching Procedure tests will be carried out on plant residues as part of the testwork programme. The Upper Kokinolakkos valley is a brownfield site with suitable characteristics for a new TMF. Plant residues should be dewatered as much as possible for water recycling. When backfill is not required or the backfill plant is down, all concentrator tailings will be directed to the new TMF at Kokkinolakkos.

Key reported parameters

Technical qualifications

The report presents a proposed hydrometallurgical design; values not reported in the source are not inferred.