The following information is an extract from The Achmmach Tin Project Small Start Option Definitive Feasibility Study Report completed in 2016.
ATS has based the Achmmach process plant design on the use of conventional cassiterite gravity and flotation concentration technology. Given the relatively fine-grained nature of cassiterite in Achmmach ore, the use of cassiterite flotation and centrifugal gravity concentration will be more intensive than it may be in similar hard rock tin recovery circuits. However, unlike some deposits, the occurrence of contaminant sulphide minerals is low, thus confining sulphide removal to simple steps ahead of gravity concentrate dressing and fine cassiterite flotation.
Opportunities to improve process efficiencies within the fine and ultrafine circuits may emerge from ongoing metallurgical work. These opportunities could result in earlier rejection of gangue and reduced material flow through the flotation circuits.
In the interests of minimising earthworks and concrete requirements, ATS specified that key process equipment had to be light enough not to require extensive foundations, thus allowing for the use of smaller scale modular components at each process stage.
The key components of the Achmmach flow sheet, described below, include:
SSO Process Plant Development
The outcomes of metallurgical testwork completed throughout the course of the March 2014 DFS and the subsequent EDFS provide the basis for the SSO DFS process design. The EDFS work refined the recovery algorithm developed during the early DFS work, and now ATS expects the higher grade of Stage 1 ore will lead to higher tin recovery during this stage of the Project.
Table 5‑1 summarises the process parameters for the SSO.
Table 5‑1: SSO Process Parameters
The total process power draw upon completion of Stage 2 commissioning will be 4.3 MW, or approximately 5.1 MVA.
ADP and ATS developed the final process flow diagram (PFD) from the earlier SSO PFS version.
Figure 5‑1 shows the overall process flow diagram.
Figure 5‑3 prepared by ADP illustrates the layout of the process plant. ADP based the layout on the installation of 2 x 250 ktpa modular process streams while providing sufficient space for a third 250 ktpa process plant for Stage 2 expansion.
Figure 5‑3: Achmmach Process Plant 3D Rendering
The principal ore flow is from the mine to the plant via the ROM pad and crushing plant (not shown above) to the EDS mills. Then via secondary ball mills to the spiral plant and regrind mills, which together comprise the gravity circuit. The residual ore flows through the flotation and fine concentrate circuit and then on to the tailings system. Dressed concentrate exits site via the concentrate load out facility. The tailings and water circuit provides process water to the entire process.
ATS elected to employ contract crushing as a means of defraying initial capital investment. ATS will provide the selected Moroccan contractor with land, power and water. The contractor will be responsible for delivering crushed ore to the fine ore stockpile at the rate and top size specified by ATS from time to time.
The crushing circuit envisioned by ATS will comprise a primary jaw crusher operating in open circuit. Jaw crusher product will be screened ahead of the secondary cone crusher stage, operating in closed circuit with a nominal 20 mm screen. Screen undersize will report to the tertiary cone crusher stage from which screened product will report to the fine ore product stream.
Historical hard rock cassiterite beneficiation practice has relied on combined rod and ball mills (rod-ball) in series to liberate cassiterite without attendant loss to overgrinding. Primary rod milling is the principal means of achieving initial grind performance to about 2 mm, followed by ball milling to target sub-millimetre sizing.
ATS takes the view that rod milling, whilst conventional is capital intensive and prone to operational disruption related to media charging and media tangling, particularly where mill operators are relatively inexperienced. In this regard ATS sought a hammer milling substitute for rod milling, based on the horizontal shaft impact machine as exemplified by the Hazemag crusher out of Germany. ADP subsequently introduced ATS to Energy and Densification Systems (EDS) of Johannesburg, South Africa who manufacture a 10-shaft horizontal impactor designed to reduce hard rock from nominally 40 mm to less than 1 mm at rates up to 100 tph. This is very similar to the operating range of the rod mill. The full size 10-shaft machine operates with 178 kW of installed power.
Comminution Circuit Description
An independent local crushing contractor will deliver crushed ore to a fine ore stockpile, from which a loader will transfer ore to the comminution circuit feed hopper. The feed hopper will split the incoming ore stream to each of two 250 ktpa process plants via a 10-shaft dry EDS mill operating in open circuit ahead of the secondary ball mill discharge hopper. EDS mill product will combine with secondary mill discharge slurry and the mill product pump will deliver the mixture to three 200 µm high frequency vibrating screens. Process water will transport screen oversize to the ball mill. The spiral feed pump will transfer screen undersize to the spiral plant cyclone system.
Figure 5‑5 illustrates the configuration of the comminution circuits.
Figure 5‑5: SSO Primary Comminution and Regrind Layout (2 x 250 ktpa circuits)
Two sets of cyclones located at the head of the gravity circuit will deslime milled ore at 10 µm, with the overflow slimes reporting to the tailings thickener. The deslime product will be classified at 38 µm with the overflow fines reporting to the flotation circuit. The +38 µm -200 µm fraction will report to the modular spiral plant for separation into tin concentrate, middlings material and coarse reject.
Figure 5‑6 provides a depiction of the classification and spiral modules.
Figure 5‑6: SSO Classification and Spiral Plant (2 x 250 ktpa Modules)
Sulphide flotation and magnetic separation will clean the gravity concentrate prior to final upgrade over shaking tables to a tin content of approximately 59% Sn. The final gravity concentrate will report to filtration and dispatch. Middlings material and table rejects will report to the regrind circuit for progressive reduction to float feed size. The spiral feed cyclones will classify regrind mill product with the underflow reporting to the head of the spiral circuit.
Classified and deslimed fine ore will report to the flotation circuit, which will employ a conventional flow sheet of rougher / scavenger, clean and reclean. Conditioning tanks will provide sufficient residence time for the addition of silicate dispersant and depressant, followed by cassiterite collector.
Figure 5‑7 illustrates the flotation circuit.
Figure 5‑7: Achmmach Flotation Circuit
A series of Falcon centrifugal concentrators operating in cleaner / recleaner configuration will upgrade fine flotation concentrate to approximately 50% tin content. Falcon cleaner tails will report to the tailings thickener. The final fine concentrate will report to concentration filtration and dispatch to combine with the final gravity concentrate.
Figure 5‑8 illustrates the concentrate filtration and dispatch facility.
Figure 5‑8: Concentrate Filtration & Dispatch (Conveyor omitted for clarity)
An agitated surge tank will blend and store concentrate slurry ahead of a plate and frame filter, which will produce concentrate cake containing approximately 8% moisture. The filter product conveyor will deliver concentrate into a truck mounted sea container in lots of up to 20 tonnes moist weight.
The container truck will reverse up to the conveyor such that the conveyor will fill to the front of the container. The truck will slowly move forward, facilitating even distribution of concentrate along the length of the container. Kasbah estimates Stage 1 container movements will approach 4 per day, increasing to 6 per day upon commissioning of Stage 2.
Concentrate containers will be transported to Casablanca Port prior to consolidation and shipping to an Asian based smelter. ATS has retained this option as proposed in the March 2014 DFS as a reasonable basis for the determination of concentrate shipping and smelting charges.
Ore mining will commence five months before ore processing to allow build-up of sufficient ROM pad stocks to allow for commissioning and process plant ramp-up to design capacity. Commencement of ore processing will be dependent on completion of the plant construction and dry commissioning phases.
The mined ore ramp-up period will be dependent on mine development strategy and timing.
Process Plant Costs
ADP prepared its estimate of process capital based on vendor inputs, Moroccan construction rates and appropriate shipping charges. The cost and exchange rate bases were H1 2016.
The sum of ADP estimated Total Project Costs was US$16.3M and Owner’s Costs was US$4.5M.
Table 5‑8 shows the build-up of processing operating costs excluding labour and power. Kasbah compiled these costs based on the outcomes of EDFS metallurgical testing, later equipment vendor quotations for maintenance costs and vendor quotes for the supply of consumables. Off-shore supply was quoted CIF Casablanca Port. Pricing and exchange rates are current within H1 2016.
Table 5‑8: Achmmach Process Operating Costs