Metallurgy
Ixtaca: One of Mexico’s Premier Precious Metal Discoveries
The Ixtaca deposit gold and silver mineralisation occurs as electrum (a gold/silver alloy) and gold and silver bearing sulphides in epithermal veins and veinlets cutting carbonate (limestone and shale) and volcanic rocks.
The actual occurrence of gold and silver in the limestone hosted portions of the Ixtaca project is in the form of high grade veins and veinlets, not, for example, as widespread low grade disseminated mineralisation. The many individual veins and veinlets within the vein swarms enclose irregular small bodies of barren limestone because these veins and veinlets branch and re-connect, as well as locally change strike and dip, pinch and swell.
The wireframe models constructed to define the overall vein zones therefore contain interspersed irregular zones of barren limestone dilution.
In early 2018, Almaden started to explore X-ray Transmission (“XRT”) ore sorting as a means to eliminate this barren waste material from the mill feed. In July 2018, Almaden released results of ore sorting tests on the key limestone lithology, which demonstrated that XRT ore sorting technology:
Increases average mill feed grades for the limestone unit by 39% for gold, and 47% for silver
Rejects 36% of run of mine as mine waste rock at the crushing stage
Results in 93% recovery for silver and 88% recovery for gold from the ore sorter
Has the potential to reduce the environmental footprint over the life of mine by:
Reducing process tailings
Reducing process water usage
Reducing process energy requirements and CO2 emissions.
How Ore Sorting Works
Sensor-based ore sorting has been used in the mining industry for decades and it is estimated that sensor based ore sorting technology is currently in use in approximately 350 mines globally including approximately 50 base and precious metal mines.
The operation of a commercial ore sorting machine is shown below. Crushed and screened mineralized rock is evenly fed over a conveyor belt. An electric X-ray tube creates a broad-band radiation. This radiation penetrates the material and provides spectral absorption information that is measured with an X-ray camera.
The resulting sensor information is then processed to provide a detailed “density image” of the material allowing it to be separated into high and low-density fractions. If the sensor detects material to be sorted out, it signals the control unit to open the appropriate valves of the ejection module at the end of the conveyor belt. The detected materials are separated from the material flow by jets of compressed air. The sorted material is divided into two fractions in the separation chamber.
Above: XRT Ore Sorting – functional principle. Source: TOMRA
Above: Commercial TOMRA COM Tertiary XRT ore sorting machine. Source: TOMRA
Ixtaca Ore Sorting Tests
The sorting tests were carried out in 2 phases. First, a First Inspection showed that an XRT sorter is able to detect high atomic density sulfide inclusions within the limestone host rock (see figure below). Sensor images showed a correlation between ore grade and sensor response and thus an indication of “sortability” for the ore.
Above: Ixtaca XRT First Inspection Images
In the second phase, a Performance Test was carried out on a commercial scale XRT machine. A 2,200 kg sample of limestone was collected from fresh drill core in the main Ixtaca zone.
The samples were prepared for sorting by crushing and screening at a McClelland metallurgical laboratory in Reno and shipped to the TOMRA ore sorting Test Center in Wedel, Germany. Fines (-12mm) are not sortable and were weighed and assayed at McClelland.
Tests were carried out at various TOMRA XRT equipment parameters at various feed size fractions. All waste and ore products from the trials were weighed and analyzed independently by ALS Global.
The results of the tests demonstrated that the XRT could successfully reject:
39% of waste rock from coarse rock (18mm to 50mm) at a grade of Au 0.25 g/t and 12 g/t Ag
52% of waste rock from midsize rock (12mm to 16mm) at a grade of Au 0.22 g/t and 12 g/t Ag
The above grades are below the anticipated mine cutoff grades.
A mass balance of the ore sorting test including consideration of the fines that will bypass the ore sorter and sent directly to mill feed is summarized in Table 1.
Almaden’s Feasibility Study (FS) on the Ixtaca Project which was announced in December 2018, incorporates ore sorting in the flow sheet, as well as the Rock Creek Mill which has now been purchased by Almaden. A simplified flow sheet is available here +
In the FS, product from the secondary crusher will be screened in to coarse (+20mm), mid-size (12 to 20 mm), and fine (-12mm) fractions. Coarse and mid-size ore will be sorted by an XRT ore sort machine to eject waste rock. Fine ore will bypass the ore sorting and is sent directly to the mill.
Ore sort waste from Limestone and Black Shale is below waste/ore cutoff grade and is placed in the waste rock dump. Ore sort ‘waste’ from the Volcanic unit is low grade ore and will be stockpiled for processing later in the mine life. Ore sorting pre-concentration increases the mill feed gold and silver grades by 32% and 31% respectively compared to run of mine (ROM) grades. The table below shows ROM grades with ore sort waste removed from the ROM, and the resulting mill feed.
Crushed ore is transported to the grinding circuit by an over land conveyor. Grinding to 75 microns is carried out with ball milling in a closed circuit with cyclones. Cyclone underflow is screened and the screen undersize is treated in semi-batch centrifugal gravity separators to produce a gravity concentrate.
The gravity concentrate will be treated in an intensive leach unit with gold and silver recovered from electrowinning cells.
The cyclone overflow will be treated in a flotation unit to produce a flotation concentrate. After regrinding the flotation concentrate leaching will be carried out in 2 stages. CIL leaching for 24 hours will complete gold extraction, followed by agitated tank leaching to complete silver leaching. A carbon desorption process will recover gold and silver from the CIL loaded carbon, and a Merrill Crowe process will recover gold and silver from pregnant solution from the agitated leach circuit.
Cyanide destruction on leach residue is carried out using the SO2/Air process. Final tailings are thickened and filtered then dry stacked and co-disposed with mine waste rock.
Average process recoveries from mill feed to final product over the life of mine are summarized in Table 4 for each ore type.
The work is being carried out at McClelland Laboratories in Reno Nevada under the supervision of independent engineers Moose Mountain Technical Services (“MMTS”) and qualified person Tracey Meintjes, P.Eng. of MMTS.
