Silver mineralisation amenable to both cyanide leaching and flotation processes
• Cyanide leaching returns silver recoveries of 70%
• Flotation produces very high concentrate grades of up to 5.5% Ag with up to 72% recovery
• Flotation followed by cyanidation of tailing stream increases total silver recoveries to 76%
• Future processing options include coarse (heap) leaching or millingflotation-cyanidation
• Further testwork will optimise processing options and maximise flotation and leaching recoveries
Azure Minerals Limited (ASX: AZS) (“Azure” or “the Company”) is pleased to report that very encouraging results have been returned from preliminary metallurgical testwork on silver mineralisation from Mesa de Plata, part of the Alacrán Project located in the northern Mexican state of Sonora.
Managing Director, Mr Tony Rovira commented, “Metallurgical recovery was always going to be critical to this deposit and these very favourable results provide us with the confidence to continue progressing the project with further development studies.”
A series of preliminary mineralogical and metallurgical tests were undertaken to:
• characterise mineralogy of the mineralisation and identify silver-bearing species;
• extract silver by cyanide leaching, flotation and gravity methods;
• identify most favourable processing routes; and
• identify options to improve processing grades and recoveries.
A master composite (head grade of approximately 130g/t Ag) and a high grade composite (head grade of approximately 600g/t Ag) were prepared. The high grade composite returned 70% recovery from the cyanide leaching process and 67-72% from flotation, while the master composite returned 52% from cyanidation and 51-55% from flotation. For the high grade composite, a combination of flotation followed by cyanidation of the tails increased total silver recovery to 76%.
Testwork was undertaken by Blue Coast Research (Nanaimo, BC, Canada) and mineralogy was carried out by Xstrata Process Support (Falconbridge, Ontario, Canada) over the period September to November 2015.The program was conducted under the supervision of metallurgist Mr. Andrew Holloway, P.Eng.,CEng, of AGP Mining Consultants (“AGP”), based in Toronto, Canada.
AGP commented, “In AGP’s opinion, the initial metallurgical testing of Alacrán mineralization has shown encouraging results and provides information to support further development, with two possible process options identified.
“Cyanide leaching results were very favorable, with greater than 70% recovery achieved from the high grade composite.
“Additionally, it is important to note the excellent performance in the flotation tests of the high grade composite material, with very high concentrate grades, up to 55,600g/t Ag (5.5% Ag), achieved in the first two minutes of flotation.
“Results of direct cyanidation on a coarse sample indicate that a low cost heap leaching option may be viable, while an alternative option of finer milling-flotation-cyanidation could become the favoured process route. Both options should be investigated to assist with the development of an economic analysis of the deposit.”
DETAILS OF METALLURGICAL PROGRAM
Sample Selection & Preparation
Sample material was selected from four Reverse Circulation drill holes with the aim of creating a “master” composite. Approximately 2kg of drill cuttings were subsampled from a series of 12 intervals in each hole, for a total sample size of 96kg.
Sample material was despatched to Blue Coast where each hole was composited, crushed to -1.7mm and homogenized, then 10kg was removed from each composite to make up the 40kg master.
In addition to the master composite, a number of high grade intervals were selected to form a high grade composite of 20kg mass and approximately 600 g/t Ag grade.
Sub-samples of master composite and high grade composite material were submitted to the laboratory for head assay. Results are given in the table below.
Table – Measured Head Assays
|Composite||Cu %||Pb %||Zn %||Sb %||As %||S %||Ag (g/t)||Au (g/t)|
A sample of master composite was shipped to Xstrata Process Support (XPS) for preliminary mineralogical evaluation. An un-sized sub-sample was measured using QEMScan and EPMA mineralogical equipment. The master composite was found to be mainly quartz (80% by mass), with alunite and various iron oxides making up a further 14% of the sample mass. Only two silver-bearing species were identified, namely:
• Bromian Chloroargyrite, making up 70% of contained silver; and
• an Sb-Pb-Fe oxide, making up 30% of contained silver.
Direct data from the EPMA shows that the Bromian Chloroargyrite mineral contains over 80% Ag, indicating that concentration of this mineral by flotation could result in high silver concentrate grades.
Cyanidation work consisted of a series of 48-hour bottle roll tests at various grind sizes, on both the master and the high grade composites. Results were encouraging with over 50% Ag recovery on the initial master composite test. Silver recovery of 70% was achieved on the high grade composite. Additional leach tests were carried out on both the high grade and master composites using whole ore leaching, and also on the flotation test tailings (ie. combined flotation plus cyanidation testing).
Whole Ore Leaching
The leach curves shown in Figure 1 illustrate the results of first tests on the master composite and the high grade composite – both composites at 75 micron grind, 45% solids, 10.5 pH and 1.5 g/l NaCN concentration. The curves highlight how most of the extracted silver leaches very quickly (less than 4 hours) with relatively little leaching thereafter. Reasons for this will be examined in future testwork programs. Finer grinds (60 micron) were tested, with very little increase in Ag extraction.
In addition to the above testing, a single bottle roll test was completed on a crushed (i.e. not milled) sample of high grade composite. This test was designed to give an initial indication of the potential for heap leach processing. The resulting leach curve was similar to those shown in Figure 1, and the final extraction of 65% of the silver was only slightly less than that achieved after a 75 micron grind, indicating the feasibility of heap leach processing.
Flotation Tails Leaching
In addition to leaching ground samples of high grade and master composites, the work program included some preliminary flotation + cyanidation tests, in which the flotation tailing slurry was subjected to the standard cyanidation bottle roll conditions. In the test on the master composite flotation tailing, 22% of the remaining silver was extracted by cyanidation, for an overall silver recovery of 62.3%. For the high grade composite, the extraction from flotation tailing increased to 26%, giving an overall silver recovery of 75.8%.
Combining the flotation and cyanidation results gives the following results (note that the cyanidation recoveries are calculated as a percentage of flotation test feed).
Table 2 – Combined Flotation + Cyanidation Results
|Composite||Flotation Recovery %||Cyanidation Recovery %||Total Recovery %|
Batch rougher flotation testwork was carried out on samples of master composite and high grade composite material. Results were encouraging, with good recoveries and excellent concentrate grades achieved in most tests. The Grade vs Recovery curves for the first four rougher flotation tests are shown in Figure 2.
Testing of the high grade composite material highlighted an excellent response with very high concentrate grades of 38,205g/t Ag (3.8% Ag) and 55,600g/t Ag (5.5% Ag) being achieved in the first two minutes of flotation. Overall recoveries (after 14 minutes of flotation) are also reasonable at 51-55% for the master composite and 67-72% for the high grade composite.
As noted in the previous section, combining the flotation process with a cyanide leach of silver in the flotation tailing slurry increases the overall recovery by 10%.
A simplified, three stage gravity separation test was undertaken on an un-sized sub-sample of the master composite using a laboratory scale Knelson concentrator. The combined silver recovery to the three products was 12.9%. This suggests that the chloroargyrite was either insufficiently liberated due to grind size, or is of insufficient density.
FURTHER METALLURGICAL TESTWORK
Further work will investigate optimising recoveries for leaching and flotation, as well as combinations of the two. In addition, more advanced gravity separation testwork will be trialled.
A short flotation process (ie. flash flotation within the milling circuit) plus cyanidation of coarsely milled ore may be a viable flowsheet for this project and will be investigated.
For further information, please contact:
or visit www.azureminerals.com.au
Information in this report that relates to Exploration Results is based on information compiled by Mr Tony Rovira, who is a Member of The Australasian Institute of Mining and Metallurgy. Mr Rovira is a full-time employee and Managing Director of Azure Minerals Limited. Mr Rovira has sufficient experience which is relevant to the styles of mineralisation and types of deposit under consideration and to the activity which he is undertaking to qualify as a Competent Person as defined in the 2012 Edition of the “Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves”. Mr Rovira consents to the inclusion in the report of the matters based on his information in the form and context in which it appears.