By Dan Oancea - Twitter
The method selection for silver analysis and the art and science of extracting silver metal from silver minerals will be briefly exposed in this little article inspired by the Vancouver hosted Silver Conference.
ALS Chemex held a short presentation regarding QA/QC and pitfalls of silver analysis. Silver methods need to be selected specifically for each sample type and exploration program. The control of detection limits, precision and upper range are based on method selection.
First of all we have to know that labs like the more frequent occurring types of silver (i.e. native, sulphide and 'horn silver') and hate the resistive forms like the silver telluride and the silver-gold telluride.
Three important methods are to be considered:
- Aqua Regia Digestion: It is a powerful solvent for sulphides, which dissolves Ag and noble metals but the resistive mineral forms are not completely dissolved; range 0.01 to 1,500 g/t;
- Four Acid Digestion: A very strong method which almost completely breaks down the resistive minerals and silicates; it has a higher cost than Aqua Regia Digestion; suitable for ore grade; range 0.01 to 1,500 g/t;
- Fire Assay: It is a fully quantitative method displaying an improved precision; sample size 30-50 g; suitable for native silver it also can determine Ag and Au together; best method for ore grade and exceptionally rich samples; 0.05 to 10,000 g/t; it has a variation named the Metalic Screen Fire Assay, a method which can accommodate a sample size up to 1 kg and at the same time can probe the presence of a nugget effect.
Typical ranges for Ag analysis are:
- Exploration - 0.1 to 100 g/t Ag;
- Ore Grade - 1 to 1,000 g/t Ag;
- Fire Assay - 10 to 10,000 g/t Ag.
To be remembered: no Fire Assay on low grade samples; better stick with Aqua Regia.
There is one more method to be considered when working with a very large sample size (up to 3 kg), a method used to determine the cyanide solubility and recovery of silver in a mining situation: the Ag by CN Leaches.
A short introduction to silver metallurgy was provided by Steve Busby of Pan American Silver.
We were quick to learn that metallurgy is an art & a science, that they are trained to use energy in various ways to treat ores and produce silver. Their study aims to figure out what constitutes ‘commercially viable ore’.
There are different factors affecting the process selection: geological, mineralogical, metallurgical, environmental, geographical, economic and political.
The process development starts with Mineralogical Studies and Preliminary Metallurgical Testwork, followed by Preliminary Metallurgical Evaluation and Detailed Metallurgical Testwork.
Proven silver extraction methods include:
- ROM Heap/Dump Cyanide Leaching – Yanacocha;
- Direct Smelter Shippment - Eskay Creek;
- Crushed Ore Cyanide Heap leaching – Rochester;
- Gravity Concentration – San Jose;
- Flotation/Smelting – Cannington;
- Agitated Cyanide Leaching/MC – La Coipa;
- Agitated Cyanide Leaching/CIL – Chatree (Thailand);
- Oxidative Pre-Treatment – Newmont (Nevada); and
- Sulfuric Acid Oxidative Pressure Leach – Sunshine.
Some 2005 processing methods include:
- Flotation concentrates shipment to smelter – Cannington, Australia & Fresnillo, Mexico;
- Cyanidation with Merrill Crowe – Luismin, Mexico; El Penon & La Coipa, Peru;
- Flotation/Gravity Concentration CN Leaching – Dukat & Lunnoe, Russia;
- Heap Leach – Rochester, Nevada & Yanacocha, Peru;
- Cyanidation with Carbon Adsorption – Newmont, Nevada & Chatree, Thailand.
Difficult to treat ores are considered refractory and require intensive treatment. Silica encapsulation provides a poor cyanide extractions or flotation concentrations at finely ground sizes.
Manganese-silver ores - the secondary manganese oxide is insoluble in cyanide and totally unsuitable to oxidation, flotation and sulfuric acid treatments. That’s why some good sized Mn - Ag ‘deposits’ cannot be considered commercially viable.
Some words of advice:
- Don’t get too excited about your new discovery. Before starting to further ‘delineate’ the deposit, do some early metallurgical tests; you don’t want to send your money down the drain because the ore is refractory, needs special treatment and the processing cost is higher than it's worth;
- A low grade deposit requires economical methods of metallurgy. The more marginal the deposit is the more money will be put in process selection.
- “Arguing with a metallurgist is like wrestling a pig in the mud; after a couple of hours you’ll realize that they like it!”
Steve’s final thoughts: “Geologists had to climb higher in the mountains and the only thing that they could get is low grade deposits; now that the metal prices are up again the geologists could get down on the beach”.
Hmm, geologists roaming free on Cancun’s sand beaches having the excuse of searching for silver rich Carbonate Replacement Deposits and… getting paid for that! It doesn't sound like a bad idea.
Back to Earth: The next GeoMine article will deal with geophysics and silver deposit types, as presented at the conference.