Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-03-23T09:55:18+00:00
  • To participate in the 911Metallurgist Forums, be sure to JOINLOGIN
  • Use Add New Topic to ask a New Question/Discussion about Pyrometallurgy and Electrometallurgy.
  • OR Select a Topic that Interests you.
  • Use Add Reply = to Reply/Participate in a Topic/Discussion (most frequent).
    Using Add Reply allows you to Attach Images or PDF files and provide a more complete input.
  • Use Add Comment = to comment on someone else’s Reply in an already active Topic/Discussion.

Gold Concentrate (5 replies)

4 months ago
Colin 4 months ago

Can someone explain the process of extracting gold once you have obtained a concentrate after putting your alluvial material through a gravity machine. We put about 200kg of soil through a gravity machine and ended up with about 2kg of concentrate. We then took this to a lab to see what the gold content was. the results showed 805 g/t in the concentrate. Once you have the concentrate what is the next process to extract the gold.

4 months ago
JRH 4 months ago

Punch in basement chemist on the Internet and read he will get you started.

Marshal Meru
4 months ago
Marshal Meru 4 months ago
4 months ago
Adamfocus 4 months ago

Boil in Aqua regia (1 par t HNO3 3parts HCL or 7.5ml HNO3 with 22.5mL HCL then times this out).

Cool and bubble through SO2

4 months ago
inOr 4 months ago
1 like by David

Look at "Recovery and Refining of Precious Metals." by CW Ammen.  It may be very expensive to buy, though there are 2 editions.  As far as I can tell, there's little difference between them.  Your library might have one.  Though there are a few errors, most are not serious.  I recommend this reference because it discusses methods applicable to the modest quantities and volumes you are dealing with.

Basically, there are 3 broad classes of recovery to choose from:  chemical leaching, amalgamation and selective dissolution and precipitation.  Leaching is used at all scales.  The most common method, with which metallurgists have the most experience, is leaching with sodium cyanide.  It's specific to the gold and silver, leaving other metals behind in the wastes.  A major disadvantage is the extreme toxicity of the cyanide, and the possibility that an accident causes the cyanide solution to become acidic.  That generates hydrogen cyanide gas, which kills in small quantities upon inhalation.  Cementation with zinc metal replaces the gold in solution, and metallic gold falls out of solution as a fine powder.  See Ammen.  Because of cyanide's antipathy toward life in general, you must save all wastes and destroy the cyanide in them before discarding into the environment.  The dilution solution does not apply here.  

 Another chemical method that may be considered leaching is chloridation.  Moist concentrate is purged with chlorine gas, which reacts with the gold and dissolves it in the water present in the form of the chloride complex.  This method is not very specific, since many metals and their ores are dissolved this way.  But compressed chlorine gas is sold in small metal bottles.  Though not as dangerous as hydrogen cyanide, chlorine is also toxic.  If done in a well ventilated fume hood, the excess chlorine released slowly into the atmosphere ( away from neighbors, you shouldn't be able to smell it). Advantage: small volumes involved, waste disposal relatively simple.  See Ammen.  

Boiling the concentrate in aqua regia (3 volumes of hydrochloric acid to each volume of nitric acid) dissolves gold and other metals quite efficiently and quickly.  The a.r. solution is extremely corrosive and evolves toxic nitrogen oxide (NOx) gases. Though not as deadly as HCN, these reagents and their handling should be done in a fume hood or in a well ventilated environment.  Wear gloves and wear a face shield to protect yourself from acids and fumes.  Actually, you should take these precautions whenever handling any of the reagents and reactants mentioned here.  As Adamfocus states, you can precipitate the gold chloride from solution with sulfur dioxide.  Again, Ammen discusses this method at length.  I don't remember if it applies to this method, but some hydrometallurgical methods require that all nitric acid be removed from the solution by repeated dilution with HCl and evaporation.  Since SO2 is a reducing agent and nitrate is an oxidizing agent, you may have to use excessive SO2 to remove it.  Once again, the SO2 is toxic, though not nearly as bad as the gases mentioned above.  I grew up in NJ and the sulfur in the nearby Bayway oil refinery occasionally caught fire, releasing sulfur oxides into the air.  In oxygen, SO2 becomes SO3, which reacts with water in the air and the lungs to make sulfuric acid.  Yummy.  I was a runner back then, and I can tell you there's nothing like running a few miles and spitting out sulfuric acid when you take a few deeeeep breaths. To this day, I wonder if I permanently damaged myself.  If there's any reliable, simple and benign method, it escapes me right now. 

Finally, there's amalgamation.  Simple, effective and selective, exposing the concentrate in liquid mercury dissolves the gold metal .  The method used most often is to coat copper plates in mercury (that makes a copper amalgam) and running a dilute slurry of the concentrate over the copper amalgam.  When the mercury is fully loaded with gold, it is scraped off of the plates and placed in a specialized still, called a 'retort', and heated to drive off the mercury as a vapor which is then collected from the cooled exit tube of the retort and recycled.  The gold remains behind in the retort in the form of a metal sponge.  I really don't like this method.  Mercury is toxic in all its forms.  You shouldn't expose your skin to the liquid, and no mercury vapor in any quantity can be allowed to escape the apparatus.  Unlike cyanide, mercury does not kill quickly, instead it slowly and irreversibly damages brain and kidney tissues, unless prompt medical treatment is successful.  The nervous system's response is particularly gruesome.  

Jorge Ganoza
4 months ago
Jorge Ganoza 4 months ago

You do not mention the gold recovery. It is important to calculate it to evaluate the metallurgical performance of the gravity circuit. I think you should clean your gravity concentrate using a shaking table (e.g. gemini table). In this way you can increase the concentrate grade, Alternatively, you can use a gold pan concentrate to clean your 2 kg gold concentrate. In both cases, if there is free gold, you will be able to see the gold particles. 

The gold concentrate from the cleaning stage should be ready for smelting. If you don't want to smelt the concentrate, you can sell it.

Please join and login to participate and leave a comment.