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The nature of the ore deposit determines the mining and mineral processing techniques applied. Oxide ore deposits are frequently of such low grade (e.g., 3 to 10 parts per million) that extensive mineral processing cannot economically be justified. In this case they are merely shattered by explosives and then piled into heaps for extraction by cyanidation (see below). These heaps can be hundreds of metres long and 15 to 30 metres in height

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gold ore roasting techniques

Alluvial deposits are either dredged from pond and river bottoms or sluiced from banks and floodplains with high-pressure hydraulic hoses. Alluvial deposits require little or no comminution; they are usually concentrated by gravity techniques such as jigging or tabling, in which a slurry is passed through jigs or over grooved or ridged tables that retain the denser gold particles while allowing the much less dense sand and gravel to pass over

Endogenetic deposits frequently contain elemental gold that is highly disseminated within a base metal sulfide mineral. These deposits are mined, crushed and ground, and then concentrated first by gravity separation to recover coarse particles of native gold before being subjected to froth flotation to concentrate the sulfide mineral fraction that contains the gold

Elemental gold (and silver as well) is soluble in mercury, so that, when particles of the metal are brought into contact with a fresh mercury surface, they are wetted and dissolved, forming an alloy called amalgam. This phenomenon is exploited for the recovery and concentration of fine particulate gold or silver

gold ore roasting techniques

Amalgamation is accomplished by passing a slurry of ore over copper plates coated with mercury, by mixing a slurry of ore and mercury in a cylindrical or conical vessel called an amalgam barrel, or by grinding the ore in a ball, rod, or pebble mill to free the gold from the mineral matrix and then adding mercury to the mill and continuing grinding until the gold has dissolved in the mercury. The denser amalgam is then separated from the now barren ore in the mill discharge. After filtration and washing to remove impurities, the amalgam is heated in a sealed retort to distill off the mercury, which is recovered for reuse

Although amalgamation is still practiced extensively in gold recovery, the very real hazards of mercury poisoning of either the operators or the environment have limited its application and compelled the use of carefully designed equipment to ensure against contamination

More gold is recovered by cyanidation than by any other process. In cyanidation, metallic gold is oxidized and dissolved in an alkaline cyanide solution. The oxidant employed is atmospheric oxygen, which, in the presence of an aqueous solution of sodium cyanide, causes the dissolution of gold and the formation of sodium cyanoaurite and sodium hydroxide, according to the so-called Elsner reaction:

gold ore roasting techniques

With ores of higher gold content (i.e., greater than 20 grams of gold per ton of ore), cyanidation is accomplished by vat leaching, which involves holding a slurry of ore and solvent for several hours in large tanks equipped with agitators. For extracting gold from low-grade ores, heap leaching is practiced. The huge heaps described above are sprayed with a dilute solution of sodium cyanide, and this percolates down through the piled ore, dissolving the gold

Immense amounts of solution and solids are associated with a vat leaching circuit, owing to the very low concentrations of gold in the ores. In order to eliminate the huge capital costs associated with the purchase and installation of solids/liquids separation equipment, techniques have been developed that circumvent the entire separation process. One of these is the addition of granular activated carbon to the ore slurry during or upon completion of gold solubilization. The dissolved gold is readily adsorbed onto the carbon, thus removing it from solution, and the granular carbon is separated from the now barren ore by running the slurry through a screen. Gold is then leached from the carbon particles by a strong solution of sodium cyanide and sodium hydroxide, and it is recovered from solution by electrowinning directly onto steel wool or by the Merrill-Crowe process. In the latter process, the gold-bearing solution is deoxygenated and passed through a filter-press, where the gold is displaced from solution by reduction with zinc metal powder

Many gold-bearing ores and concentrates are not readily amenable to cyanidation, owing to the presence of substances that consume the cyanide reagent before it can dissolve the gold, preferentially adsorb the gold as it dissolves (a phenomenon called preg-robbing), or completely surround the gold particles in such a way as to prevent access by the cyanide leach solution. Such ores are referred to as refractory, and they frequently contain the sulfide minerals pyrite, pyrrhotite, or arsenopyrite. Gold can be freed from these ores or concentrates by treating them with various oxidizing processes. The most common method is to roast gold-bearing minerals at temperatures of 450° to 750° C (840° to 1,380° F) to destroy the interfering sulfides. Oxidation can also be accomplished by the use of high-pressure reactors called autoclaves, in which the minerals in an aqueous slurry are treated at high temperature and pressure with oxygen-bearing gases. After oxidation is complete, cyanidation, as described above, is employed to solubilize and extract the gold

gold ore roasting techniques

rock crushing methods and how to recover gold from ores

The classic old mortar and pestle was the method used by early day miners to crush their ore specimens for testing. You can still buy these things today at most prospecting stores - they are made of cast iron and last a long time and work fairly well. It just takes a lot of elbow grease and work to break up the rock

A slightly different version of mortar and pestle work is a variety called hand stamp crushing. A heavy cylinder of iron is welded to a long rod and placed inside a pipe or tube which is just a bit larger than the diameter of the iron cylinder. This iron cylinder is picked up and dropped onto the ore, much in the manner of the old time stamp mills. This method seems to be popular in Australia

A more expensive but certainly faster and easier method is to use a small jaw crusher, such as is used in preparing samples for assaying.  I have one of these in my garage and it is pictured at the right. For those thinking about processing larger quantities of ore such as hundreds or even thousands of pounds of rock from mines, there are several manufacturers that sell small portable hammer and impact mills. These mechanized crushing systems can generate some very significant dust, and so when using them is important to consider methods for dust control or elimination

rock crushing methods and how to recover gold from ores

If you're screaming of course you need to pick out the screen size you're going to use.  It is really not necessary to go to super small screens like 100 mesh. This reduces most of your rock quite literally to powder. The size of screen I use is 18 mesh and I think anything in the range of 16 to 20 mesh is a good size. I would not recommend going with anything larger than 10 mesh or smaller than 30

Finally, once the rock is fully crushed we get to the part about separating out the gold from the crushed rock. Often, this is accomplished simply by very careful panning. Panning is the most suitable method if you only have an amount of rock less than about 15 or 20 pounds. On the other hand if you are going to be processing large quantities of rock on a regular basis such as would come from the workings of a hard rock mine, you will need some sort of automated system to process your crushed materials.  If you really are going to be working with ores from a hard rock mine, perhaps the best method for the small minor is the use of the a small shaking table.  A good table is fairly bulletproof in one set up operates well on its own, and does an efficient job of capturing small gold particles. Such equipment isn't cheap of coarse but if you're going to be handling thousands of pounds of rock, it's just too much to consider panning as an option for processing

Another possibility that needs to be considered is the use of mercury in capturing the finest gold particles from your ores.  In most instances if you are just handling a few gold Quartz specimens that you find here and there, the use of mercury will not be worth your time.  In some cases the gold in the ore is extremely fine sized, and using mercury to capture it makes sense.  This is also true for operators processing large quantities of hard rock material. I've done up a web page on using Mercury to capture fine gold and you can view it here. (link to page)

rock crushing methods and how to recover gold from ores

For handling another purpose as many operators will be interested in melting down  the gold they recover in crushing their specimens into dore buttons and bars (Dore is a term for unrefined, impure gold and silver metal). I've done up a web page on melting and pouring your gold and you can view it here. (Pouring Your Own Gold)

Ø       Eye protection. it is normal that one crushing rock small particles fly through the air. If a small sharp piece of quartz lands in your eye, it could do some serious damage.  It is important to always wear goggles or protective glasses when crushing. Goggles are the better choice of the two

Ø       Dust mask: the process of crushing rock always creates airborne dust.  If your lungs are already compromised by smoking or other problems, you definitely don't want to expose them to additional hazards. Silicosis is a very nasty and incurable problem which is brought on by long-term exposure to quartz bearing dust. Using a dust mask when crushing is a good habit and make sense especially if you're doing it a regular basis

rock crushing methods and how to recover gold from ores

Ø       If using mechanized equipment, always be very careful around any moving parts.  A good number of accidents in the mining industry occur when people get hands or clothing caught in moving mechanized parts.  The precautions you take will be well worth your time.

Want to know a little bit more about this crazy prospector guy? Well, here's a little bit more about me, and how I got into prospecting: Chris' Prospecting Story   Interested in seeing more gold? Here are some interesting photos of beautiful  Gold Nuggets

goldprocessing -mining and concentrating| britannica

The nature of the ore deposit determines the mining and mineral processing techniques applied. Oxide ore deposits are frequently of such low grade (e.g., 3 to 10 parts per million) that extensive mineral processing cannot economically be justified. In this case they are merely shattered by explosives and then piled into heaps for extraction by cyanidation (see below). These heaps can be hundreds of metres long and 15 to 30 metres in height

Alluvial deposits are either dredged from pond and river bottoms or sluiced from banks and floodplains with high-pressure hydraulic hoses. Alluvial deposits require little or no comminution; they are usually concentrated by gravity techniques such as jigging or tabling, in which a slurry is passed through jigs or over grooved or ridged tables that retain the denser gold particles while allowing the much less dense sand and gravel to pass over

Endogenetic deposits frequently contain elemental gold that is highly disseminated within a base metal sulfide mineral. These deposits are mined, crushed and ground, and then concentrated first by gravity separation to recover coarse particles of native gold before being subjected to froth flotation to concentrate the sulfide mineral fraction that contains the gold

Elemental gold (and silver as well) is soluble in mercury, so that, when particles of the metal are brought into contact with a fresh mercury surface, they are wetted and dissolved, forming an alloy called amalgam. This phenomenon is exploited for the recovery and concentration of fine particulate gold or silver

goldprocessing -mining and concentrating| britannica

Amalgamation is accomplished by passing a slurry of ore over copper plates coated with mercury, by mixing a slurry of ore and mercury in a cylindrical or conical vessel called an amalgam barrel, or by grinding the ore in a ball, rod, or pebble mill to free the gold from the mineral matrix and then adding mercury to the mill and continuing grinding until the gold has dissolved in the mercury. The denser amalgam is then separated from the now barren ore in the mill discharge. After filtration and washing to remove impurities, the amalgam is heated in a sealed retort to distill off the mercury, which is recovered for reuse

Although amalgamation is still practiced extensively in gold recovery, the very real hazards of mercury poisoning of either the operators or the environment have limited its application and compelled the use of carefully designed equipment to ensure against contamination

More gold is recovered by cyanidation than by any other process. In cyanidation, metallic gold is oxidized and dissolved in an alkaline cyanide solution. The oxidant employed is atmospheric oxygen, which, in the presence of an aqueous solution of sodium cyanide, causes the dissolution of gold and the formation of sodium cyanoaurite and sodium hydroxide, according to the so-called Elsner reaction:

goldprocessing -mining and concentrating| britannica

With ores of higher gold content (i.e., greater than 20 grams of gold per ton of ore), cyanidation is accomplished by vat leaching, which involves holding a slurry of ore and solvent for several hours in large tanks equipped with agitators. For extracting gold from low-grade ores, heap leaching is practiced. The huge heaps described above are sprayed with a dilute solution of sodium cyanide, and this percolates down through the piled ore, dissolving the gold

Immense amounts of solution and solids are associated with a vat leaching circuit, owing to the very low concentrations of gold in the ores. In order to eliminate the huge capital costs associated with the purchase and installation of solids/liquids separation equipment, techniques have been developed that circumvent the entire separation process. One of these is the addition of granular activated carbon to the ore slurry during or upon completion of gold solubilization. The dissolved gold is readily adsorbed onto the carbon, thus removing it from solution, and the granular carbon is separated from the now barren ore by running the slurry through a screen. Gold is then leached from the carbon particles by a strong solution of sodium cyanide and sodium hydroxide, and it is recovered from solution by electrowinning directly onto steel wool or by the Merrill-Crowe process. In the latter process, the gold-bearing solution is deoxygenated and passed through a filter-press, where the gold is displaced from solution by reduction with zinc metal powder

Many gold-bearing ores and concentrates are not readily amenable to cyanidation, owing to the presence of substances that consume the cyanide reagent before it can dissolve the gold, preferentially adsorb the gold as it dissolves (a phenomenon called preg-robbing), or completely surround the gold particles in such a way as to prevent access by the cyanide leach solution. Such ores are referred to as refractory, and they frequently contain the sulfide minerals pyrite, pyrrhotite, or arsenopyrite. Gold can be freed from these ores or concentrates by treating them with various oxidizing processes. The most common method is to roast gold-bearing minerals at temperatures of 450° to 750° C (840° to 1,380° F) to destroy the interfering sulfides. Oxidation can also be accomplished by the use of high-pressure reactors called autoclaves, in which the minerals in an aqueous slurry are treated at high temperature and pressure with oxygen-bearing gases. After oxidation is complete, cyanidation, as described above, is employed to solubilize and extract the gold

goldprocessing -mining and concentrating| britannica

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