How to assay gold ore25.07.2020
How to Assay Gold & Silver Ore
You can use the following method to determine the weight per ton of gold in the ore. Mass assays: calculating the content. Weight of your sample in grams / grams per ton = weight of your sample in tons. Weight of your sample in carats / carats per troy ounce = wt. of your sample in ounces. For extracting gold from low-grade ores, heap leaching is practiced; huge heaps are sprayed with a dilute solution of sodium cyanide, and this percolates down through the piled ore, dissolving the gold. There are very well defined rules for the safe and responsible use of cyanide – as laid out in the International Cyanide Code. Gold refining.
Last Updated: June 12, References. This article was co-authored by our trained team of editors and researchers who validated it for accuracy and comprehensiveness. There are 11 references cited in this article, which can be found at the bottom of the page.
This article has been viewedtimes. Learn more The naked eye cannot determine the purity of metals. This holds true for ore and jewelry alike. In order to determine the percent composition of a sample of gold, the sample must be assayed. Gold can be assayed in one of three ways: with fire, with aqua regia, and with energy dispersive X-ray fluorescence spectrometry.
To assay gold using fire, start by placing the gold into a heat-resistant crucible. Then, add lead oxide, sodium bicarbonate, potassium carbonate, and flour to create a flux, and heat everything to between 1, and 1, degrees Celsius.
Next, pour off the top layer of molten glass, and pour the metal underneath into a mold to cool. Finally, cupel the metal to vaporize any lead, and then submerge it in nitric acid to dissolve any silver so you're left with just gold. To learn how to dissolve gold in aqua regia, scroll down!
Cookie Settings. Learn why people trust wikiHow. Download Article Explore this Article methods. Tips and Warnings. Related Articles. Article Summary. Method 1 of Prepare your equipment. You will need a crucible to put the sample into.
You will need a heat source such as a torch or furnace to bring the sample to high temperatures. You will also need other reactants such as the additives to create the flux, the bone ash to cupel the metal, and some sodium nitrate to extract residual silver.
You will also need molds to pour hot metal into. Also wear goggles, heat resistant gloves, and ideally a fireproof suit. Place the sample in a crucible. The crucible needs to be able to withstand high temperatures. The sample will be exposed to enough heat to melt all of the metals and separate them from other minerals. Clay or ceramic crucibles can withstand tremendous heat. Combine any additives. Additives such as lead oxide, sodium bicarbonate, potassium carbonate, and flour are used to form a flux.
The flux reacts with the same how long to cook snapper in foil ore to promote melting. Different ratios of each additive will produce slightly different flux compounds. Heat the reaction to completion. The flux reaction needs to be heated to completion.
When the reaction is complete, you will see two separate layers. Depending on the laboratory and the additives used, you will typically heat to between 1, and 1, degrees Celsius 2, - 2, degrees Fahrenheit. The top layer is molten glass that contains no valuable minerals. The bottom layer contains your molten precious metals. Pour off the top layer. Carefully discard the top layer of molten glass. It will be of no further use in the assay.
In the mold, the metal can cool until it once again reaches a how to assay gold ore state. This metal is how to pronounce words in japanese to english comprised of gold, silver, and lead.
Cupel the metal. A cupel is a porous container made of bone ash that will readily absorb lead oxide. To cupel the metal, you place it in the cupel and blast it with hot air. This will oxidize the lead. The lead oxide will then vaporize or be absorbed by the bone ash. After cupling, you will have a metal sample that is composed of gold and silver. Dissolve the silver. Submerge the metal in nitric acid.
The acid will not dissolve the gold, but it will dissolve the silver. You can then pour the solution through a filter to separate the gold. Wash the gold. Wash the gold with water to remove any excess nitric acid.
Pat the gold dry with a soft towel. At this point, you should have a sample that is nearly pure gold. Weigh the gold. With all contaminants removed, you can weigh your gold on a scale. By comparing the weight of the gold to the weight of the original sample, you can determine the percent weight of gold in your ore or scrap. This completes the fire assay of the gold piece. Method 2 of Gather the needed reagents. You will need hydrochloric and nitric acids.
You will also need a how to dress corporate casual to filter out contaminants. Finally you are going to need an oxidizing reagent. Wear goggles and gloves when working with this method.
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Mix acids to form aqua regia. To make it, mix three parts hydrochloric acid and one part nitric acid. Use gloves, goggles, and caution when making and using aqua regia. It is highly corrosive and toxic. Aqua regia cannot be stored well. A new batch must be made for each use. Dissolve the sample. Submerge the metal sample in aqua regia. Stir and swirl to dissolve the sample.
Non-metal minerals and silver in form of silver chloride may not dissolve. These minerals will form a sludge. Filter the sample. Pour the sludge solution through a filter.
The sludge will remain on one side of the filter and the aqua regia solution containing the metals will pass through to the other side. The solution is usually greenish in color and will contain many dissolved metals such as gold, and copper.
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The mineralogical data is validated by supporting chemical and metallurgical data (including gold mass- balances). A wide range of metallurgical techniques are used to test the extractability of gold from gold ore. The most common metallurgical tests employed are direct cyanidation, gravity separation and diagnostic leaching.
Before you get going too far, take a look at the following information The goal of assaying and testing any potential gold and silver bearing materials is to determine their metals content.
While it is sometimes possible to look at a sample and guess its possibilities, more often than not, in order to have an accurate idea of how valuable in ore is it must be analyzed. The results of these tests are usually reported in the form of parts per million or also as ounces per ton. The modern assay office is a true analytical laboratory, with the capacity to analyze many types of materials to determine their metals content. A century ago, the fire assay and a few other methods were the only ones used.
Today advanced equipment analyzes a variety of samples not just by fire assay, but by atomic absorption, mass spectroscopy and other specialized techniques. When you are doing anything with hard rock materials, sampling and assaying are the key concepts.
Before any material is taken to be assayed, the sample must first be collected. Accurate samples, ones which reflect the true average of the material must be obtained. If the sample does not reflect the average nature of the material, no amount of technical accuracy in processing the sample will provide a correct result. The first step in any assay is sample preparation I have a separate web page on methods for crushing and pulverizing rock samples and specimens.
If the sample is wet, it is first dried, then crushed. It is split into a reasonable fraction, and then pulverized to a powder. Splits of the pulverized powder are then set aside for various analysis methods, depending on what analysis is to be done. Only a small part of the actual sample is actually analyzed. Between each sample, the crushing and pulverizing equipment is cleaned off to prevent cross contamination of the samples. The famous fire assay has been around for more than years, but it still is the best method for determining the quantity of gold in a rock matrix.
The fire assay process works by fusing or dissolving the rock sample in a crucible using a lead glass flux. Fluxes of various types are used on various types of ores. Most flux mixes contain sodium bicarbonate, potassium carbonate, borax, litharge lead oxide and flour. A typical flux mix might contain five parts sodium bicarbonate for parts potassium carbonate to parts borax one part flour and eight parts litharge. The flour servers to reduce some of the litharge to lead metal.
This flux works for most oxidized ores. For ores rich in sulfides, the flour may be omitted and from one to four nails added according to the amount in nature of the sulfides. In this type of flux, the nails serve to reduce the litharge to lead as well as the reduction of the sulfides. The art of selecting the correct flocks for fire assay test is not extremely difficult but it does take some understanding of minerals and their reactions.
Once the fluxes selected it is mixed with a known quantity of the pulverized ore sample, and the two are heated in a crucible to a bright red heat the amount of pulverized sample used is often With the reactions are complete, the crucible is removed and the molten glass formed of the flux and ore sample is poured off.
The remaining lead metal at the bottom of the crucible is poured into a mold where the lead settles to the bottom and the whole mass cools and solidifies. Once cool, the molds are turned over and the lead buttons removed.
This mass of lead metal contains all the gold and silver was present in the sample. The mass of lead metal is then placed on a cupel made of bone ash. This material is such that it readily absorbs lead oxide which has properties like glass, but does not absorb metal. The lead button is placed in the cupel and heated.
During this process areas allowed into the furnace in the lead metal is oxidized to lead oxide. The oxide then is either absorbed into the cupel, or volatized off into the atmosphere. What is left is a tiny bead containing all of the gold and silver that was in the original sample.
The bead is then weighed, and the total weight of the gold and silver together is known. After this, the gold and silver are parted by flattening to bead and putting it in a solution of dilute nitric acid. Dilute nitric will dissolve the silver but not the gold. The remaining purified gold is then weighed. The weight of the silver in the assay is calculated by subtracting the weight of the gold from the weight of the bead that contained both the gold and the silver.
Using the known original weight of sample that was used in the assay, the weight of the gold in the weight of the silver, the content of the ore sample in ounces per ton or other units can be calculated. Although fire assay processes have been around for many centuries, they are still somewhat of a mystery to many miners.
While improper fire assay methods or incorrect fluxes can cause problems, when properly done a fire assay is extremely reliable method of determining the gold and silver content of an ore sample. The details of how to do a fire assay correctly, and the proper fluxes and how they are made, have been written about and published in many books. They are not secret in any way. These include mercury, arsenic, antimony, thallium and others.
Increased levels of these elements may indicate valuable gold or silver deposits nearby. To measure these elements, the sample must be digested and put into a water solution. There are several methods to put the elements into solution, but the most common is to dissolve them in a heated solution of aqua regia acid aqua regia is a combination of Hydrochloric and Nitric acids.
The resulting leach liquid, no matter the digestion process, is analyzed with special equipment. The instruments used in these analyses measure very small quantities of these indicator elements, and include atomic absorption, x-ray fluorescence, inductively coupled plasma with atomic emission spectroscopy, mass spectroscopy and infrared spectroscopy.
Assay results from a modern lab are not cheap to obtain. Just for comparison sake, recently dropped off seven samples to be analyzed by fire assay for gold and by atomic absorption for suite of pathfinder or indicator elements.
In addition, because of the high price of gold and silver, and the resulting increase in exploration activity, the processing time for my samples was about four weeks. Turn around times are a lot longer these days as the assay labs are quite busy.
A new type of test which is been done in recent decades, is the bottle roll test. This is a preliminary test to determine the leach ability of an ore by cyanide. In this type of test, a known quantity of pulverized sample is placed in a bottle with dilute cyanide solution. The bottle is placed on an apparatus that rotates the bottle keeping the sample and cyanide solution mixing together at all times. After certain amount of time, usually 24 hours, the solution is sampled in the amount of gold which contains is determined.
Usually a portion of the sample is treated by fire assay as well, so calculations can be made to determine the percentage of gold recovered by the bottle leach process. Similarly, there are also processes where a large diameter plastic pipe is filled with crushed ore material, and cyanide solution is percolated down through a vertical pipe and collected.
After certain amount of time circulating the cyanide solution down through the material in the pipe, the solution is tested for its gold and silver content. This process is known as a column leach test. Each of these two cyanide leach tests have their own specific benefits.
The old time prospectors when they were out searching in the field for new veins would do a rough assay by crushing or samples and carefully panning the results. With some experience and a careful examination of the amount of gold present, they could make some excellent visual of the estimates of the amount of free gold in the ore. This method has its limitations, because it is possible to have significant gold which is not free, or too small to be panned and seen by the prospector.
Still it was a handy and quick way to estimate the gold content and know quickly if they had struck it rich without having to carry hundreds of samples in to the nearest assay office. In many ways this is like extracting the gold from high grade specimens, but the difference is in the calculation of values.
The first step in this process is to determine the weight of the sample you are going to process. Realistically, your sample should be in the range of no less than 5 pounds up to about 15 pounds. This may take a number of steps as you weigh parts of the sample on your scale and add them up until you get the weight of the total sample. The greater the weight of your original sample, the more accurate the results will be, but you will also be doing a lot more work to crush the sample to size.
Be sure to write down the exact weight of the sample because you will need this number later. The next step is to crush and pulverize the sample. There are a number of ways to crush a sample down, and I have a separate web page discussing methods for crushing and pulverizing rock samples and specimens.
Take a look at this webpage to get an idea of what methods you might want to use on your sample. You can find it here: Rock Crushing page. You will need to get your sample crushed down to a fine sand and powder size. This is necessary to be sure that you will recover all the gold in the sample. The next step is to carefully pan through your sample. You need to be very watchful about what how you do this to be certain that you do not loose any fine gold. The whole purpose here is to get down through to a final fraction which has nothing but gold.
Pan down until you only have about a couple teaspoons of material left. Of course, you can use magnets to help remove magnetite. The blob should weigh about 15 carats or so around 3 grams.
Be sure to write down the exact weight and then add it to your pan. Shake your pan until all the gold is collected by the mercury. Don't over do it and shake the pan so hard the mercury breaks all up into little pieces. Then remove the mercury and allow it to dry. Re-weigh the dry mercury which now contains the free gold from your sample. Subtract the original weight of the final weight now containing the gold.
This gives the weight of the gold. See my web page on amalgamation to see more information on the hazards and use of mercury to recover gold. You can find it here: Using Mercury page.