The de-silverizing of base bullion is carried out in accordance with the principles of the well-known Parkes process.
The base bullion produced by the blast furnace department carries (apart from silver values) impurities, the chief of which are copper, antimony, and arsenic.
In order to more clearly indicate the grade of the base bullion, the following analysis, representative of a half-year’s production of base bullion, is submitted:
Before proceeding with the actual operation of de-silverizing, it is necessary that the impurities, copper, antimony, and arsenic, should be eliminated. This is accomplished in the following steps :
Removal of Copper from Bullion
Copper is largely present in the base bullion as dissolved sulphide, and this is removed by liquation in a reverberatory type furnace, of which the following are the essential details:
Hearth area…………………………………220 sq. ft.
Grate area……………………………………..31 sq. ft.
Ratio hearth area to grate area……………….7:1.
Working temperature………………..750° to 850° C.
Coal consumption per 24 hours…………2 tons.
The hearth of the furnace is encased in a steel tank 22 ft. 4 in. long by 13 ft. 4 in. wide by 2 ft. 10 in. deep ; the bottom of the tank is covered by a depth of composition conforming ….Read more
The purest gold obtainable is required for use as standards or check pieces in the assay of gold bullion. The following method of preparing it is now in use at the Mint. Gold assay cornets from the purest gold which can be obtained are dissolved in nitrohydrochloric acid, and the excess of nitric acid expelled by evaporation with additional hydrochloric acid on a water bath. The blackish-red fused product, smelling of chlorine and consisting chiefly of AuCl3. HCl, or HAuCl4 (chlor-auric acid), is then poured in a thin stream into a large glass vessel full of distilled water, and a solution of about 1 oz. of gold in each pint of water (1 gramme of gold in 20 c.c. of water) is formed in this way. After vigorous stirring the solution is left to settle, and at the end of about a week, the whole of the precipitated chloride of silver will have subsided to the bottom. The progress of the subsidence is easily watched. The particles fall at the rate of about 3 or 4 inches per day. The clear bright supernatant liquor is now removed by a glass siphon, and diluted to about 1 oz. of gold per ….Read more
The bead of silver and gold obtained by cupellation is squeezed between pliers, or flattened by a hammer on a clean anvil, to loosen the bone ash adhering to its lower surface, and is then cleaned by a brush of wires or stiff bristles. It is then weighed, the silver removed by solution in nitric acid, and the weight of the residual gold taken, when the difference between the two weighings represents the silver. If the bead contains more than one-fourth its weight of gold, more silver is added to it as otherwise some of the silver will remain undissolved, being protected from the action of the acid by the outer layers of gold. The amount of silver to be added is calculated from the (approximately) known composition of the bead, or guessed from its colour. A pale yellow bead always contains more than 60 per cent, of gold, but a perfectly white bead may not “ part” completely. The addition of the silver is sometimes effected in the case of small beads by fusion on charcoal by the blowpipe, but it is better to cupel the bead with the additional silver, wrapped in as small a piece of lead ….Read more
Cupellation is conducted in a muffle furnace, the construction of which is shown in Figs. 62-63. The fire is lighted, a little bone-ash is sprinkled on the floor of the muffle to prevent its corrosion by litharge in case of the upsetting of a cupel, and the cupels introduced as soon as a bright red heat is attained. The cupels are cleaned by gentle rubbing or blowing before being charged-in, and are again cleaned with bellows before the lead is charged-in. They are placed in the furnace one by one, or, better, charged-in together on a tray, see below.
Cupels are little cups made of bone-ash, and are either round or square. In their manufacture the bone-ash is finely powdered so that it will pass a 40-mesh sieve, then slightly moistened with water (to which a little carbonate of potash is sometimes added), put into a mould (Fig. 65), and compressed by the blows of a mallet, or, better still, of a screw press, so as to cohere firmly. The mould is preferably made of steel, as gun-metal wears sooner, and an uneven surface is disadvantageous. It is kept clean, bright, ….Read more
“Highgrading” a polite word for stealing gold and silver for your refinery, has been a way of life since the metals in native form have been mined or produced. Unfortunately, in the past, highgraders were not always severely prosecuted or even looked upon unfavorably in their communities. Recent high metal prices with attendant world wide publicity, however, has made highgrading or thievery again a serious problem in mines and plants.
When a small button of gold the size of the end of one’s little finger has a value equal to about one week’s wages, the safeguarding of gold production by enforcing tight security measures becomes a prime responsibility of management. The higher the gold price, coupled with unrestricted ownership, the greater the temptation to “get-your-cut”, and diligence by management must become an integral part of plant security.
Primary security begins by knowing the expected gold production and the sampling of a gold ore mill feed for this determination has proved not reliable. However, solution flows can be accurately sampled and measured thereby closely predicting the gold recovered each day as precipitate. The progressive record of total ounces of gold contained in the precipitation press, ….Read more
Fire assaying, in essence, is a miniature or small scale smelting process which recovers and reports the total gold content of the assay sample, including gold combined with other elements or locked in the ore particles. Because of this, a assay may report values that cannot be recovered by placer methods and it cannot be too strongly stressed that when dealing with gold placers, the sample values should not be determined by fire assay. Furthermore, no credence should be placed in placer valuations or reports that are based on the results of fire assays. Although this should be common knowledge among mineral examiners, a surprising number seem unaware that fire assaying although accurate per se yields misleading results when applied to placers.
There are other reasons: First, consider the small quantity of material used in a fire assay. The usual amount of sample taken for a crucible charge is either 29.166 grams (one assay-ton) or half of this amount. Next, consider that a particle of placer gold only 1/32-inch in diameter may weigh about ¼ milligram. Now if the bank-run material from which it came averaged 10c per cubic yard, the ¼-milligram gold particle ….Read more
The Gold Refining Process by Aqua Regia was introduced at the Pretoria Mint after the Miller process had been tried and abandoned owing to the alleged difficulty of treating the gold bullion extracted by the cyanide process. In the aqua regia process the gold is dissolved and precipitated. It is made very difficult if the silver exceeds 100 parts per 1,000, and at Pretoria bullion was not treated if the silver exceeded 50 parts per 1,000. Mill gold with 80 to 110 parts of silver, and cyanide gold if it contained more than 50 parts, were melted with gold obtained by the chlorination process to reduce the silver to less than 50 parts, and granulated. Charges of 500 grammes of the granulations were then placed in each of 40 boiling flasks of 3,250 c.c. capacity, and treated with a mixture of 6 parts HCl to 1 part HNO3. Some gold was always left undissolved to avoid loss of acid. The flasks were heated on sand baths, and at the end of the day poured into porcelain vessels holding 100 litres each.
Next morning the clear liquid was siphoned off, leaving the silver chloride at the bottom of the vessel, and transferred ….Read more
The Moebius Process of Purification of Gold by Electrolysis is now in successful operation and is said to be specially suitable for refining copper bullion containing large proportions of silver and gold with small quantities of lead, platinum, and other metals, but is chiefly used in parting dore silver containing not more than 20 per 1,000 of base metals.
The apparatus required consists of a number of wooden vats coated inside with graphite paint, and filled with a solution containing 1 per cent, of nitric acid, which constitutes the electrolyte. The anodes consist of plates of bullion of about ½ inch thick, 18 inches long, and 10 inches wide, which are hung in muslin bags destined to catch the insoluble impurities after the silver, copper, &c., have been dissolved. The cathodes consist of plates of pure silver, slightly oiled to prevent adhesion of the deposited metal. These plates are continually scrubbed by a mechanical arrangement of wooden brushes by which their surfaces are kept free from loose crystals of electrodeposited silver. The loose silver falls on to trays placed below, which are removed at intervals, and the silver collected from them.
The current should have an electromotive force of from one to three volts ….Read more
The guard pot, with the clay pot in it containing 2 or 3 ozs. of fused borax, is placed in the furnace, and is heated gradually until the bottom of the clay pot is dull red. The ingots (of which the larger are slipper-shaped) to be refined, amounting in all to 650 to 720 ozs. in weight, are then placed loosely in the pot, the furnace filled with fuel, and the dampers opened. As soon as the gold is melted, which generally happens in about one and a-half hours, the boraxing of the pots being also affected at the same time, the perforated lid is put on, and the pipe-stem, previously brought carefully to a red heat to prevent cracking or flaking, is pushed to the bottom of the pot. As the pipe is being inserted, the chlorine is gently turned on to avoid stoppage of the passage through the stem by the solidification of metal in it. The supply of chlorine is controlled by the glass stopcock over the furnace, and the amount is adjusted so that the whole of the gas is absorbed and no globules of metal can be thrown up. This can usually be ascertained by ….Read more