Before melting the retorted metal, the black-lead or graphite crucible must be annealed by driving off all the contained moisture, or the sudden heating of this moisture through its expansion and the formation of steam will burst the pot. For this purpose the pot is kept in a warm place, such as under or over a stove or boiler for a week or more, when it is placed directly in the stove or in the boiler fire for some time, being in this way slowly and gradually brought to a very high heat, after which it can be used with safety. Between melts the crucible should be kept in a dry, hot place, for when below 250F. The crucible tends to absorb moisture, and its life is largely dependent upon the thoroughly annealed condition in which it is used. The pot is placed in the furnace fire and when sufficiently heated to melt the metal, the flux is added. After the flux has become molten, the retorted metal sponge is added in pieces as fast as it melts down until the whole is melted, employing a long scoop or tongs in handling the pieces of gold sponge. The quantity of the flux and its character will depend upon the cleanness of the sponge after retorting and the nature of the impurities. The amount of flux used and the proportions vary with each melter and can be determined only in an empirical way by knowing the theory of fluxing and then judging the amount and character of the impurities.
Should the amount of flux appear too small, as melting proceeds, more can be added at any time, while an excess does no harm, nitre excepted. The pot should be provided with a cover which is kept in place during melting, except when removed for observation or for stirring the melt. The experienced melter on a clean retorted metal will use little or no flux, while the novice may use, on a somewhat base retort, flux to the amount of 5 or 10% of the metal. The average melter employs borax-glass and bi-carbonate of soda in approximately equal proportions by bulk. The professional melter confines himself largely to borax-glass. The principal impurities to be fluxed off are oxide of iron, sand, and a little sulfur. Borax dissolves the metallic oxides forming borates of the bases ; soda acts as a desulfurizer and forms sodium silicates with the sand ; together they slag off the impurities and cause the metal to melt down rapidly. It is preferable to use an excess of borax-glass. For taking care of small or medium amounts of iron, in addition to the use of borax, silica may be used to form an iron silicate. Where the retorted metal contains a large amount of iron, sulfur should be added to the surface of the molten metal at the sides of the melting pot, and stirred in with a plumbago stirrer to form a matte of sulfide of iron.
Should the amalgam have contained some sulfide, the molten metal should be 'poled' by allowing a heated iron rod to remain in the pot for a little time to slag off the sulfur as iron-matte (ironsulfide). If the amalgam contained much metallic iron this 'poling' will not be required. If the quantity of iron sulfide formed is small, it will be dissolved by an excess of slag; if large it will form a matte between the bullion and slag. This matte should be saved and after a quantity from several melts is collected, should be fused with borax and soda to form a button of gold and a bar of clean matte, or should be cast into a bar and shipped. Nitre (potassium-nitrate) is used to oxidize the base metals that they may pass into the slag, but it also oxidizes the carbon of the crucible while its base potassium combines or slags with the clay used in the manufacture of the crucible, corroding it badly, consequently nitre should only be used by the experienced melter. Silica tends to increase the grade of the bullion, but if not used in the right proportion the slag is liable to become viscous and contain shots of gold. An excess of soda makes a liquid slag and one that separates easily from the bar; a large excess can easily be detected in cold slag, especially when slacked or chilled in water, from having the characteristics of soda. An excess of soda will attack the crucible while an excess of borax will not.
A silicious slag one containing an excess of silica is stringy, can be pulled into long strings when cooling, and is glassy and brittle when cold. When the slag contains such an excess of silica that it becomes thick and viscous it will still be stringy it should be thinned by the addition of soda to unite with the silica as a sodium silicate. A basic slag one containing less than a normal amount of silica or acid flux is 'short,' cannot be pulled into strings when melted or cooling, and is dull and stony looking when cold. The slag made in melting retort metal is usually of a very basic nature, borax being relied upon to slag the impurities and thin the charge, but too great an excess of borax will make the slag thick. If the slag is too thick and yet is basic, and it is deemed inadvisable to add more borax, then silica should be added, which may be in the form of fine quartz tailing free from slime, since the slow settling slime is mainly clay silicate of alumina rather than pure silica. The appearance of graphite in the slag indicates that the crucible is being attacked, and usually means that more silica should be added.
For melting an ordinary retort sponge, a small amount of flux consisting of two or three parts by weight of borax-glass and one of soda, and 'poling' with an iron rod, if the amalgam contained much sulfide, is all that will be required in the way of fluxing. After the metal and slag have subsided to a quiet fusion, the mass is stirred with an iron rod that has been previously heated red hot that no gold may adhere, the object being to settle any shots of metal in the slag and to render the gold homogeneous. The crucible is then lifted from the fire by means of suitable tongs and its contents are poured into an iron mould, which has previously been well coated on the inside and heated. The slag rises on top of the metal and may overflow the mould without doing any harm, if it be quite fluid, while the gold by its greater weight or specific gravity sinks down through the molten slag and is retained in the mould. It is improbable that any shots of gold that will not settle while in the furnace will do so after pouring, so all slag from gold melts should be carefully examined for shot gold. The mould should be smooth and clean on the inside, all rust, old slag, or metal should be removed. It should be given a coating on the inside, preferably of carbon. This may consist of a mixture of lampblack and lubricating oil having the consistence of soft butter.
Or it may be a coat of soot given by inverting the mould over burning pitch pine, resin, or oiled waste. Whitewash can be used. Thick oil has been used, but it sputters while pouring and afterward burns with a disagreeable smoke and odor. The purpose of this coating is to prevent the gold from sticking to the sides of the mould and to allow the bar to come out easily. The mould should be well warmed, but not excessively, before being used, that it may not be cracked by the introduction of the hot metal, and that the gold and slag may not be suddenly chilled, interfering with forming a neat smooth bar. The mould is finally leveled that the bar may be of an even thickness. Usually the mould is placed at a right angle to the flow from the melting pot, but a neater, easier pour can be made by setting the length of the mould in the line or direction of the pour. Greater homogeneity can be given the bar for the purpose of sampling by continually moving the entering stream of metal up and down the length of the mould in pouring.
After the gold and slag have cooled sufficiently to become solid, they are dumped out of the mould into a tub or sink of water, which usually causes the slag to separate easily from the gold. The bar is cleaned by knocking and scrubbing off any bits of slag, or by setting back in the melting pot until hot and then plunging it, first into dilute sulfuric acid, and then into water. Nitric acid is also used. If the bar looks very base and dirty, it may be re-melted and refluxed. Two opposite corners are chipped off for assay, or it is bored in from four to eight places with a %-in. drill, rejecting the surface borings; the latter method of sampling is to be preferred. Some use graphite rods for stirring the molten bullion; these are either purchased or are made by cutting a section out of an old or condemned crucible in the shape of the lower part of a golf club. A small hole is bored in the toe of the stirrer. After stirring the bullion, the gold caught in the hole, amounting to half a gram or more, is poured into a basin of water ; this is repeated three or four times and the bullion assay made from the granules obtained in this way. A dip sample taken in this way is more accurate than any bar sample.
Recovering Gold from Slag, Old Screens, Etc. The slag from the meltings, likewise old melting crucibles, are saved and eventually run through the clean-up barrel in a separate charge to recover any shots of gold. The slag can be sent through the battery if there is no clean-up barrel available, but the crucibles should be crushed otherwise and panned, as the graphite is harmful to the plate amalgamation. After this treatment the tailing should be assayed, as it may still contain sufficient gold to warrant shipping to a smelter; it has been cyanided, but with poor extraction. The wood removed from the mortars, together with old screen frames, and all wood or canvas likely to contain any amalgam should be burned and the ashes put through the clean-up barrel, or the mortars in lieu of a barrel or pan. The burning is sometimes accomplished in the stove installed for the comfort of the mill men, the ashes of which are regularly emptied into the mortars. This method cannot be used where cyanidation follows because of the tendency of carbon to precipitate gold and silver. The worn-out screens should be thoroughly scrubbed and pounded after being taken from the frames, to remove any amalgam, and then placed in a heap. Shoes and dies and pieces of iron from the mortars should be scrubbed and hammered and the 'eyes' of amalgam in the blowholes picked out by means of old round files tapered down to a point, finally being consigned to a pile. The fine iron removed from the amalgam should be placed in shallow tubs. The oxidation of these screens and coarse and fine iron and steel should be promoted by occasionally adding salt and frequently wetting with water. After being reduced to rust as far as possible, that material which will enter the clean-up barrel should be ground up in it with a small amount of mercury and finally dropped into water, puddled, and the finer iron removed by a magnet. It may be necessary to re-wash this finer material. The screens and coarse iron receive a thorough scrubbing and pounding before being finally thrown out. Roasting or burning the screens and fine iron is a quick way to loosen the adhering amalgam and to promote oxidation.
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Hard Rock Quartz Mining and Milling
