Precipitating The Gold

100. Precipitating The Gold. On beginning the leaching, about a dozen gallons of the solution of iron sulphate is let into the gold tub (for exceptions see article on precipitating gold, under "Addenda.") The gold is precipitated as a brown powder, which, however, requires many hours to settle. The object of putting the precipitant into the gold tub in advance, is to decompose the gold solution, and precipitate the gold, as soon as it enters the tub, so as to lessen the loss by absorption into the wood, by an accidental leak, or an overflow; also to neutralize the free chlorine in the lixivium.

101. When the solution flowing from the leaching vat has become colorless, or before, if much copper is present, a little of it is received from the hose in a glass vessel, such as a beaker, tumbler, or ore sample bottle, and some solution of iron sulphate is added. If gold is present, a dark cloud is produced, either instantly or after the lapse of a few seconds. When only a slight discoloration is produced in the lixivium, by the addition of iron sulphate, it is better to divert the stream into one of the extra gold tubs, or "gold wash-tubs," because the metal obtained from very weak lixivium is so extremely fine as to settle with difficulty, and even to pass through a filter. The leaching is continued until not a trace of gold can be detected in the lixivium coming from the vat.

102. I will here call attention to a circumstance which was observed in my works. After the leach ceased to show gold by the iron sulphate test, if the discharge was stopped for a few minutes, and again started, the solution would again give a considerable precipitation on being tested, but in a few minutes would again fail to show even a trace, and this could be repeated several times. It was due to unequal percolation, and is mentioned here in case the same might occur in other works.

103. The solution in the gold tub is well stirred with a wooden paddle, and, after waiting for a few minutes, to allow the gold to settle a little, a sample is taken in the glass, and tested by adding some iron sulphate. If any discoloration is produced, more iron sulphate is needed in the tub. If none, yet, as an excess does no harm, it is best to make sure that there is enough of the precipitant, by emptying the glass, rinsing it once or twice with the liquid from the gold tub, taking another sample, and testing it by adding some of the strong solution of gold, which, it will be remembered, was set aside at the beginning of the leaching. If a dark cloud is now produced, it is certain that enough of the iron sulphate has been used in the gold tub. If not, it is safest to add another bucketful, mix thoroughly, and try again. By taking the trouble to make this check test, an expensive accident may be avoided.

104. The gold settles better if the stirring is repeated after an hour or two, and it is advantageous to add a few pounds of sulphuric acid. If, in stirring, a circular motion is imparted to the liquid, nearly all the gold will settle near the middle of the tub. The gold requires from 24 to 48 hours to settle.

105. The weak leach in the wash-tub may be treated with some of the iron sulphate, to throw down the small quantity of gold which it contains, which must then be allowed abundant time for settling; or it may be raised by means of a wooden or leaden pump, and used to begin the leaching of the next charge of ore. A third way, which suggests itself, but has not been tried, except, as will be seen hereafter, in connection with the extraction of silver, is to precipitate the gold, together with any remains of copper, etc., which may be present, by an addition of calcium polysulphide, the preparation of which will be described in its place. A great excess of the precipitant should be avoided, because the gold tersulphide which it produces is somewhat soluble in calcium polysulphide. The precipitate obtained in this way would probably settle more rapidly than the other.

106. Iron Sulphate Solution. This precipitant for gold may be prepared by dissolving common copperas, or green vitriol, in water. A quantity of the crystals is placed in the vat prepared for it, which, it will be remembered, contains a filter, and water is added. There should always be an excess of copperas on the filter, in order that the solution may be saturated. It may also be prepared by dissolving scrap iron in diluted sulphuric acid. Several days must be allowed for the preparation of the solution by this method, and an excess of iron must always be present in the dissolving tub, the supply of solution being maintained by the addition of dilute acid from day to day, and of iron as fast as it disappears.

107. When the liquid in the gold tub has become perfectly clear, the discharge pipe is freed, with its float, and the liquid is allowed to flow through a filter of sand or sawdust, because some fine gold always remains in suspension, even after 48 hours' settling. If a sand filter is used, it is, after a time, taken up and chlorinated, like ore. If if is of sawdust, it is thrown into the furnace with ore, and burned, or burned by itself and the ashes collected.

If the discharge pipe of the gold tub does not sink deeply enough to draw, a piece of board is laid over it, which keeps it below the surface, and follows it down. If it tends to draw too deeply, by being sucked down, a larger float should be used; or a cord attached to the float, and turned around a nail in the side of the tub, will secure it; but it is then necessary to slacken the cord from time to time. If the hose is rather stiff, and well adjusted, it will give no trouble. The construction of the float, encircling the end of the hose with a thickness of two inches of wood, obviates all danger of a loss of the precipitated gold by drawing too near the bottom of the vat. Other arrangements for drawing off the liquid will be found under "Addenda."