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To silver copper plates, that is, to amalgamate them on the face with mercury, is really a most simple operation, though many batterymen make a great mystery of it. Indeed, when I first went into a quartz mill the process deemed necessary was not only a very tedious one, but very dirty also.
To amalgamate with silver, in fact, to silver-plate your copper without resort to the electro-plating bath, take any old silver (failing that, silver coin will do, but is more expensive), and dissolve it in somewhat dilute nitric acid, using only just sufficient acid as will assist the process. After some hours place the ball of amalgam in a piece of strong new calico and squeeze out any surplus mercury.
About an ounce of silver to the foot of copper is sufficient. To apply it on new plates use nitric acid applied with a swab to free the surface of the copper from oxides or impurities, then rub the ball of amalgam over the surface using some little force. It is always well when coating copper plates with silver or zinc by means of mercury to let them stand dry for a day or two before using, as the mercury oxidises and the coating metal more closely adheres.
Only the very best copper plate procurable should be used for battery tables; bad copper will always give trouble, both in the first "curing," and after treatment. It should not be heavily rolled copper, as the more porous the metal the more easily will the mercury penetrate and amalgamate. I cannot agree that any good is attained by scouring the plates with sand and alkalies, as recommended in some books on the subject; on the contrary, I prefer the opposite mode of treatment, and either face the plates with nitrate of silver and nitrate of mercury, or else with sulphate of zinc and mercury, in the form of what is called zinc amalgam. If mine water, which often contains a little free sulphuric acid, is being used, the latter plan is preferable.
The copper should be placed smoothly on the wooden table and secured firmly thereto by copper tacks. If the plate should be bent or buckled, it may be flattened by beating it with a heavy hammer, taking care to interpose a piece of inch-thick soft wood between hammer and plate.
To coat with mercury only, procure some nitrate of mercury. This is easily made by placing mercury in an earthenware bowl, pouring somewhat dilute nitric acid on it, and letting it stand till the metallic mercury is changed to a white crystal. Dense reddish-brown fumes will arise, which are injurious if breathed, so the operation should be conducted either in the open air, or where there is a draught.
Having your silvering solution ready, which is to be somewhat diluted with water, next take two swabs, with handles about 12 inches long, dip the first into a basin containing dilute nitric acid, and rub it rapidly over about a foot of the surface of the plate; the oxide of copper will be absolutely removed, and the surface of the copper rendered pure and bright; then take the other swab, wet with the dilute nitric of mercury, and pass it over the clean surface, rubbing it well in. Continue this till the whole plate has a coating of mercury. It may be well to go over it more than once. Now turn on the water and wash the plate clean, sprinkle with metallic mercury, rubbing it upwards until the plate will hold no more.
A basin with nitrate of mercury may be kept handy, and the plates touched up from time to time for a few days until they get amalgamated with gold, after which, unless you have much base metal to contend with, they will give no further trouble.
It must be remembered, however, that an excessive use of nitric acid will result in waste of mercury, which will be carried off in a milky stream with the water; and also that it will cause the amalgam to become very hard, and less active in attracting other particles of gold.
If you are treating the plate with nitrate of silver prepared as already mentioned, clean the plate with dilute nitric acid, rub the surface with the ball of amalgam, following with the swab and fairly rubbing in. It will be well to prepare the plate some days before requiring to use it, as a better adhesion of the silver and copper takes place than if mercury is applied at once.
To amalgamate with zinc amalgam, clean the copper plate by means of a swab, with fairly strong sulphuric acid diluted with water; then while wet apply the zinc-mercury mixture and well rub in. To prepare the zinc-amalgam, clip some zinc (the lining of packing cases will do) into small pieces and immerse them in mercury after washing them with a little weak sulphuric acid and water to remove any coating of oxide. When the mercury will absorb no more zinc, squeeze through chamois leather or calico (as for silver amalgam), and well rub in. The plate thus prepared should stand for a few days, dry, before using. If, before amalgamation with gold takes place, oxide of copper or other scum should rise on this plate a little very dilute sulphuric acid will instantly remove it.
Sodium and cyanide of potassium are frequently used in dressing- plates, but the former should be very sparingly employed, as it will often do more harm than good by taking up all sorts of base metals with the amalgam, and so presenting a surface which the gold will pass over without adhering to. Where water is scarce, and is consequently used over and over again, lime may be added to the pulp, or, if lime is not procurable, wood ashes may be used. The effect is two-fold; the lime not only tends to "sweeten" sulphide ores and keep the tables clean, but also causes the water to cleanse itself more quickly of the slimes, which will be more rapidly precipitated. When zinc amalgam is used, alkalies would, of course, be detrimental.
When no other water than that from the mine is available, difficulties often arise owing to the impurities it contains. These are various, but among the most common are the soluble sulphates, and sometimes free sulphuric acid evolved by the oxidisation of metallic sulphides. In the presence of this difficulty, do one of two things; either /utilise/ or /neutralise/. In certain cases, I recommend the former. Sometime since I was treating, for gold extraction, material from a mine which was very complex in character, and for which I coined the term "polysynthetic." This contained about half a dozen different sulphides. The upper parts of the lode being partially oxidised, free sulphuric acid (H2SO4) was evolved. I therefore, following out a former discovery, added a little metallic zinc to the mercury in the boxes and on the plates with excellent results. When the free acid in the ore began to give out in the lower levels I added minute quantities of sulphuric acid to the water from time to time. I have since found, however, that with some water, particularly West Australian, the reaction is so feeble (probably owing to the lime and magnesia present) as to make this mode of treatment unsuitable.
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