Before an ore is tested for its amenability to the cyanide process, it should be subjected to a preliminary examination in order that the experimenter may become familiar with its general physical and chemical characteristics. Knowledge so obtained will be of value in laying out the most effective testing program and may point to special methods of attack should the ore not respond readily to simple methods of cyaniding.
A representative sample is taken from the ore to be tested. The size of this sample will depend somewhat upon the amount of ore available, but it need not exceed ½ to 1 lb. If the ore is in lumps coarser than 1″, examination may reveal the nature of the ore and gangue minerals, the degree of crystallization, the extent to which crushing may be required to liberate a portion of the ore minerals, and other pertinent information. The sample is then ground to about 20 mesh, and a weighed amount panned. The concentrate from panning is roughly weighed and then examined under a low-power microscope. The amount and size of any elemental gold and silver are noted, and the nature and amount of any sulphides or heavy minerals are determined. Copper minerals and tellurides especially should be sought. A portion of the concentrate (or some of the crude ore) is given a rough qualitative chemical examination.
From such a procedure, information as to the desirability or necessity for any treatment such as amalgamation or concentration (gravity or flotation) preceding cyaniding may be obtained. Also, there will be some indication as to the degree of grinding necessary.
After being ground to a suitable size, the ore is now sampled for assay. This size will depend upon several factors, such as the nature and amount of material available. It is essential that the assay sample be thoroughly representative, and the exact procedure must be adapted to the conditions prevailing. The amount of analytical work done will depend upon the extent of the testing work to be undertaken and will range from determination of gold and silver only to a practically complete analysis.
In connection with the analytical work it is generally advisable to make a qualitative test for water-soluble salts and if any be present to determine their nature.
Where the ore is known to contain coarse gold or “metallics,” it is essential that these be removed first from the head sample and separately weighed or run down in an assay crucible and collected in lead by standard assay procedure.
A known weight of, say, 500 to 1,000 grams is crushed to about 100 mesh and then either panned or put over a laboratory jig for removal of metallic gold. The tailings can then be dried and assayed in the usual way, while the metallic gold recovered is calculated back on the basis of the weight of total sample taken.
Preparing Sample for Gold Cyanidation Tests
The most successful treatment process is the one that yields the greatest net profits. As the complexity of a treatment increases, so also does the number of factors to be considered before arriving at a decision as to the methods best adapted to the ore under examination.
The following operations may precede cyanidation:
- Amalgamation. Amalgam converted to bullion; tailing cyanided.
- Corduroy or blanket strokes. Concentrate amalgamated; tailing cyanided.
- Gravity concentration. Concentrates may be
b. Amalgamated with, or without cyanidation of tailing.
c. Cyanided, raw or roasted; tailing cyanided.
- Roasting. Calcine cyanided direct or after amalgamation or concentration on blanket or corduroy strakes.
- Acid wash to remove soluble harmful constituents such as ferrous iron and copper compounds.
- Aeration with lime solution.
- Flotation. Concentration treated as in 3; tailing cyanided or discarded.
Laboratory testing of the foregoing operations may be conducted as follows:
Several lots of the ore are prepared by grinding to such different sizes as may seem desirable. A 200-gram sample of ore is put into a bottle with 100 cc water, 1 gram NaOH, and 30 to 50 grams mercury. The bottles are placed on rollers and revolved for 1 to 2 hr. The pulp is then panned, and the mercury separated. Pouring the partially cleaned mercury from one clean beaker to another and washing with a strong stream of water will be found to facilitate final cleaning. A small amount of sodium amalgam will be found useful in causing all the mercury globules to coalesce. The tailings are dried and assayed. The mercury is dissolved in nitric acid, and any remaining gold washed, dried, and wrapped in lead foil with enough silver for parting and then cupeled. An alternative method is to dissolve the mercury down to a small globule, which is then run directly into the assay crucible.
Sodium amalgam can be prepared by forcing small shavings of metallic sodium beneath the surface of mercury. Care should be taken to see that the mercury is dry. As the reaction is rather violent, due precautions should be taken in making the amalgam.
Amalgamation tests can also be made in a grinding pan or an iron mortar, particularly when the ore contains much silver. Copper sulphate and salt may be added to the pulp. The ore may first be given a chloridizing roast.
Corduroy or Blanket Strakes “Sluice” Test
In testing the use of blankets or corduroy the pulp is wet ground to various sizes, and the pulp at a dilution of about 4 or 5 to 1 is passed over the strake set at an inclination of about 1½ to 1¾ in. per ft. Whenever possible, the strake should be not less than 10 to 12 ft. long; the width is not so important. If for any reason it is not possible to use such a length, or if the amount of ore is limited, some idea of the effect of the strakes can be obtained by repeatedly passing the pulp over a short strake. In laboratory tests the weight of concentrate is disproportionately large as compared with plant practice.
The tailings are collected, weighed, and assayed. The blankets are washed, and the concentrate collected. To make a complete recovery of the concentrate, the blanket after being carefully washed should be burned and the ash added to the bulk of the concentrate.
Gravity Concentration Test
If enough ore is available, a laboratory Wilfley table may be used. For small lots of ore a gold pan or a small laboratory jig such as the mineral jig is convenient and gives reasonably good results. Sufficient concentrate should be prepared so that the various methods of treatment as already noted can be tried.
In handling concentrate the experimenter should be particularly careful to avoid losing any fine free gold.
Both concentrate and tailing can be tested with and without regrinding. The concentrate should be dried at low temperatures to avoid oxidation of sulphides, weighed, and assayed, and calculation made as to the ratio of concentration.
In the laboratory, roasting experiments may be carried on by using fire-clay roasting dishes or heavy sheet-iron pans, preferably in an electrically heated muffle. The charges, period, and temperature should be carefully noted. Modern methods of temperature control have made possible greatly improved results in this field.
If the ore contains much arsenic, the amount should be determined before and after roasting. If it is desired to reduce the arsenic to an extremely low amount, the calcine is reroasted with charcoal. The calcine may be passed over blanket or corduroy strakes and then cyanided after removing soluble acidic salts or neutralizing them with lime.
Some ores contain soluble iron or copper compounds which cause a high consumption of cyanide. A water wash or a wash of dilute sulphuric acid or sulphurous acid may remove these compounds to permit of economical treatment of the ore by cyanidation. Washing has usually to be avoided following a chloridizing roast, however, owing to dissolution of gold chlorides, unless provision is made to recover them from the wash water. While acid pretreatment has been used in a few cases, most careful investigation should be made before adopting it.
On some refractory ores and concentrates a preliminary intense aeration in a strong lime solution before the addition of cyanide has been found beneficial. The maximum solubility of lime, CaO, in the ordinary cyanide mill solution is about 2.5 lb. per ton of solution. However, as much as 50 to 100 lb. lime per ton of ore may be used, depending upon dilution and chemical consumption.
Flotation testing of ores containing precious metals may be for the purpose either of determining the applicability of the process to an ore under investigation or of determining the possibilities of improving the work of an operating flotation plant.
In the latter case the work is best done at the mill itself, samples taken directly from various points of the mill stream being used for comparison. In this way it is possible to obtain a pulp that has been ground under operating conditions as well as to use the mill water, which may be of great importance.
When the tests are made to evaluate the application of the process to an ore, the work can, of course, be done anywhere. However, it should be made certain that the ore used for experimentation is thoroughly representative of that which is to be milled. Furthermore, if possible, a sample of the water to be used in the mill should be obtained, and its effects noted.
Difficulties are frequently encountered owing to surface oxidation of sulphides, particularly pyrhotite, and where flotation results cannot be duplicated after an interval of several days or weeks, this condition may be suspected. On this account, it is also desirable to ship and store the ore sample at as coarse a size as is compatible with good sampling, crushing to under that portion which is to be used for immediate testing.
One point that should always be kept in mind is the determination of the proper place flotation may fill in securing the maximum net profit from the ore. On some ores an all-flotation treatment might be indicated; usually, however, better results may be obtained by combining flotation—if it be used at all—with other methods such as amalgamation, concentration, or cyanidation. Again, if all-flotation seems to be best, the possibility of separate treatments of different fractions of the pulp should be considered. For example, the so-called primary slime might advantageously be separated and floated in a separate circuit.
Naturally, such points cannot be determined in advance, but if the operator will keep them in mind as the work progresses, he will be enabled to obtain a better perspective of the basic problem, which is to produce the maximum profit from the operation as a whole, and determine the proper balance between extraction and cost.