In both cyanidation and flotation practice it is necessary to keep a close check on the working solutions, since the concentration of various soluble salts derived from, the ore or supplied as a protective measure to the system is usually critical.

Alkalinity

Because of the need in most instances for maintaining a definite protective lime alkalinity in cyanide circuits in order to prevent loss of cyanide as HCN by hydrolysis and/or reaction with atmospheric CO2, the usual titration methods give effective control (see Appendix). In some instances a soap titration giving a measure of the degree of hardness in terms of calcium units has been found to be a better yardstick of control than the customary acid titration.

Ammonium Sulphate

McLachlan, Ames, and Morton found that at Noranda the presence of sulphates, either added as (NH4)2SO4 or present in the barren solution as CaSO4, increases the free settling rate as compared with use of lime water. They also found that a well-aerated pulp settles faster than a poorly aerated one and that the combined effect was beneficial to cyanidation. They consider that the sulphate radical acts as a buffer against the oxygen demand of the pyrrhotite and pyrite.

Other authorities state that lime can interfere with the extraction of gold owing to the formation of insoluble cyanogen or other coatings and that the use of ammonium chloride or soda ash will improve extraction in certain cases.

There are also instances where, owing to the presence of various cyanicides in the ore, it is necessary to carry solution alkalinity below the range of convenient titration using ordinary indicators, for instance, at Morro Velho in Brazil. In such cases the use of pH measurements becomes of special importance. Some mills also make it a practice to test regularly the pH of the raw water used for make-up.
In the case of flotation, pH control, whether by indicators or recording meters, is very general practice (see Chap. II for principles involved).

pH control is applied mainly to pulps in which certain reagents known as regulators are used. These reagents are capable of changing the pH concentration of the circuit and thus influence other factors involved in flotation. Their ability to depress or float selected minerals is closely associated with the degree of alkalinity (or acidity) of the medium. Frequent pH tests should be made. The use of excess lime can be avoided by proper pH control, and it may be pointed out that the use of too much lime not only wastes this material but may have a harmful effect upon the operation. In particular, lime tends to inhibit the flotation of metallic gold. The positive influence of hydrogen-ion concentration likewise holds true in the use of regulators other than lime; moreover, the degree of active alkalinity or pH exerts a characteristic influence throughout the processing not only on the regulators but also on the other reagents used.

Hydrogen-ion determinations should be performed on the individual raw ores, because these substances vary widely in their natural reaction, and a record of such characteristics will be of value to the operating engineer. Furthermore, pH determinations are of value in the control of the water used in flotation. Differences in pH always will be found in the raw waters, and these changes often cause variations in the operating results of the flotation machine which cannot be explained otherwise.

Alkalinity Control 

Successful cyanidation of Sub- Nigel ore, containing about 1 per cent pyrrhotite, requires pre-aeration with close and constant control of pH between 9.6 and 10.0.. As neither glass nor antimony electrodes of the usual industrial pH meters can be used directly in pulps containing abrasive solids or cyanide, it was necessary to devise some method of continuously measuring pH which was sufficiently simple and foolproof to be used by the ordinary operator.

It was found by experiment that the agitation in the Pachuca tank would continuously remove the cake from a leaf filter suspended in the pulp, so that clear solution could be withdrawn almost indefinitely without interruption.

The apparatus developed consists of a 15-in.-square filter leaf made of ¼-in. pipe and 1/8-in. screen covered with 15-oz. filter , cloth. The filter leaf is suspended in the pulp from the rim of the Pachuca tank and is connected to the Merrill-Crowe vacuum through a small solution receiver mounted at the top of the Pachuca.

As pH varies indirectly with temperature, the bottom discharge from the solution receiver is carried down through the pulp to prevent change of temperature from the filter to the electrode assembly and pH meter, located under the Pachuca tank.

The electrodes are wiped with a soft cloth once every two or three days to remove any deposit that may have formed; other than this no attention is needed; and over a period of more than a year no maintenance has been required.

The filtering rate through the leaf filter is 160 to 200 cc per min. of clear solution which is sufficient to displace the solution in the electrode assembly every 2 min.

A complete description with diagram and comment on some of the difficulties encountered is given in a paper by H. E. Cross presented at the May meeting of Chemical, Metallurgical and Mining Society of South Africa and published in Vol. 48, No. 11, of the journal of the society.

Soluble Salts

In flotation circuits it is seldom necessary to analyze the solution for soluble salts because higher concentrations of impurities, within the limits of the alkaline circuits employed, can usually be tolerated. In cyanidation work, however, the serious effects of cyanicides have already been discussed. Poor extractions are not usually the direct result of the presence of such salts, but they are indicative of elements in the ore that consume cyanide before the gold can be dissolved and necessitate abnormally large additions of this not inexpensive reagent. Cyanide solutions are therefore frequently tested, especially during starting-up periods for such elements as sulphur, copper, and iron or the thiocyanates, ferrocyanides, etc., determined by direct titration (see Appendix).

In addition to the problem of high cyanide consumption, the presence of soluble elements in cyanide solutions that interfere with precipitation can seriously affect metallurgical efficiency and costs. High concentrations of zinc and copper are well known for their inhibiting effect on gold precipitation with zinc, but relatively low concentrations of nickel and thiocyanate, such as are encountered at the Kerr Addison Gold Mines, Ontario, Canada, can also cause precipitation difficulties. The serious effect of chromium is discussed under “Cyanicides.” High concentrations of arsenic resulting from the treatment of roasted arsenopyrite concentrates can inhibit precipitation completely.

Precipitation

An article appearing in Trans. 50, C.I.M. and M., 558, 1947, “An Apparatus for Comparing Various Zinc Dusts for Gold and Silver Precipitation” by D. J. A. Dahlgren, describes a method and device that may be useful to mill operators when problems of precipitation arise. Since flows of pregnant solution as low as 4 liters per hr. are handled, the apparatus has possibilities as a precipitation unit for use in cyclic laboratory cyanide tests.

Provision is made for de-aeration, the metering of the solution at a constant head past an orifice through which a regulated suspension of zinc dust is fed by means of a novel reciprocating feeder, filtration, and the metering of filtration and control of vacuum. The efficiency of precipitation is obtained by dividing the number of ounces of gold or silver precipitated per ton of solution by the number of pounds of zinc dust fed per ton of solution.