TESTS
From what will be said later in regard to the limited applicability of these methods, it will be evident that anyone undertaking the study for the first time of either the application of the processes to a particular ore, or a study of the theory of the processes themselves, should begin with an ore that is easily treated by flotation. What is needed is an ore that presents the fewest special problems. An ideal mixture for the experimental work of a beginner would be one composed of 60% quartz and 40% brown blende, or say, 70% quartz and 30% of pyrite or chalcopyrite. These mixtures are not common in nature, but approximations thereto occur frequently. With the above proportions in mind as ideal ores for the beginner, it is well to ascertain that the ore chosen does not contain more than 2.5% of calcium carbonate or other carbonates. A series of experiments for the beginner who has had no previous knowledge of the processes can easily be made without special apparatus, and such experiments will serve to illustrate some of the phenomena of concentration by floating part of an ore at the surface of a liquid. At the very outset a standard test should be selected for trying each new ore studied or each new oil investigated. The behaviour of the new material under these standard conditions can then be compared with previous results, thus giving the investigator a point of departure in his explorations. A good standard experiment is 1,000 parts of ore crushed to 6o-mesh, 3,000 parts of water, 10 parts of sulphuric acid, and i part of oil, all by weight and at a temperature of 70 C. Variations can then be made from these proportions in conformity with the special characteristics of each ore under examination, or the special nature of the oil used, or of any other variable that may be under examination. In beginning the investigation of any ore, the above is a good combination with which to start the first experiment, and then the proportions may be varied by gradual steps until the combination giving the maximum result is reached.
In the following experiments the ore can be weighed in an ordinary balance ; the water can be measured in cubic centimetres, calling I cc. a gramme ; the acid can be measured from a burette, but the oil should be measured by counting drops from a glass tube drawn to a medium fine point, previously having determined by trial how many drops of a given oil go to make up one-tenth and one-hundredth of a gramme, because the preliminary experiments will be with small amounts of materials.
EXPERIMENT
Take 10 grammes of ore.
30 grammes of water.
o.1 grammes of acid.
o.o1 grammes of oleic acid.
Temperature 75 C.
Place these in a test-tube of 75 to 100 cc. capacity ; warm contents to about 75 C ; close the tube with the thumb, and shake energetically for about a quarter of a minute. The only measure that can be cited for the amount of shaking necessary is to shake the tube until the arm aches. Upon allowing the tube to stand a few seconds there should be decided indications of a separation into layers. At the bottom should be a layer of quartz or gangue much lighter in colour than the original ore ; then a layer of gradually settling gangue-slime ; then a small layer, gradually increasing, of sulphides dropping through the liquid ; then a considerable layer of dirty water ; and finally, on top of all, a layer of froth composed of bubbles of air and sulphide particles. By varying the amounts of oil or other ingredient of the mixture a series can be secured for comparative inspection. Some of the froths made with certain oils are of wonderful persistence, and if undisturbed will stand without alteration for months.
The next step is to take a larger vessel, say, a wide-mouthed bottle holding 200 cc., one that can be grasped comfortably by both hands and held in front of the operator with the thumbs over the cork. The shaking must also proceed to the aching point by the energetic use of the arms and shoulders. The charge in a series of tests of this kind will be :
Ore (6o-mesh) 30 grammes.
Water 90 grammes.
Acid 0.3 grammes.
Oil 0.03 grammes.
Temperature 75C
Shake and allow to settle. By carefully preparing a set ol bottles for comparison, or by familiarity with a long range of similar tests, the operator can make a rough estimate as to the result. In both the case of the test-tube and the bottle it is difficult, if not impossible, to get quantitative results.
The question of securing, on a small scale, quantitative results that would bear some known relation to actual milling operations, and at the same time be under ready control as to the various factors, has occupied considerable attention. Small machines built exactly like large machines do not always duplicate the large-scale operations. Perhaps the best all-round apparatus for a study of ores and the influence of oils, acids, temperatures, etc., on surface-tension results is a small machine,* suggested originally by the author to H. L. Sulman and John Ballot, first made by H. F. K. Picard. and improved to its present form by James M. Hyde and others of the Minerals Separation, Ltd., Staff, as illustrated in the accompanying photograph. The driving mechanism to the left of the picture does not require any description, and can with advantage be discarded in favour of a small motor. The motor also has the advantage that exact speeds can be duplicated, as well as exact lengths of time of agitation. The portion of the apparatus on the right can be better described by reference to Fig. 10, which is a section drawn through the apparatus parallel to the plane of the photograph. Fig. n is a transverse section, and Fig. 12 a plan of the machine.
This machine is made in two parts, an upper one A, and a lower one C, both sliding easily on a rubber cushion B between the two parts, or, better still, having the bearing surfaces highly polished. The rectangular section of the interior is 4^ inches. The upper part A has a tail D, the purpose of this being to prevent leakage of the froth when the upper part is slid to the right for the purpose of removing the froth. Windows of glass in each of the two parts enable the operator to watch the progress of the test. At E there is a simple agitator with four arms at right angles. The shaft driving this agitator goes through the bottom G of the lower part C and is packed with a small stuffing-gland to prevent leakage. At F is a hole communicating with the interior at the bottom, closed with a valve. This hole is for the withdrawal of tailing. The agitator-arms are 2 inches long from the centre of the shaft, thus describing a circle of 4 ins. when revolving, and so leaving a clear space of J in. wide between the ends of the arms and the sides of the interior. The normal speed of this agitator is about 1,500 r.p.m. The machine should be made of some metal that gives a clear casting. The lower surface of A, which rests on the rubber cushion B, and the upper surface of C, upon which the rubber cushion rests, should be planed and polished smooth. The cushion is a piece of rubber insertion about one-sixteenth of an inch thick, and just wide enough to fill the space between the edges H and J of the tray-like top of C. The cushion can be discarded if the surfaces are accurately planed and polished.
The equipment necessary for the tests will also include a tin boiler, with the proper connections to make steam at a low pressure, the steam being introduced by a tube hanging from the top of the machine almost to the agitator inside. An assortment of agateware dishes, burettes, pipettes, etc., is an obvious necessity. Having arranged everything in order, the method of making a test is as follows . Measure 1,500 cc. water and place in the machine. This will fill it to about J inch above the bottom of the upper window. There will be no leak at the plane of junction between the upper part A and the lower part C, because the planed surfaces adhere to the rubber sufficiently to prevent leakage under the slight head. Start the agitator at half-speed, and then admit the steam. While the agitator is running, steam in large volume can be admitted without any ' bumping.'* Weigh 500 grammes of properly crushed ore, and put it into the agitating water ; test the temperature, and when it is at 75 C add the oil and acid in such quantity as the test requires. Turn on the motor to full speed, and let it run, say, five minutes. Stop the motor and agitator, and let the whole experiment set for one half-minute to allow time for the froth to collect on the surface of the water and for the gangue to settle. If the experiment is a success, the gangue will be much lighter in colour than was the original ore, and will mostly settle to the bottom of the machine, where it can be seen through the lower window. Immediately above the heaviest sandy gangue will be seen the finer gangue and slime rapidly settling. At the bottom of the upper window will be a gradually increasing layer of nearly clean water, and on the surface of the water will be from J to 1 1 inches of a dense sulphide froth, the amount of froth depending on the proportion of sulphides in the ore and the success with which the experiment is conducted.
The purpose of the peculiar construction of the apparatus will then be apparent, for the upper part A can be slid along the cushion B, carrying with it the froth and that portion of the clean water above the plane of junction of the two parts. When the upper part has been slid to a position where the screws in the righthand side of the upper window are directly over the lip K, the froth and clear water will run out into a dish placed under K. The tailing, slime, and remaining water can be given a further agitation, but enough water should be added to bring the water up to a line above the bottom of the upper window. After the second period of agitation, a further amount of froth will be formed ; this can be removed in a similar manner, and a third and fourth also, if desired. After all the froth has been secured, the tailing, slime, and remaining water can be removed at F, and the two products, concentrate and residue, examined for weight, size, or composition. In this machine an average of 20 tests per day can be made, and the following series is suggested as instructive :
Take 100 Ib. of ore crushed to 6o-mesh, such as described above as being suitable ; mix thoroughly and put in a dry place for use in the whole series of experiments.
A useful series of observations can be secured by simply observing the weights of concentrate produced. More interesting data, however, can be obtained by sizing both the tailing and the concentrate on a standard series of screens and then assaying the sizes. When the results are plotted from the weights of the various concentrates secured in this series, the curve will be of the nature of Fig. 13. This curve shows that for this ore under these conditions the maximum effect is secured with 0.8 gramme of oil. More than that amount is detrimental, which fact would be proved by still further increasing the amount.
According to the amount of carbonate in the ore, the curve of the plotted results will be something like Fig. 14. This ore needs but little acid, the maximum result being secured with 3 grammes. It will probably be ascertained that if the acid is increased sufficiently the recovery is less, because a point is reached where H2S gas begins to be evolved, and this gas is generally fatal to flotation.
Other series can be built upon the two illustrations already given by varying at regular intervals the temperature, the proportions of ore and water, the nature of the oil, the length of time of agitation, and the speed of agitation. Then having exhausted-trie
The above series of tests, it will be observed, is by means of a process that includes the use of oil and acid and the addition of air by violent agitation. The general principles are the same as in any other combination, and as they are easily applied and approach working conditions, and the results are comparable with those secured in large plants, they are of considerable value. Over a period of four years, when thousands of small tests were made on an ore that was being treated in two plants of 1,000 tons per day, the results from the small tests averaged 5% less recovery and 3% lower-grade zinc in the various concentrates than were secured in the large plant.
This machine is a useful addition to the ore-testing laboratory, whether flotation processes are in view or not, because, by the use of it and the expenditure of sufficient acid and agitation, the sulphide contents of almost any ore can be determined accurately.
Another machine of simpler design and cheaper construction is shown in section in Fig. 19, and in plan in Fig. 20. It is made of easily procurable materials, and can be constructed in the laboratory. It has the disadvantage, however, that it requires some skill in removing froth after the agitation. The froth can be removed by careful skimming with a spoon. Some froth will unavoidably be sunk during the skimming operation, but the agitator can be started again, followed by a second skimming. If there appears to be still further chance of a useful result, a third and fourth period of agitation and skimming can be tried. If this machine is made 4^ inches square, inside measurement, and 10 inches deep, it will be the right size for tests on 500 grammes of ore. The equipment necessary for making tests will also include a tin boiler, with the necessary connections for introducing steam into the apparatus. The agitator should be so belted to the motor that it will run about 1,500 r.p.m. The agitator should be about 4 inches in diameter, thus leaving J inch clear all round its path of rotation. The method of conducting a test will be just the same with this machine as with the other, except in the matter of removing the froth, and in this instance it will have to be removed by an ingenious hand. Carefully dip as much as possible of this froth off the surface of the water with a large spoon. The design and material of this spoon can be left to the ingenuity of the experimenter, as each man will fancy one of his own invention. Some froth, as said before, will be sunk during the skimming operation, but the agitator can be started and run for one minute previous to another skimming. A third and a fourth, if necessary, will give a fair recovery, provided good mineral-frothing conditions have been established. This machine will yield much instruction, but is not as direct in its results as the one in Fig. 6.
An interesting series of experiments can also be conducted in an ordinary soda-water syphon. The cap of the bottle should be altered so that the pressure can be raised in the bottle with a bicycle pump. The necessary alterations are quite easily done, and can be effected in the laboratory. The charge will be 100 grammes of ore, 300 gm. water, with acid and oil to suit. The air-pump should be one with a pressure-gauge attached, so that pressures can be noted. After pumping to 30 Ib. pressure, the bottle with the charge should be shaken a few times, so that the air will dissolve. Then the lever being depressed, most of the charge can be drawn into a beaker, and the froth and tailing examined at will. It is worth noting in this connection that excellent mineral froths can be made at normal atmospheric temperature.
FURTHER TESTS ON OTHER PROCESSES | TEST FOR MACQUISTEN PROCESS
