CHAPTER VII DISCREPANCIES BETWEEN ACTUAL RECOVERY SECTION II TONNAGE COMPUTATION In some mills every car or the belt conveyor if such be in use, passes over an automatic weighing and recording scale and this method comes nearest of any to giving a correct determination of the tonnage, but even here an error may creep in due to faulty correction for moisture. The percentage of moisture in the current ore is usually taken as a fixed quantity, and the amount is more often guessed at than ascertained. Even when an honest attempt is made to obtain a correct average figure for moisture, the determination is by no means easy. Ore from different parts of the mine is liable to vary over wide limits in its moisture content, and the season of year may make a difference in the ore from any given stope. And if there is an opening for discrepancies in tonnage under the weighing system, what is to be said of the more common practice of taking the buckets or cars at a fixed and arbitrary weight and multiplying by the number passed into the mill during each day? Probably the best way to estimate average moisture in the ore fed to the mill is to weigh a whole bucket load at intervals and at once dry it on a flat-sheet supported on bricks, over a slow fire, weighing it again when dry. If two or three buckets are treated in this way every second day for a week or ten days a fairly good idea may be obtained of what correction ought to be made, and an average thus arrived at. Similar determinations should be made both in the dry and the wet seasons. If ore in different parts of the mine varies greatly in its water content, separate estimations of each kind of ore should be made and the number of cars of such ore during the day or month noted. If a sample of this dried ore be taken and tested for moisture after. being pulverized in the laboratory, a further reduction in weight may often be recorded amounting to 1% or over. Another method commonly made use of for arriving at the monthly tonnage is to estimate it from the ore pulp in the cyanide plant, the sand (if leaching is in use) being calculated from the cubic contents of each charge, and the slime by the specific gravity and measurement of the slime charges. If these operations are performed with proper precautions and due care, a close approximation to the correct tonnage may be obtained but it is very easy for serious errors to creep in. Regarding the sand charges the weight of a cubic foot of sand varies over wide limits according to the method of filling and the specific gravity of the ore The same ore may vary from 19 to 30 cubic feet per ton according to (1) whether the charge is measured in the tank in which it was collected or after being transferred to another tank, (2) (if measured after transferring) whether such transfer is made. by shovelling or mechanical devices, (3) whether such measurement of transferred sand is made before or after applying solution, (4) (if measured in the collecting tank) whether it be collected by one or another of the various methods described, (5) whether the sand be charged dry or moist. Thus, in order to avoid error the weight per cubic foot should be carefully and frequently determined. Another source of error may arise in the measurement of vertical depth of each charge owing to the fact that the surface is never level as a whole, nor is it free from small irregularities at the point of measurement making it difficult to decide within an inch or so at what point to begin measuring. In the slime treatment it is difficult to estimate tonnage with the continuous system but in the charge system it may be done very closely, if the following precautions are observed: 1. The specific gravity of the dry slime must of course be accurately determined. 2. In ascertaining the ratio of liquid to solid in any given charge the sample taken must be representative. As a rule the thicker a pulp is, when in agitation, the more homogeneous it is. From a slime charge with a dilution ratio of 3 or 4 to 1 in an ordinary mechanical stirring tank, samples may be taken at different points which show widely differing ratios of liquid to solid. 3. Correct measurements of the cubic content of the charge must be made. This cannot be done while the charge is in motion. Moreover, allowance has to be made for permanent solid material occuping space in the tank, especially hard rings or cones of slime, sand, etc., and also (in the case of flat bottom tanks) a certain depth of treated pulp which it is impossible to remove each time the tank is emptied. Tonnage of Slime Pulp, as Ascertained by Density Determination. This method depends on the cubic capacity of the slime charge and its density as determined by specific gravity test. Specific gravity may be explained as the ratio between the weight of a given substance and the weight of the volume of distilled water at 4 degrees C. displaced by it. The first requisite is to find the specific gravity of the dry ore-pulp. For this purpose dry some of the powdered material over a water bath or at a temperature not exceeding 100 deg. C. (212 F.), and at once weigh up carefully 250 grams and place in a dry litre measuring flask which has been previously weighed and the weight recorded. Then add about 500 cc of distilled water and mix well. The dry ore nearly always contains a large quantity of air adhering to the particles which is very difficult to detach and therefore Clennell1 recommends immersing the flask in boiling water and keeping it at that temperature for from one to two hours in order to expel all the air. It is then cooled, and distilled water added up to the mark, after which the flask with contents is weighed and the weight of the empty flask deducted, the object being to ascertain the weight of the water which has been displaced by the ore. To do this the weight of a similar volume of distilled water 1 The Cyanide Handbook, page 557 (Second Edition). must be known. A litre flask contains nominally 1000 grams of water but it is better to empty the ore pulp out of the flask and after washing well to refill with distilled water at the same temperature as was the water used for mixing with the ore sample, and after drying the exterior of the flask to weigh carefully. This figure less the recorded weight of the dry flask is then compared with the weight of the same volume of the ore-and-water mixture previously found. Let a = Then and and b = weight of dry ore taken weight of water in the flask when filled to the mark with water alone C = weight of mixture of ore and water at the same mark d = weight of water in the mixture W = weight of water displaced by the dry ore s = specific gravity of dry ore If great precision is required, the mixture of ore and water, and also the water alone, should be cooled to 4 deg. C. in the flask and weighed at that temperature, the level of course touching the mark in each case, but for ordinary purposes it is sufficient that the temperature at both weighings should be that of the atmosphere. To determine the specific gravity of the wet pulp composing the slime charge it is more convenient to use a litre flask whose neck has been cut off about an inch above the mark, or better still to procure such a flask properly finished off by the manufacturer. This is weighed empty and then filled to the mark with the pulp to be tested. A representative sample should first be taken in a bucket and the ratio of water to ore should be low, that is, the pulp should be as thick as possible to ensure homogeneity. The sample is then thoroughly mixed and kept in agitation while portions are dipped out with a small beaker, and poured into the flask taking care that no settlement takes place in the beaker during the operation. This is continued until the level reaches the mark on the neck. The flask is then dried (if wet on the outside) and weighed, and the weight of the empty flask deducted from the figure found. For calculating the quantity of ore in the charge from this figure, Clennell1 gives the following formula: where V1 = Number of cu. ft. of pulp to 1 short ton of dry slime. S 1 P = W. A. Caldecott's pulp table with one or two slight changes in the formulas is here given in full, based on a specific gravity of 2.6 for the dry ore. A second table follows, showing the changes in the various formulas in the preceding table for specific gravities from 2.5 to 3.5, so that a complete table may be drawn up for each mill according to the specific gravity of its ore. 1 The Cyanide Handbook, page 559 (Second Edition). |