CHAPTER VII. THE ASSAY OF TIN ORES THE student is required to determine the percentage of metallic tin by fireassay in (a) Clean Stream Tin. (b) Poor Ore containing Wolfram and other impurities. (a) Clean Stream Tin, SnO2 Principle of Assay.-The student will find, on consulting advanced textbooks, that various methods of assay are recommended; methods varying in quantities of ore and fluxes, and, to an unaccountable degree, in temperature and time of fusion. Here only one method will be given, and the student must practise this method till accurate results are obtained; then, if time permits, he may experiment with other methods. As in photography the amateur who changes about from one developer to another seldom does good work, so here the student who attempts all methods seldom masters any. The method here described depends on the reaction expressed in the following equation— SnO2+2KCN=Sn+2KCNO Apparatus.-The wind furnace, pulp scales, and minor accessories. Reagents. The sole reagent is potassium cyanide, that sold as gold cyanide being most suitable. Details of Operation. The stream tin is sampled and reduced as usual, the final assay sample being passed through an 80 sieve. The following charge is weighed out in duplicate— In the bottom of each crucible (E) ram about 5 gms. KCN, then add the above charge, and finally add about 5 gms. KCN as a cover. Transfer the crucibles, and carefully bed them in a bright red (not a white) fire. Fuse for twenty-five minutes. Remove the crucible, and if the edges are still bright red pour the charge; if not, allow it to remain in the crucible till cold. Break the crucible and remove the button. Generally, the student will find that by quick and careful pouring good results may be obtained. The slag may be (and should be) examined by grinding and panning (see "Panning "), and any prills obtained are added to the result. Note the weights of buttons obtained. The student must continue his practice till uniform results are obtained, and to do this he must have the furnace well under control. Calculation of Results.-As the result is generally required in percentage of tin, the weight of button in grams is multiplied by 10, e.g. a sample of stream tin gives buttons averaging 7:32 gms.; the percentage of tin is therefore 73.2. Accuracy of Results.-Care and much practice are necessary in firing tin ores. Under good conditions, that is, with a clean ore and proper firing, the error is not greater than 0.5 per cent., e.g. if an ore by fire-assay yielded by the Cyanide method 72.3% tin, the true tin contents are not more than 72.8%, nor less than 71.8%. (b) Poor Ore containing Wolfram and other impurities. Method of Treatment.-As arsenic and sulphur may be present besides wolfram and decomposed granite or other rock, it is evident that if the oxide of tin in a fairly clean state is required, some method of purification must be adopted. As and S will be eliminated by roasting, the light gangue by panning, and any oxide of iron and tungsten by treatment with suitable chemicals. The remaining oxide of tin is then treated as before. Apparatus.-Furnace for melting and roasting. A prospector's 'gold' dish or other 'panning' or 'vanning' appliance. Chemical apparatus, pulp scales, etc. Reagents.-Potassium cyanide, ammonium carbonate, hydrochloric acid, ammonia (20E. NH1HO), rain (or distilled) water. Details of Operation.-The ore is crushed and sampled till one pound or 400 gms. is obtained. This sample is passed through a 60 sieve. If S and As be present, add a little ammonium carbonate, and roast the sample in a sheet-iron vessel on top of the wind furnace, the temperature having been raised to a bright red. During roasting add a little more ammonium carbonate. When the S and As have been removed, transfer the charge to the gold pan, under which is placed a larger basin to catch the tailings.' Run into the gold pan about a quart of water, and with one hand work the charge about till thoroughly moistened (takes two or three minutes), allow the sediment to settle for a minute, and carefully pour off the liquid into the basin below. This second basin is set over a sink to conveniently dispose of subsequent overflow. 'Panning' or 'vanning' proper now begins. The lip of the gold pan is dipped in the second basin till enough water is lapped up to cover the ore about half an inch. The ore and water are then agitated by a peculiar circular and jerking motion (best learned by practice), with the object of settling the heavier particles of tin ore and keeping the light gangue partially suspended. Some operators allow a continual discharge to proceed by inclining the pan; others work the ore down to the angle of the pan, or between the angle and the ripple near the lip, and then wash away the top layer (about the total depth). This operation is continued till the dark coloured oxide of tin begins to show through the lighter gangue. The remaining grains of sand are removed as far as possible by further panning. The student should now take the tailings in the second basin and again pan them in the gold dish. He will generally find that in the first operation he has allowed some oxide of tin to escape, and even on the third repetition he may still find that he is losing tin. (For practice in panning the student is recommended to take three small specks of gold, place them in a few pounds of earth, and pan off the mixture till he separates the three specks.) From an ore containing approximately 2% tin a residue of about 14 to 20 grams will be left as the result of the concentration. This residue is now transferred to a beaker or porcelain basin with cover, and gently boiled with strong HCl to dissolve iron salts and wolfram. Half an hour's boiling generally suffices. If wolfram be present, a yellow residue of tungstic acid (WO) forms. The HCl is removed by decantation, and then the tungstic acid is removed by solution in excess of strong NH HO, a soluble tungstate being formed and removed by decantation. The residue is now dried, weighed, and fluxed with the same proportion of cyanide as before. Calculation of Results.-The percentage of tin is found thus:-If 400 grams of ore give, for example, 8 grams of metallic tin, then 100 grams give 2 grams of tin; that is, the ore contains 2% of tin. Accuracy of Results.--The main source of loss in this assay lies in the operation of concentration. The only remedy is continual practice and repeated treatment of the tailings, and even then ores may be met with which even experienced vanners find difficult to treat. The purification process by acid and ammonia is very desirable where a clean button of tin is desired. The button, when pure and formed in a mould of fair depth, should be silvery white in appearance, and have a dimple on the top. If these conditions are not present, the student should hammer out the button and subject a portion of it to qualitative analysis, the result of which will give him a clue to the impurity; and his method of treatment must be varied accordingly. It is difficult to estimate the degree of accuracy of the results of this assay, but with careful concentration and purification of the concentrates, and proper firing, the error should not be greater than two points in the first decimal place; thus, an ore containing 1.8% tin should yield results not lower than 1.6%. CHAPTER VIII. THE ASSAY OF LEAD ORES. THE student is required to determine by fire-assay the percentage of lead in (a) The sulphide, Galena (PbS). (An oxidised ore containing Cerussite (PbCO), Anglesite (PbSO4), etc. (a) The sulphide, Galena Methods employed.-Regarding the methods employed authorities vary widely; here two methods are given for sulphide ores, the Soda and Argol (or Nail) method, and the Cyanide method. The reactions are expressed as follows 7PbS+8NaHCO3 = 4Pb+ 3(Na,S,PbS) + Na2SO4 + 4H2O+8CO2 Na,S,PbS+ Fe = Pb+ Na,S+ FeS PbS+ KCN= KCNS+ Pb. First Method (Soda, Argol and Nails). Apparatus. The wind furnace, pulp scales, crucibles, etc. Reagents. Soda, carbonate of potash, argol, salt, nails or wire. Details of Operation. The ore (clean galena) is reduced, sampled, and an assay sample obtained which has passed through an 80 sieve. Weigh out in duplicate the following charge— Thoroughly mix the charges on glazed paper, and transfer them to their crucibles (F). In each crucible insert two loops of fencing wire, tied together at their bends with iron wire, and of such a length that they come to about a quarter of an inch from the top of the crucible. When the loops are inserted add a cover of 5 to 10 gms. salt. Carefully bed the crucibles in a good coke fire, and build the coke well above the top edges of the crucibles. Place covers on the crucibles and a few lumps of coke on the covers. Keep the fire between dull red and bright red (that is, a red), and never let it rise above a bright red. The time of fusion varies with the fire and ore, but at the end of ten minutes the cover may be removed and the charge inspected now and then, replacing the cover and coke after inspection. When frothing ceases, allow the charge to remain in the furnace for five minutes. Then pour into a hot, greased conical mould, after withdrawing the wire loops, any adhering prills of lead being removed by washing the wires in the slag. If the top edges of the crucible have cooled below a red heat the charge should not be poured. The button is in this case removed by breaking the crucible when cold. Detach the slag; hammer the button into a cube and weigh it. Note the result and all details of firing, etc. Calculation of Results.-The percentage is obtained by multiplying the weight of the button in grams by ten. error + or Accuracy of Results.-A difference of opinion exists as to the merits of this and of the Cyanide methods. Neither is as accurate as the gravimetric estimation. With a fairly clean Galena both methods give results with an of one unit, e.g. if the gravimetric process gives 75% of lead, these processes will give from 74% to 76%. On the whole, the evidence is slightly in favour of the Cyanide method, but before proceeding further the student must continue his practice till his results are uniform and satisfactory. Even if he has to run through thirty or more assays, the time so employed is amply repaid by the confidence gained. The following tabulation indicates the causes of inaccuracy:— Briefly, the fire-assay of lead is unreliable when other reducible metals are present, and when these are absent it is accurate to about one unit on rich ores. Apparatus.-As before. Second method (Cyanide). Reagents. Potassium cyanide salt. Details. Weigh out in duplicate the following: Mix charge, cover and insert in the furnace as usual. between a red and a bright red, firing as before. buttons. Calculations.-As before. Fuse for 15 minutes Accuracy of Results.-Here again practice and care are necessary. With a clean rich galena good results are obtained, but when impurities such as copper, antimony, bismuth, etc. are present the results are worthless. To |