May, 1913
T H E JOURNAL OF I N D U S T R I A L A,VD E-YGI-1-EERISG C H E M I S T R Y
RAPID DETERMINATION OF SULFUR I N PYRITES CINDER B y A . B. CONNER Received December 9, 1912
I n THIS JOURNAL,^ Mr. C. C. Nitchie described a rapid method for the determination of sulfur in roasted blende which is carried out by roasting the sample in a boat in a n electric combustion furnace, in a current of pure dry air, absorbing the acid gases in standard sodium hydrate solution and titrating the residual alkali with equivalent standard acid. Nitchie suggested t h a t the method could be applied to the determination of sulfur in pyrites cinder and immediate trial was given this suggestion in the writer’s laboratory, with entire success; it has been in constant use daily for the past ten months with results t h a t are all t h a t could be desired. As the article referred to is very complete in its directions, i t will only be necessary t o mention in outline such modifications or changes as have been found satisfactory during the period of its use. Instead of the furnace mentioned, a Hoskins tube furnace such as is in common use for the direct combustion of carbon in iron and steel was a t hand and proved well suited for the work. This furnace takes a fused silica tube three-quarters of a n inch by twenty-four inches or even one of larger bore if desired. The three-quarter inch size seems best for this work and is made quite rigid in the furnace by a turn or two of one strand of three-eighths inch pure asbestos rope packing gently packed around the tube in each opening of the furnace. The furnace covers one foot of the tube leaving six inches of each end projecting; the ends do not need to be water-cooled. The asbestos shield described for the outlet stopper proved sufficient protection and the gases only just warm the absorption bulb, unless the aspiration is too rapid. An ordinary water filter pump or aspirator is used for drawing the gases through the train. A Muencke’s glass filter pump, operated by compressed air instead of water, with a needle valve on the air supply, gives finer control and is preferred by the writer. The pump is connected t o the six-bulb Murray potash bulb by rubbertubing and a further control of the aspiration is obtained b y the use of a screw pinch-cock on this tubing. The Murray bulb tubes are furnished by E. H. Sargent & Co., and those with an dpening for a No. 3 rubber stopper are most easily handled. It is necessary t o specify the opening since they are also made for No. o stoppers. A ten-bulb Meyer tube of similar type may be used, but is a trifle more cumbersome. The air is freed from CO, and dried by bubbling through a bottle containing about zoo cc. of I : I KOH and then through a calcium chloride tower filled with quarter-inch granular CaCl, and is led t o the combusion tube by a foot or more of rubber tubing connected b y glass t o a rubber stopper for closing the entrance end. This stopper need not be protected. When first started, the furnace2 should be heated 1
THISJOURNAL, 4, 30.
The temperature used is 1000° C. and is regulated by a rheostat f u r nished with the furnace. The setting of the rheostat is determined by a thermo-couple. This the makers of the furnace can also supply together
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up gradually by cutting out resistance from the rheostat and the silica tube should be bedded with a layer of zo-mesh silica sand which has been digested for some hours in strong HC1, washed, and dried in a n oven, or with 20-mesh grained alundum such as is furnished by the Norton Co. for carbon combustions. This bedding should be done before the first heating up and before the introduction of the couple, if this is put into the tube. I t is easily accomplished by filling a boat with the silica or alundurri and introducing it into the tube after this is in the furnace in position for working. The boat is pushed into the far side of the furnace zone of the tube, slightly inclined to spill a portion of the sand and gradually withdrawn, a t the same time tipping the boat; until, as it reaches the near end of the tube, i t has been completely overturned and the sand has been distributed in a n even layer along the bottom of the tube and back to the position occupied by the entrance rubber stopper. This serves as a medium on which the boat slides easily and lessens the likelihood of breakage of the tube. No breakage, however, has occurred with this equipment. A one-gram sample is put into a No.j 7 3 3 alundum boat which has been bedded with a layer of about onesixteenth inch of silica sand, washed and dried as above mentioned and previously ignited in the furnace. If the sample rests on unignited sand the decrepitation of some of the grains of sand will throw the cinder out into the tube, xhich will be attacked by the iron oxide. Should this occur or should a sample be spilled in the tube, the furnace should be shut off, allowed * t o cool, and the tube emptied, brushed out with a burette brush, replaced and “rebedded.” A convenient way of handling the work is to switch on the furnace the first thing in the morning. About a n hour and a half to two hours is required for the furnace t o come to maximum heat with the rheostat set for I O O O O C. and in the meantime the sample may be prepared and the bulb filled. While the article b y Mr. Nitchie states that roasted blende samples need be ground only fine enough to secure an average sample in one-gram, 60 to 80 mesh seems necessary with pyrites cinder and I O O mesh or even finer material requires apparently only two or three minutes longer for complete oxidation; so that if the sample has been prepared in a disc grinder and this has been set a little closer than was intended, it is only necessary to allow about five minutes’ longer aspiration. Twenty-five cc. of standard alkali ( I cc. = 5 mg. S) are run into the bulb (preferably from an automatic pipette) and diluted as described in the paper cited, so t h a t on aspiration the liquid extends into the fifth bulb. The necessary amount can be ascertained by trial b y inclining the tube so that the liquid runs into with the necessary leads and ammeter which are specially designed for continuous use with t h e furnace, This is quite desirable if the voltage of the electricsupply is notconstant since the life of the furnace winding is shortened when the temperature is carried higher t h a n necessary. I f the voltage is constant the temperature can be taken with the regular thermo-couple in the combustion tube a t t h e time the furnace is first p u t in use and the setting of the rheostat made thereby. From two weeks t o a month later the setting of the rheostat should be checked in the same way. and from time t o time thereafter as the winding changes slightly in itsresistance with use.
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the bulbs from the large bulb. Twenty-five cc. are used instead of the seven to ten mentioned, since this gives a larger excess alkali and a trifle greater accuracy. The bulb is placed in position, the aspiration started and regulated to twenty-five bubbles in ten seconds. The stopper is removed and the boat pushed into the center of the tube by the rod which is provided with a distance guard of fine wire. The stopper is replaced and the aspiration continued for ten or twelve minutes for moderately fine samples, or fifteen minutes for those of Ioo-mesh grain or over. Almost immediately on placing the boat in the furnace the sulfur gases are seen to commence passing over and they continue to be visible for several minutes. In most cases the fume seems to pass entirely through the set of bulbs. Actually a trace of SO, does pass them, but it is negligible. If the ore contains even a few tenths of one per cent. of zinc this increases the white cloud and may lead to the belief t h a t SO, is being lost. The results below show that for practical purposes the loss need not be given further thought. At the end of the specified time the furnace is shut off if no more samples are to be tested, the bulb is disconnected and its contents are washed into a tall 500 cc. beaker and titrated, using phenolphthalein and equivalent standard acid, the formula for calculation being: cc. alkali - cc. acid = per cent available S in cinder.
Vol. 5 , No. 5
used and it is with this in mind that we may discuss availability. RAPIDDETERMINATION O F SULFUR IN PYRITES CINDER Per cent S. Per cent S. Regular method Combustion method 0.77
1.49 1.96 2.45 2.46 3 .OO 3.32 3.36 3.43 3.47 3.66 4.10 4.14 4.23 5.21 6.29 6.68
0.75 1.60 2 .oo 2.43 2.30 3.05 3.45 3.43 3.65 3.55 3.53 4.13 4.15 4.15 5.30 6.15 6.50
RMULTSOF TOO RAPID ASPIRATION Per cent S. Per cent S. Comb. meth. Comb. meth. 25 bubbles in 10 sec. Nearly twice as fast 2 .SO 4.13 5.15
1.80 3.85
4.70
The original available sulfur is t h a t occurring as sulfides, oxidizable by combustion with air, and any sulfates soluble in the dilute hydrochloric acid used in getting the baked residue into solution in the original analysis of the pyrites. Any sulfur compounds, such as BaSO,, or equivalent barium compounds 2 (carbonate, etc.), do not enter into the question of It is customary, a t our works, t o test one average availability, either in the pyrites or in the cinder, sample from the burners daily or, if more than one and hence must not be estimated in the cinder. This kind of pyrites is being burned, a sample of the cinder ' precludes the use of a fusion method, either peroxide from each. This, in connection with an exact figure or carbonate, where there is any possibility of these for the available sulfur in the pyrites and the chamber compounds occurring in the pyrites. They are nearly and tank "dips," daily, gives a reliable check on the always present to a greater or less extent, hence fusion production of the plant. The sample is obtained by methods are practically out of the question except for taking a part shovelful from each barrow or truck total sulfur, which is not what is wanted in the usual of cinder as it is wheeled out, and placing in a box. cinder test. Each morning the accumulation is mixed and quartered I n burning pyrites some sulfur is oxidized and comby the head furnaceman and a pailful sent to the lab- bines with the iron compounds to form sulfates. These, oratory, where it is ground in a Sturtevant disc pul- of course, constitute available sulfur in the original verizer after being crushed in a roll jaw crusher of pyrites and must be estimated along with the unoxilike make. Fifteen to twenty minutes is required dized sulfide, etc. A simple extraction with cold to prepare the sample for analysis. water will prove their presence when the extract is The following table shows the comparison between treated with barium chloride. The writer has always the regular (Br-KBr-"0,) gravimetric method for found them present in all the kinds of cinder ever residual available sulfur in cinder and the results by examined, and it is safe t o conclude that they are this combusion method. The figures include results always present. With many kinds of pyrites the major on a number of foreign and domestic ores and are well portion of the available sulfur left in the cinder is scattered over the period t h a t the method has been in the form of soluble sulfates. I t also depends quite in use. largely on the conditions during burning. With The majority of the figures given are selected from one Spanish ore as much as 80 per cent of the remainthose showing sulfur considerably above the average, ing available sulfur has been found in the form of soluas these were the ones most frequently checked, and ble sulfates. Sulfites are also frequently present and they show the closeness of the two methods. must be included. The following method and check methods mentioned It is certain from complete analyses of quite a number are intended to give available residual sulfur in cinder of varieties of Spanish and domestic ores that lime is but it must be borne in mind that the meaning of this a n almost constant constituent, but in nearly all cases term must take into consideration the available sulfur only a few tenths of a per cent are found, seldom exin the pyrites and the method of its determination. ceeding 0.5 per cent and more frequently only 0.2-0.3 The wet or Lunge method is the one most commonly per cent.
T H E J O U R N A L OF I h T D U S T R I A L A N D ENGINEERIATG CHEMISTRY
May, 1913
As recently pointed out by A. M. Smoot,I this lime as sulfate is not insoluble in the course of the regular wet assay of pyrites and its sulfur is estimated as available sulfur. This being so i t should rightly be included as such in the cinder, but from recently published data concerning the decomposition temperature of sulfates i t would seem as though the combustion method would not include this. The temperature given for the decomposition of CaSO, is 1 2 0 0 ~ C . a Mr. Nitchie finds t h a t CaSO, is not decomposed in his method of analyzing roasted ginc blendes. If so, the sulfur equivalent to the lime should remain in the furnace with the ferric oxide obtained b y roasting pyrites cinder. My check analyses b y gravimetric means indicate t h a t if the lime is combined with sulfur in the cinder, the combination is broken during the test and the sulfur evolved as gas. I t is probable t h a t the ferric oxide plays some part in this reaction. If, however, the lime retained its equivalent of sulfur i t would, as seen above, amount with most kinds of pyrites t o only one or two-tenths of one per cent. Two grades of Spanish pyrites with which the writer is quite familiar contain several per cent of zinc and a number of others smaller amounts. By the water test above referred it is easy t o determine t h a t some of the soluble sulfate is present in combination with zinc. What Mr. Nitchie says about zinc sulfate in roasted blende applies t o pyrites containing zinc, but the reactions may include the action of iron sulfide a n d oxide on zinc sulfate for the results indicate a complete decomposition of this last salt. DETROIT, MICH.
40 1
separation is essential. The reason for requiring a higher factor than the theoretical is believed t o be due t o the lead peroxide carrying one-half molecule of water. Solution of the spelter sample is effected by dilute hydrochloric acid according t o Oswald Gunther' or dilute sulfuric according t o Eliot and Storer, until nearly all zinc is dissolved. This procedure makes it possible to operate on a large sample and each 0.1 cc. of potassium permanganate (0.568 gram t o the liter) equals 0.001 per cent lead, when 19.2 grams spelter is taken. The insoluble l.ead, cadmium and some zinc is filtered off, washed, dissolved in nitric acid, diluted, ammonia and ammonium persulfate added t o peroxidize the lead. .After filtering, the cadmium may be precipitated in the filtrate b y hydrogen sulfide. The substitution of trichloracetic acid, according t o Fox,zfor any one of the mineral acids usually employed in separating cadmium and zinc, has been found advantageous, since it is not dissociated to the same extent as hydrochloric or sulfuric acids, and even rather concentrated solutions do not prevent the complete precipitation of cadmium. I n the case of mineral acids the strength must be kept within narrow limits, a s is well known. A slight excess prevents the complete precipitation of cadmium, while insufficient acid causes zinc also t o be precipitated, making several reprecipitations necessary. By means of trichloracetic acid usually only one re-precipitation is called for, unless it is accompanied by more zinc than was aimed a t originally in dissolving. ANALYTICAL M E T H O D
A NEW TECHNICAL METHOD O F SPELTER ANALYSISa
Weigh out r 9 . z grarm of the spelter and place in B y ERIC JOHN ERICSON No. 3 beaker. Add 2 0 0 cc. ordinary water and 43 cc. At the Sixth International Congress in Rome, 1906, concentrated hydrochloric acid (1.2 sp. gr.), or I O O cc. chemists were invited t o publish their methods of water and 50 cc. dilute sulfuric acid ( I : 3 ) ; allow spelter analysis for comparison and possible unification. t o stand over night whenever cadmium is t o be deHence I now present a scheme, involving the applica- termined; if lead alone is wanted a few hours' time tion of a n original method for lead, which, a t the same will do. Filter off metallic, consisting chiefly of time, effects a separation from cadmium. The funda- lead, cadmium and some undissolved zinc, and wash mental reactions were described in the Journal of the with hot water. Transfer whatever metallics are on American Chemical Society, September, I 904, and the filter paper back into the beaker by means of a subsequently in the Engineering and Mixing Journal, jet of water. This can be accomplished without loss July 2 j , 1908, and May 2 2 , 1909, giving its application if done a t once before the paper gets dry. Now add t o ores. It is based on the following reactions: I O cc. concentrated nitric acid and boil unaii brown Pb(XO,), 4NH40H (NH,),S,O, fumes cease t o come off. Filter and wash if traces = PbO, 2iYH4N0, 2(NH,),SO, 2H,O of tin or antimony are indicated, which is very seldom PbO, H,O, 2 H N 0 , = Pb(NO,), aH,O 0, the case. If filtered and washed the volume will jH,02 aKMnO, 6HN0, probably be large enough; if not filtered, add 1 0 0 cc. = zICNO, 2Mn(NO,), 8H,O 50, distilled water, 30 cc. concentrated ammonia and Hence: zKMnO, = 4H,O, = SPb 5-10 grams ammonium persulfate, depending on size of precipitate. Should lead be unusually high, Since the theoretical factor 5Pb = 1.851 gives too say over 1.00 per cent, it is best .to add half of the IoFe persulfate before adding the ammonia. Boil five low results, the empirical factor 1.92 was chosengiving results agreeing exactly with the standard gravi- minutes and allow ten more for the precipitate t o settle. metric method-whenever a preliminary separation Then filter while still warm through double 11 or 12.5 of lead is not necessary, and 1.95 for ores where such cm. No. I F filters. Wash four times with a hot I O per cent ammonia solution and five times with hot
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Eng. Mzn J.. 94. 412 Met. and Chem. En0 , 10, 172 Paper presented a t t h e Eighth International Congress of Applied Chemistry. S e n York, September, 1912. 2
1 Bericht der Internationalen Analysen K o m n i s s i o n an dqr V I International Kongress f u r Angewondte Chemie i n R o m , 1906, p. 5 1 . 2 Jour. Chem. Soc.. London, 1907, p. 964.
