M a y , 1914
I
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
405
LABORATORY AND PLANT THE PYROMETER IN THE ASSAY MUFFLE' B y FREDERIC P. D E W E Y ~
r S t a n d i n g alone, b y itself, a pyrometer reading has absolutely n o value as a control of assay operations i n a muffle or a s a guide t o t h e assayer in carrying on such operations. T h e reasons for this are varied a n d complex. ( I ) T h e temperature t h a t controls t h e success of t h e operation is t h a t of t h e lead b u t t o n undergoing oxidation. At present we have no means of learning this temperature under practical working conditions, so t h a t some suitable place must be selected within t h e muffle for t h e location of a pyrometer. ( 2 ) Unfortunately, however, there is absolutely no approach even t o a fixed relation between t h e pyrometer reading a t a n y given point available a n d t h e temperature of t h e oxidizing button. T h e oxidation of t h e lead supplies much h e a t t o t h e b u t t o n , b u t its effect upon t h e pyrometer is negligible. One factor governing t h e a m o u n t of heat utilized b y t h e b u t t o n is t h e r a t e of oxidation of t h e lead, a n d this in t u r n is, within wide limits, largely influenced b y t h e passage of t h e air over t h e b u t t o n , so t h a t t o fully utilize a n d apply t h e pyrometer reading we must also know t h e height of t h e barometer a n d t h e effect of variations in t h e barometer readings upon t h e draft of t h e particular muffle under consideration. F u r t h e r a n d most i m p o r t a n t , from a practical standpoint, is t h e freedom of entrance for t h e air t o t h e muffle. I n other words, by manipulating t h e door or t h e stopper of t h e muffle, widely varying differences between t h e b u t t o n temperature a n d t h e pyrometer reading m a y be produced. T h e effect of t h e door conditions is twofold. It affects t h e supply of air t o t h e b u t t o n a n d also t h e actual temperature of t h e bottoms of t h e muffle on account of t h e varying amounts of air t h a t have t o be heated there in passing through t h e furnace. Finally t h e relation of t h e position of t h e b u t t o n within t h e muffle t o t h a t of t h e p y rometer is vital. Therefore, t o intelligently utilize a n y s t a t e d pyrometer reading i t is essential t o have exact information upon a variety of other conditions surrounding t h e operation. Bradford3 pointed out t h e inconsistencies of various s t a t e m e n t s regarding pyrometer readings in assaying a n d well established facts, such for instance as advising a temperature of 700' t o 750' for cupellation when i t requires at least 906' t o fuse litharge. I n a series of tests he demonstrated t h e large amount of heat supplied b y t h e oxidation of t h e lead a n d t h e higher temperature thereby attained b y t h e b u t t o n . His arrangement of a p p a r a t u s was ingenious, b u t risky t o t h e pyrometer couple a n d not applicable t o routine work. He gives an excellent description of t h e conditions immediately surrounding a cupellation. Fulton, Anderson, Goodner a n d Ossa4 determined Presented a t the 49th Meeting of the A. C. S., Cincinnati, April 7-10, 1914 and published by permission of the Director of the Mint. Published simultaneously by the American Institute of Mining Engineers. Assayer, Bureau of the Mint. THISJOURNAL, 1, 181. West. Chem. M e t . . 4, 31.
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t h e difference in temperature between a n e m p t y cupel a n d t h e cupelling lead in a n adjoining cupel as 14j0, a n d also give various other temperature determinations, under t h e conditions employed b y t h e m . For a long time I have been engaged upon a n investigation into t h e conditions surrounding t h e assay of gold bullion as affecting t h e accuracy of t h e results obtained. Xaturally t h e question of t h e temperature of cupellation early a t t r a c t e d attention, b u t there were so m a n y other conditions t o be investigated where our information was meagre, while t h e temperature question seemed t o be under fairly good control b y t h e eye of experienced cupellers, t h a t t h e use of t h e pyrometer was not actively t a k e n up until recently. I n t h e early days of t h e investigation various points regarding temperature were carefully considered a n d some of t h e problems were worked o u t . Some of t h e problems presented themselves with emphasis. I n this connection a careful distinction should be drawn between t h e problem of ascertaining t h e effect of t h e various conditions of t h e cupellation upon t h e t e m perature of t h e cupelling bead a n d t h e problem of t h e regulation a n d adjustment of these conditions so as t o produce t h e best possible conditions for cupelling, a n d t h e final problem of establishing a suitable indicator or guide t o show t h a t t h e proper conditions are being maintained, a n d especially a n indicator which m a y be applied in different muffles a n d under varying conditions. I n a broad a n d general way t h e time required t o work off a given weight of lead is a crude indication of t h e temperature of t h e cupellation. When carrying on uniform work in q u a n t i t y t h e decreasing size of t h e b u t t o n is a general guide for t h e temperature a n d a rough notation of t h e t i m e will often be useful in explaining irregularities of t h e results. If t h e general conditions remain uniform, a prolonged cupellation indicates lack of heat, a n d a rapid one a n excess of heat. I n making t i m e observations i t is essential t o adopt some fixed point in t h e operations t o begin taking t h e time a n d another t o stop. If all t h e other conditions could be rigidly controlled a n d t h e time be very carefully observed, i t would furnish a good guide t o t h e temperature, b u t i t would be available only a t t h e finish of t h e r u n a n d could not be used t o change conditions during t h e run. Also, i t could n o t be applied at other times or places or under different conditions. Again it furnishes n o preliminary evidence t h a t t h e furnace is in good condition before starting t h e work. It is, however. hardly ever possible t o control t h e other conditions a n d sometimes accidental variations creep in. On one occasion, when t h e conditions, including t h e temperature, appeared t o be normal, i t became evident t h a t t h e lead was not oxidizing fast enough. An examination showed t h a t in setting a new muffle t h e workman h a d not p u t t h e slit in t h e back of t h e muffle exactly opposite t h e chimney outlet. This choked off t h e draft a n d retarded t h e oxidation. T h e retardation of t h e work was, of course, excessive, b u t
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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
this experience emphasizes t h e natural effect of changes in t h e barometer upon cupellation. A resetting of t h e muffle corrected t h e difficulty. Another post f a c t o temperature indicator is t h e amount of gold absorbed b y t h e cupel in gold bullion assaying, high absorption under similar conditions, indicating high temperature. Here again controlling t h e other conditions is difficult a n d I have found this indicator t o be of value largely in emphasizing t h e fact t h a t variations in t h e other conditions may falsify t h e pyrometer reading. It makes a difference whether the pyrometer is rising or falling. If we could hold t h e pyrometer a t t h e same point for a long time before making t h e cupellation this cause of difference would be minimized. b u t this is impracticable in every-day wolk. I t makes a difference if t h e muffle be new a n d in'good condition or old a n d nearly worn out, a n d it must not be forgotten t h a t a new muffle may be poor a n d leak more t h a n a n old one, which was of good quality when new T h e only feasible place t o p u t a pyrometer in a n assay muffle is close t o t h e t o p of t h e arch of t h e muffle a n d for convenience i t must be inserted from t h e back. We all know, of course, t h a t closing t h e muffle increases t h e temperature a n d t h a t on moving toward t h e back of t h e muffle t h e temperature rises, as practical everyday working facts. I n order t o get a more exact idea as t o t h e difference in t h e temperature in different part? of t h e muffle a n d t h e relations between these temperatures a n d t h e fixed pyrometer readings, a second portable pyrometer was placed on t h e bottom of t h e muffle in varying relations t o t h e fixed pyrometer as follows: directly under t h e fixed pyrometer, a t t h e right side a n d a t t h e left side in t h e same cross-section as t h e fixed pyrometer, close t o t h e front in t h e middle a n d on eadh side of t h e muffle. I n some of t h e arrangements e m p t y cupels were placed beside t h e bottom pyrometer. There are three principal causes for differences in t h e two pyrometer readings: position within t h e muffle, freedom of entrance of air t o t h e muffle, and condition of t h e burners on either side of t h e muffle. I n general, t h e door conditions, governing t h e entrance of air t o t h e furnace, exert a powerful influence upon t h e temperature within t h e muffle, a n d often cause wide differences in temperature in different parts of t h e muffle. Under t h e conditions of these tests, on opening t h e closed muffle, either pyrometer may fall 100' or more in t e n minutes, a n d a further 10' or z o o before becoming steady. I n t h e closed muffle t h e two pyrometers registered alike in only one instance, a n d differed 40' in one. I n every instance t h e movable pyrometer, on t h e bottom of t h e muffle, fell more t h a n t h e fixed pyrometer, a t t h e t o p of t h e arch, on opening t h e closed muffle, a n d in one instance 40' more. A point of grave concern is t h e stability of t h e pyrometer. I n t h e above tests both of t h e pyrometers were practically new a n d may be depended upon, b u t t h e effect of long use, for continuous periods, in t h e litharge-laden atmosphere of t h e muffle upon t h e instrument is unknown. It is known t h a t t h e hot litharge fume is destructive t o t h e t u b e , a n d it is only a question of time when it will affect t h e enclosed couple. At present there are no ready means for testing t h e accuracy of t h e pyrometer from time t o time, a n d
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it is difficult for a n assayer t o judge when i t is beginning t o fail. I n conclusion, I would say t h a t , notwithstanding t h e objections I have described, t h e pyrometer occupies a useful field a s a general guide t o t h e heat conditions in t h e assay muffle. I n t h e old a n d established practice of assaying in t h e Mint service, in t h e large laboratories, one or two men do practically all of the cupelling a n d they grow t o be very expert in judging t h e heat of t h e muffle a n d t h e condition of the cupelling bead b y t h e eye, b u t t h e carefu1,and proper use of a pyrometer would often help t h e m , while t h e man who cupels only intermittently .will find i t a good general aid. But too much dependence must not be placed upon t h e pyrometer, a n d t h e man who depends upon it entirely will never be a good cupeller. BUREAUO F THE MINT TREASURY DEPARTMENT, WASHISC~TON
APPROVED BUREAU OF MINES EXPERIMENT STATION AT PITTSBURGH Plans for t h e proposed $ joo,ooo Experiment Station of t h e United States Bureau of RIines t o be located in Pittsburgh, have been approved by t h e commission appointed by Congress for t h a t purpose. Congress, a year ago, in t h e Public Buildings Bill, authorized a new home for t h e Bureau of Mines t o cost $joo,ooo. It is now expected t h a t Congress, in its present session, will make a specific appropriation SO t h a t construction work may begin. It is hoped t h a t contracts may be let by July 1st a n d t h e buildings completed in t h e fall of 1915. T h e S t a t e of Pennsylvania has appropriated $ 2 5,000 for cooperation in establishing this Experiment Station. The group consists of three main buildings facing Forbes Street. T h e central building of t h e group, t h e Mining Building, will be three stories in height, flanked by two main buildings, one t h e Mechanical a n d t h e other t h e Chemical Building. I n t h e rear of these a n d enclosing a court will be t h e Service Building. Beyond t h e Service Building a n d spanning what is known as Panther Hollow a n d t h u s connecting t h e Bureau of Mines Buildings with t h e Carnegie Schools, will be two buildings over t h e roofs of which will pass t h e roadway from Forbes Street t o t h e Carnegie School Buildings and Schenley Park. Between t h e main group a n d t h e power and fuel group will be t h e entrance t o a series of mine shafts. One of these will be used as a n elevator t o carry heavy material a n d passengers from t h e lower level t o t h e upper; another will be for tests of hoisting ropes a n d similar mining appliances; another will be am entrance t o tunnels extending under t h e buildings a n d in which mining experiments, such as fighting mine fires. will be conducted. T h e portion of Panther Hollow above t h e Power Buildings will be arranged a s a Miners' Field, t h e slopes of t h e ravine being utilized as a n amphitheatre which will accommodate zo,ooo spectators who may assemble here t o witness demonstrations a n d tests in mine rescue a n d first-aid. The Mining Building will contain t h e administrative offices, and those of t h e mining force. I n it will be
