INDUSTRIAL A N D ENGINEERING CHEMISTRY
130
reaction. It will be noted that for all intermediate temperatures the curves are of the same shape-in fact, they practically coincide. This indicates that the heat generated is actually proportional to the oxygen absorbed. The oxygen recovered in the products is also plotted; it will be noted that it is of the order of one-tenth of the total oxygen used. 0
6
0
4
drops off, indicating a rapid breaking up of the solid oxygen compounds. In order to test the stability of these compounds at lower temperatures, coal that had been oxidized for 40 hours at temperatures up to 100" C. was heated in a stream of nitrogen at 160" for 2 hours. It was found that 83 per cent of the oxygen absorbed by the coal during oxidation was recovered as gaseous oxidation products, and the weight of these products agreed with the loss in weight suffered by the coal sample. Clearly, the so-called solid oxygen compounds of coal are unstable at comparatively low temperatures. This supports the peroxide theory of the mechanism of spontaneous heating, which is, that heating is accelerated by the formation of easily decomposable solid oxygen compounds at comparatively low temperatures. These break down t o form the normal gaseous oxidation products which otherwise would not be formed, or a t best very slowly at Iow temperatures. Two stages of reactions are therefore involved in the spontaneous heating of coal (1)mechanical absorption and the formation of solid oxygen compounds, which predominates at low temperatures and generates a relatively small amount of heat per unit of oxygen used; and (2) breaking up of the unstable oxygen compounds formed in the iirst stage, with the evolution of 60 to 70 per cent of the total reaction heat. Apparently, the amount of oxygen involved in both stages is proportional to the heat generated.
r
so
28 0
2
a
E
2
so
0 3
u
E
E
B Bo F z
4s
B O
6 8
d
V w
B
B
5
du GO 0
S u m m a r y and Conclusions
4 2
The spontaneous heating of coal was studied from the standpoint of the rate of heating of coal exposed to excess oxygen under experimental conditions, such that all of the heat generated by the oxidation of the coal was used to raise its temperature-adiabatic conditions were maintained. Rate of heating was therefore a measure of the spontaneous heating tendency of the coal under test. With samples previously dried in natural gas at 140" C., the followingresults were obtained:
2
0
40
50
Figure 7-Oxygen
70 80 TEMPERATURE, 'C
90
100
Absorbed and Reaction Heat
It is difficult to estimate how the reaction heat is distributed between the initial reactions and those evolving gaseous oxidation products, owing t o the difficulty of accurately determining the small amounts of the latter obtained. However, using Dulong's formula for calculation of the heat of reaction to gaseous products, this was found to be from 61 to 72 per cent of the total obtained by the heating rate method. This leaves from 30 to 40 per cent to cover absorption of heat and heat of formation of the solid oxygen compounds of coal with oxygen. Stability of So-Called Solid Oxygen Compounds of Coal
Porter and Ralston3 have shown that on heating coal at gradually increasing temperatures, its weight increases, owing to oxygen absorption, until a temperature around 250" C.is reached; from that point on the weight of the sample rapidly
Vol. 17, No. 2
I
1-The heating tendency of coals high in oxygen is greater in general than for those of low oxygen content, although the heating tendency is not in all cases proportional to the oxygen content. The heating tendencies of anthracite coal and of peat are small. 2-The heating tendencies of coals of similar rank are approximately the same, as indicated by four Pennsylvania and Ohio coals. 3-By extrapolation, 25.26' C. is found to be the temperature at which spontaneous heating of a Pittsburgh coal begins, although the present apparatus is not sufficiently sensitive to induce heating of this coal previously dried as noted above, below 35O 4-The rate of generation of heat during spontaneous combustion is directly proportional to the amount of oxygen used. 5-The temperature limits within which spontaneous heating will take place were estimated for a typical coal, and the relation between heat generated and heat dissipated was roughly calculated.
c.
1924 Arsenic Production Breaks All Records The production and sales of arsenic in the United States in 1924 broke all previous records, according to a statement issued by the Department of the Interior, Nine companies that produced white arsenic in the United States in 1924 reported to the Geological Survey their sales, which amounted to 14,500 short tons. The sales of thecrudeand refined product represented 71 per cent of the total white arsenic produced, and the products were sold at an average price of 9 cents a pound. During 1924 about 9000 short tons of white arsenic were irnported into the United States, as shown by the actual figures for ten months and an estimate for the remainder of the year. Most of the white arsenic imported was produced in Mexico, but considerable quantities, in the order named, originated in Germany, Japan, Belgium, England, Canada, China, Dominican Republic, and Australia. No imports of calcium arsenate were reported. The total white arsenic bought for consumption in )924 from domestic and foreign sources was about 23,000 short tons, valued at 9 cents a pound. Manufacturers of insecticides purchased the
largest quantities of white arsenic, and glass makers bought heavily during the last half of the year. Newly constructed plants near points of distribution produced a larger quantity than formerly of sodium arsenite, the weed-killing solution made from crude white arsenic. Twelve domestic arsenic plants were in operation in 1924. Two of them recovered white arsenic from ores and used it for making calcium arsenate, and one plant treated oxidized arsenical ore from which calcium arsenate was made directly. General' interest in arsenic was evident near the end of 1924, but buyers who were looking for lower prices had been influenced by reports that large surplus stocks were being carried in the United States. That there are considerable stocks of arsenic is true, but they constitute only a small fraction of a good season's demand. Considerable ore containing arsenic is stockpiled a t several points and can be held better in the crude ore than as the manufactured product. Most of the producers of white arsenic are in a position to produce heavily when there is a demand.
