THERMODYNAMIC PROPERTIES OF FLUORAPATITE
Dec., 1951
5581
(CONTRIBUTIONFROM THE RESEARCH SECTION, DIVISIONOF CHEMICAL ENGINEERING, TENNESSEE VALLEY AUTHORITY]
Thermodynamic Properties of Fluorapatite, 15 to 1600°K.l BY EDWARD P. EGAN,JR., ZACHARYT. WAKEFIELD AND KELLYL. ELMORE The heat capacity of crystalline fluorapatite, Calo(P04)sF2,from 15 to 300'K. was determined. At 298.16"K.the entropy is 185.5 i 0.2 cal. per mole per degree, and the heat content, H " - H t is 30,340 cal. per mole. The heat contents above 25' were measured to 1600°K. The equations expressing the heat content, heat capacity and entropy of fluorapatite are HT HW.M = 226.04T 14.44 X 10-8TZ 48.82 X 1O'T-1 85,050, cal./mole (298.16' - 1580°K.; f0.4%) Cp = 226.04 28.88 X lO-*T 48.82 X 106T-2,cal./mole/deg. ST S 2 9 8 . 1 ~= 520.48 log T 28.88 X 10-8T 24.41 X 1PT-2 1324, cal./mole/deg.
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Fluorapatite, Calo(P04)sFz,is a major component of phosphate rock, the chief source of phosphatic fertilizers. This paper presents results of measurements of the low-temperature heat capacity, the entropy and heat content a t 298.16'K. and the high-temperature heat contents of fluorapatite. The heat capacity and heat contents were used to derive a table of thermodynamic properties for the range 298.16 to 1600'K.
Apparatus.-The low-temperature and high-temperature calorimeters have been described.'*b In the present work, the mirror-to-scale distance of the galvanometers was increased from 1 to 2 meters, and a group of six saturated standard cells, certified by the National Bureau of Standards and maintained at 25O, replaced the three unsaturated standard cells as the laboratory reference of potential. The temperature of the water-bath used with the high-temperature calorimeter was maintained at 25'.
prevent loss of fluorine from the apatite. the fluorapatite is given in Table I.
TABLE I1 HEAT CAPACITY OF FLUORAPATITE AT 5' INTERVALS, CALORIES PER MOLEPER DEGREE T,O K . Cp T, O K . T,O K . CP CP 215 149.7 15 0.98 115 90.32 220 151.8 2.47 20 94.33 120 25 98.18 125 225 153.8 4.81 130 101.9 230 155.8 7.87 30 235 157.8 105.5 35 135 11.67 240 159.8 109.0 140 15.86 40 245 161.7 45 112.3 145 20.51 250 163.6 50 115.6 150 25.62 255 165.4 55 118.7 155 31.01 260 167.2 60 121.8 160 36.58 265 168.9 124.7 42.08 65 165 170 127.6 47.47 270 170.6 70 175 75 130.3 52.73 275 172.2 80 132.9 58.03 180 280 173.9 85 63.22 285 175.6 135.5 185 290 177.2 138.0 68.15 90 190 295 178.8 140.5 72.87 95 195 142.9 298.16 179.73 77.42 100 200 145.3 105 205 81.85 147.5 86.16 110 210
Low-Temperature Heat Capacity.-The observed heat capacities, AQ/ AT, for the empty calPreparation of Fluorapatite.-Crystalline fluorapatite was orimeter and for the calorimeter filled with 85.7 g. prepared by heating tricalcium phosphate and calcium (0.085 mole) of fluorapatite were plotted, and arbifluoride. In the preparation, monocalcium phosphate, recrystallized from hot phosphoric acid solution, was heated trary smooth curves were drawn through the exslowly to 500' to form j3-calcium metaphosphate. Calcium perimental points. Values read a t 5' intervals carbonate was precipitated by addition of ammonium car- from the smooth curves were used in the calculation bonate to a hot solution of recrystallized calcium nitrate. Tricalcium phosphate was prepared by heating a stoichio- of curvature corrections from second metric mixture of calcium metaphosphate and calcium car- Equations were fitted to the corrected heat capacbonate a t 1150" for 5 hours. Calcium fluoride was pre- ities, a deviation plot was made, and the values pared from calcium carbonate and hydrofluoric acid. A calculated from the equations were corrected from stoichiometric mixture of tricalcium phosphate and calcium the deviation plot. A tabular presentation of the fluoride was heated for 30 minutes a t 1370' in a current of dry nitrogen; a platinum boat containing calcium fluoride observed heat capacities on a mole basis is availwas placed upstream from the apatite mixture to maintain able.' The heat capacities a t 5' intervals are given an atmosphere of calcium fluoride over the mixture and thus in Table 11. The analysis of
TABLE I QUALITYOF FLUORAPATITE Constituent
Found, wt. %
CaO PzOr F MgO Si02 SrO NaZO AlnO: CUO FezO:
55.6 42.2 3.75 0.02 .03 .12 < .02 < .02
<
