Interphase Evaporation of Sodium Sulfate Solutions ROBERT F. SCHULTZ' AND IRVIN LAVINE2 University of North Dakota, Grand Forks, N. Dak.
URING the summer of 1934 eight deposits of naturally occurring Glauber salt were discovered in the northwestern part of North Dakota by a survey party under the direction of one of the authors (6). All of the deposits were found in lake bottoms having no drainage outlets, where the mineral-bearing waters had evaporated for years and left concentrated brines and deposits of crystals. The lake bottom is usually the lowest depression of a large area and receives the drainage from the surrounding region. The deposits appear white at most temperatures, but above 92' F. the surface may liquefy for a few inches. Innumerable springs are found over the entire area of the deposits. Information concerning these deposits is given in Table I. The mineral found in North Dakota is of an unusually high purity as indicated by the analyses in Table 11.
o,
I>
50
P
3
o
40
0-
3o
% 220
2 'lo
60
80
100
120 140 TEMPERATURE
160
180
200
OF.
F I Q U R1.~ SOLUBILITY OF SODIUM SULFATE
?
Pure sodium sulfate decahydrate, commonly known as Glauber salt, conMax. tains approximately 56 per cent water Depth of hydration. The necessity for the of PerDistance, Miles dehydration of the North Dakota maAv. nent D e t h Total Calod. To To material may be more fully realized Location in North Area Bed, of ged, Quantlty, railligby consideration of the fact that the No, Name Dakota Acre; Ft. In. Tons road nite 1 Grenora No, 1 Northwest of Gren500 30 30.6 1,750,000 7 12 distance from the deposits to the ora D m d e County nearest important markets (pulp and T. i a o ~ .R. , IO~W.', secs, 15 16, 20 paper mills) in Minnesota, Wisconsin, 2 Grenora No. 2 Northweit of Gren500 80 144 11,000,000 5.5 12 and Michigan, ranges from 800 miles ora D m d e County, T. ON.. R. I O ~ W , upward. These mills now obtain by8808 23 29 32 33 3 Miller Lake Sonthwes't of Aikabo, 550 50 84.8 5,000,000 2 8 product salt cake from various sources Divide County, T . in Illinois and Ohio; a small quantity 162N., R. 102W.. 880s 5 6 17 18 of natural salt cake has been shipped 4 Stanley No. 1 Northeak 'of hanley 250 10 68.3 1,000,000 7 15 in Mountrail Count;, from Canadian deposits, which enters T. 157N., R. 89W.. this country a t Portal, N. Dak. secs 21 22 27 28 2.5 15 200 5 30 600,000 East bf SLaniey, IT. 5 Stanley No. 2 The problem of producing an156N., R. SQW.,seas. 4 5 8 9 hydrous sodium sulfate from Glauber 3,000,000 5 10 Ndrtdert'st of Alkabo 275 20 120 13 McKone Lake Divide County, Elk: salt involves: (a) the removal of t h e horn tp., R. 102W., water of hydration under such condi8ecs. 11 13 23 7 North Lake Southwest D i v i d e oi C oAlkabo, unty, 200 80 40 1,000,000 1.5 8 tions that the anhydrous material is. obtained with a minimum of mechaniWestby tp. R. IOZW., 8ecs. 8 and 17 cal difficulties; (6) the removal of thia water under the most favorable ecoTABLE11. PERCENTAQE ANALYSESOF SAMPLES OF GLAUBER nomic conditions. SALT(MOISTURE-FREE BASIS) Numerous difficulties have been encountered in the comDeposit No. I 2 3 4 5 6 7 mercial dehydration of Glauber salt, due to the properties of Av. No. Samples 61 128 83 45 56 18 12 o,55 o,45 o,57 the salt. The decahydrate melts a t 90.32" F. (32.4' C.) a t NaCl AlrOx 0.32 0 . 3 3 0.15 0.37 o 22 o 97 0.16 which temperature the solid anhydrous sodium sulfate and its 0.00 0.00 0.00 0.00 0.00 0.00 0.00 FeiOa Trace Trace 0 . 0 6 7.21 3 . 2 5 Trace 0 . 1 6 saturated solution exist simultaneously. Figure 1 shows NaaCOs NaHCOs y::: y:;: that the solubility curve is inverted beyond this transition Cas04 MgSOi 1.21 2.06 2.72 1.30 1.16 1 . 2 1 4 . 1 0 point. AS a result, a heavy scale of anhydrous salt forms Nazi304 94.64 9 4 . 0 8 93.50 86.00 91.03 94.00 94.17 rapidly on the heating surfaces of equipment used to evaporate the saturated solution. A survey Of the m%ny different methods proposed for the 1 Present address, Hooker Electrochemical Company, Niagara Falls. N . Y. dehydration of this material was made by Kobe and Hauge 2 Preaent addreas, Induatrial Researoh Service, Dover, N. H. TABLEI. DEPOSITS OF GLAUBER SALTIN NORTH DAKOTA
::::
t.:! ;:30
59
60
INDUSTRIAL AND ENGINEERING CHEMISTRY
(3). A more recent development, submerged combustion, was applied experimentally t o the dehydration of Glauber salt by Kobe and co-workers (4),and has now found commercial application in Texas. A description of the process and the equipment was given by Douglas and Anderson (9). The process may be described “as a method of heating a liquid by the direct contact of the flame from a burner which projects the hot gases of the flame directly into and a t any depth below the surface of the liquid”. An evaporation system operating on sodium sulfate solutions was studied by Badger and Caldwell(1). Under normal operating conditions excessive crystallization made it necessary to clean out the evaporator within 1hour. However, by withdrawing the solution continuously, superheating, and flashing under the tubes, seed crystals formed. The vigorous circulation so obtained permitted a 10.5-hour operating period, followed by a 1.5-hour blowout period.
The Present Study The development of the Korth Dakota deposits has been given considerable attention by state, local, and other officials. Several large companies have also investigated. It became evident that any process to be applicable would have to remove the water a t a relatively low cost. The presence of unlimited quantities of lignite a t the deposits focused attention on the use of this fuel for the dehydration process. The direct use of lignite in rotary kilns was considered, but this method has not proved entirely satisfactory in commercial operation because of the tendency of the anhydrous salt to cake on the inside surface. Submerged combustion could n& prove economical in North Dakota unless a cheap source of heating gas was available. Therefore, it was decided t o study a new method, interphase evaporation, in which hot combustion gas, formed by burning lignite in a suitable furnace, is used as the heating medium. The process has been found t o work satisfactorily, and the results obtained appear to indicate that the cost is not excessive. The term “interphase evaporation” was adopted to describe the present process because it represents a system in which heat and material transfer occur between two phases, gas and liquid, which are in intimate contact. I n this particular adaptation of the process, the sodium sulfate solution constitutes the liquid phase and the lignite furnace gas constitutes the gas phase. The hot gas is pulled through the solution by an ejector or any other suitable means. The vapor leaving the solution, containing all of the water of evaporation, is then carried away by the gas stream, and the precipitated anhydrous salt settles to the bottom of the evaporator.
Vol. 34, No. 1
(Applications covering this unit have been filed with the United States and the Canadian Patent Offices.)
Experimental EQUIPMENT. A drawing of the pilot plant used is shown in Figure 2. The two principal pieces of equipment are the evaporator, 1, and furnace, 2. The evaporator was constructed of two steel 55-gallon oil drums, welded end to end and provided with a conical bottom, 3, t o which was attached a 3-inch screw conveyor, 13. The driving mechanism for the conveyor (shown in the left-hand photograph of Figure 3) consisted of a 1-horsepower motor, a suitable cone and pulley, and a reduction gear. The circular evaporator head, made from ‘/*-inch boiler plate, was held in place securely by eight 3/s-inch studs which were welded t o a flange attached to the evaporator body. A short 4-inch nipple, 11, was welded to the evaporator body about 18inches above the cone. Connection to chimney 4 was made by means of flanges and suitable lengths of pipe. A 4-inch hot gas inlet pipe (Figure 3, left) was offset vertically to permit the operation of scraper 12. A gas inlet nozzle was machined from a 6-inch length of shafting. The threaded end of the nozzle, 2 inches in diameter, was screwed into a disk that was cut to match the flanges on the 4-inch hot gas line near the evaporator, thus holding the nozzle in position and facilitating its removal. An internal taper on the discharge end of the nozzle reduced the diameter of the nozzle mouth to 11/2 inches. An external taper was also provided on the discharge end t o produce a sharp edge. The scraper was cut from a ‘/r-inch steel plate to fit the shape of the nozzle. A 1/2-inch rod, entering a 4-inch tee through a stuffing box, was welded to the scraper. This permitted the scraper to be thrust forward into the nozzle, rotated, and withdrawn into the 4-inch tee so as not to obstruct the nozzle. A liquid-gas separator, 8, of the cyclone type, connected to the evaporator by a 3-inch discharge line and a 8/4-inch liquid return line, served to reduce entrainment losses. A 21/2 N. P. A. Worthingtonsteam ejector, 9, designed to operate on 10-pound gage pressure, was connected to the separator, 8, and discharged through a 21/2-inch line to the atmosphere. The furnace, 2, was built entirely of firebrick. A 24 X 35 x 42 inch combustion chamber was separated from the rear part of the furnace by one course of bricks which served as a baffle; this was installed to reduce the fly ash in the furnace gases. The furnace was fired by an automatic stoker, 10, of the domestic type; its capacity could be varied from 10 to 50 pounds of lignite per hour. The 20 X 20 inch firebrick chimney, 4, extending about 8 feet above the floor was lined with a 6-inch tile pipe to prevent leakage. The temperature of the gases leaving the
I
about 8 inches beiow connection 11. A ga“s sampling tube was also inserted a t this point. An emergency stack, 6, was attached by means of flanges, 5, to the top of the chimney.
I
to the atmosphere. Four glass windows and a 4-fOOt liquid gage glass served as a means for observation. Three insulated 60-gallon tanks (righthand photograph of Figure 3), connected to a centrifugal pump by means of a 1-inch l i e . served as feed units. One tank was
,FIGURE 2. PILOT PLANT FOR INTERPHASE EVAPORATION OF SODIUM SULFATE SOLUTIONS
solutions; the other two were merely storage tanks. The motor-driven pump,
FIQURI 3. V~nws OF TEI PILOT PLANT
vertical gas pipes extending beneath the liquid surface from the head of the evaporator. The ends of these pipes (2 inches in diameter) were threaded so that various types of nozzles might be attached. Several runs were conducted without any attachments. I n another case short, I-inch, sharp-edged nipples were threaded into 2 X 1 inch reducers, and the resulting assemblies were attached to the three gas pipes. Another experiment was conducted with sharp-edged orifices bolted to 2-inch screwed flanges into which the gas pipes were threaded. Scrapers, similar in aonstruction and operation to those described previously, were used to clear the orifice plates of crystallized material. I n another run small cones, containing thirty-five '/d-inch perforations, were attached to the insidesurface of 2-inch couplings and the assemblies were threaded to the 2-inch gas pipes. I n the final modification a single horizontal nozzle was used. MATERIALS.The materials used were Glauber salt and lignite. The raw salt was shipped from Grenora, N. Dak., in 100-pound sacks. Several tons of this material were obtained, sacked, and shipped from a stock pile near the Grenora No. 1 deposit. One of the better grades of North Dakota lignite, stoker size, was obtained from the University of North Dakota power plant, sampled, and analyzed. A representative analysis of this fuel, as fired, follows: water, 33.7 per cent; carbon, 42.0; hydrogen, 2.9; nitrogen, 0.7; sulfur, 0.4; oxygen, 13.0; ash, 7.3; total, 100; heating value, 7379 B. t. u. per pound.
liquid feed to simplify the operation. Essentially saturated solutions of sodium sulfate were prepared in the feed tanks by dissolvingweighed quantities of Glauber salt in the requisite amount of hot water. Each batch or tankful thus prepared was mixed thoroughly by circulation of the solution. Approximately 4 hours were required to build the h e and to bring the furnace gas temperature within several hundred degrees of that attained during normal operation. At the end of this period the feed solution was pumped into the evaporator to a predetermined level. High-pressure steam was then admitted to the ejector, which caused the furnace gases to be drawn into the evaporator through the 4-inch hot gas line connecting the evaporator and the chimney. The rate of flow was kept constant by regulating the steam pressure manually. When a constant rate was established a blank was placed between the flanges on the emergency stack. During operation it was necessary. to break away the scale deposited around the mouth of the nozzle by means of the scraper at intervals of about 15 minutes. I n the course of a run the following data were recorded: furnace gas temperature, liquid temperature, gas discharge temperature, vacuum (in inches of mercury) in both the evaporator and at the ejector intake, ejector steam pressure, weight of salt discharged, weight of fuel 6red, specific gravity of feed, and temperature of feed. The volume of feed was determined by measuring the depth of liquid in the feed tanks. From a previous calibration it was found that 1 inch of liquid represented 0.230 cubic foot. From the volume and specific gravity data, the weight was computed with a fair degree of accuracy. The stoker was operated continuously but the saturated solutions were pumped to the evaporator intermittently.
Method of Operation The process chosen for investigation was primarily one of evaporation. Although preliminary calculations showed that the thermal requirements are much less for a system operating on solid feed (decahydrate), it seemed advisable to use 61
62
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
During operation the liquid level in the evaporator was permitted to drop about 0.5 inch from the predetermined mark before fresh solution was added. The average results for several runs are given in Table 111. Detailed data for one run (NO. 11) are given in Table IV to show the operation characteristics of the pilot plant.
Vol. 34, No. 1
variation of the grain size of the anhydrous salt produced under both of these conditions may be obtained from Figure 5, photomicrographs of crystals obtained when scaling took place and when it was absent. Several factors might influence this condition, including the degree of agitation and the presence of impurities. An effort was made to correlate the length of operation with the concentration of iron, calcium, and magnesium in the solution within the TADLE 111. AVERAGEEXPERIMENTAL RESULTS evaporator, but because of insufficient Total data no definite conclusions could be Water Temperature, Evapd./ drawn. F. Fuel VacUsed, Feed, D’:,”%:, Lk$yl Gas A n a h i s , % DetaiIed heat and material balances Run Time, uum Furnace No. Hr. In. HL gas Liquid Lb. Lb. Lb. (Uncor.) COa 0s Na were calculated for several of the runs. 1130 160 285 627 1.6 148 5 10 ... E:; The results for runs 11 and 12 areper283 1080 158 718 161 1.4 6 10 8 14.75 1.94 1255 167 475 1403 372 1:96 9.1 10.5 80.4 haps more indicative of the process 9 20.5 2.69 1410 166 721 2296 153a 2.21 1 3 . 1 6 . 8 80.1 5.58 1090 150 165 ... 1ikS ... than those for earlier runs. The ef10 9 2.4 1300 162 5493 5493 2:%5 9 . 0 id.*3 8d.’7 ficiency of the process, based on the 11 45 12 7 1.8 1650 178 1093 1093 116 3.35 15.0 4 . 9 80.1 heat input to the evaporator, was a Large quantity adhered to screen. calculated to be 63.4 per cent for run 11 and 92.0 per cent for run 12. The water evaporated per pound of fuel was Discussion of Results 2.38 pounds for run 11 and 2.4 pounds for run 12. The lower water evaporation, in relation t o the efficiency, obtained in I n the &st six runs the hot furnace gases were introduced run 12 as compared with run 11 was due to the lower heat through the three vertical 2-inch pipes. These runs showed value of the fuel; the lignite used in run 11 had an average that vertical gas pipes were not satisfactory because of exheat value of 7379 B. t. u. per pound as compared with 6711 cessive caking of salt which gradually closed the orifice openB. t. u. for the lignite used in run 12. ing. This was probably due to the alternate heating and wetting of the external surface of the pipe, caused by bubbles Estimated Cost of Production by Interphase of gas rising in contact with the pipe. The resulting scale forProcess mations are shown in Figure 4. The proposed location of the plant is a t the deposit near As a result of this difficulty, which permitted runs of only Grenora, N. Dak. The capacity will be 50 tons of dried salt a few hours before excessive crystallization necessitated the per 24 hours, and it will operate 360 days per year with three shutting down of the unit, it was decided to try horizontal 8-hour shifts. A flow sheet of the proposed plant is shown in gas pipes. We believed that the scaling would be minimized Figure 6. because the gas bubbles would leave the pipe without coming The crystals will be harvested by a small mechanical shovel in contact with the outside surface of the pipe. Therefore placed in a trench in the deposit. Cars, pulled by a suitable the vertical 2-inch pipes were replaced by a single 4-inch horigasoline locomotive, will bring the crystals to the plant. The zontal pipe, to the end of which was attached a suitable tapered crystals will be washed in a log washer and, after draining, will nozzle. This change practically eliminated the difficulty of be fed continuously into the evaporator through suitable feed scaling on the orifice opening, and all the subsequent runs locks. A slurry of about 5 per cent anhydrous salt and were made with this design of gas pipe. saturated solution will be withdrawn continuously from the A sharp-edged, properly shaped nozzle was found to be evaporator and sent to settling tanks. The overflow will be superior to any of the several other designs tried. Such a returned to the evaporator and the thickened slurry, of about nozzle possesses a twofold advantage in that it presents a rela70-75 per cent solids, will be filtered in centrifugals; the tively small surface upon which a cake may adhere and build anhydrous crystals, containing about 6 per cent water, will up, and it permits the gases to attain a maximum velocity a t the nozzle mouth. High velocities decrease the wetting of the inner surface of the nozzle a t the mouth, which results in less crystallization on the inside surface. Scale deposition was found to occur more readily on or around surface irregularities, such as seams, joints, corners, etc., than on the smooth portions of the ’evaporator surface. This indicates that a commercial unit would have to be constructed of a metal capable of a very smooth surface finish. The presence of fly ash in the furnace gas presented no particular difficulty. On several occasions the salt discharged from the evaporator was analyzed for insoluble matter, which was found not t o exceed 0.2 per cent. Some of this was probably introduced as scale or sediment from the steel tanks and piping. In a number of the earlier runs a dense hard scale was formed on the evaporator wall. This scale was relatively pure anhydrous sodium sulfate but was not readily soluble in hot water. When such scale deposition occurred a relatively coarse-grained product was formed. During most of the later runs no deposit was formed on the walls, and in these cases a ON NOZZLES WITH VERTICAL FIGURE 4. SALTCAKING PIPES he-grained product was obtained. An indication of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1942 a
TABLEIV. EXPERIMENTAL RESULTSFOR RUN 11 (APRIL10, 11, 12, 1941) (Steam pressure at ejector, 30 pounds per square inch gage; liquid level, 8 inches) Vacuum, In.H g Temperature, O F. Gas Analysis, % Feed Wt, of pue1 EX. EvaDora- EjecTemp., Sp. Slurry, Added Gas Liquid haust 002 0: tor tor 'F. gr. Lb. Lb.
-
Time
10:26 P. M. 10:45 11:oo 11:30 12:oo
...
...
ii6
1:i
1.8 2.0 1.9
2 :b 2.6 2.8 2.7
1000 960 1040 1090
144 146 160 168
152 156 156
2.0 2.0 2.0 3.2 2.1 1.9 1.7 1.0 3.0 2.7 1.8 2.4 2.6 2.5 2.0 2.5
3.6 2.8 2.9 3.9 3.0 2.8 2.5 2.7 3.7 3.5 2.6 3.2 3.4 3.2 2.8 3.2
1130 1110 1190 1240 1285 1270 1210 1200 1190 1200
160 163 158 164 168 162 160 160 162 160 158 156 156 156 156 157
158 158 162 162 160 162 158 152 157 158 160 160 160 155 159 158
1.6 2.4 2.6 2.1 2.9 1.7 2.7
2.4 3.2 3.3 2.9 3.6 2.6 3.5
1290 1300 1310 1290 1330 1370 1370
152 149 144 144 143
162 162 162 161 164 161 165
12:30 P. M. l:oo 1:30 2:oo 2:30 3:OO 3:30 4:OO 4:30 5:00 5:30 6:OO 6:30 7:OO 7:30
3.4 2.2 1.8 3.1 3.3 3.1 1.6 1.8 3.3 3.6 2.7 2.0 1.7 1.7 3.1
3.9 2.9 2.8 3.7 3.9 3.6 2.6 2.5 3.9 4.2 3.3 2.8 2.5 2.5 3.6
1360 1360 1360 1360 1390 1450 1460 1490 1480 1400 1380 1460 1470 1480 1480
...
162 166 165 162 166 167 166 167 166 165 156 165 163 165 165
8:OO P. M. 8:30 9:oo 9:30 1o:oo 10:30 11:oo 11:30 12:oo 12:30
3.0 2.2 3.2 1.8 2.7 1.6 1.6 2.7 2.7 2.3
3.6 2.7 3.8 2.5 3.3 2.3 2.2 3.2 3.2 2.9
1480 1470 1470 1440 1520 1490 1460 1460 1460 1590
1.6 2.6 2.9 2.7 1.6 2.2 3.0 1.8 1.7 1.6 2.1 1.8
2.3 3.3 3.5 3.5 2.3 2.9 3.7 2.6 2 4 2:4 2.8 2.6
1510 1470 1430 1380 1400 1380 1360 1430 1400 1350
7:OO A . M . ,7:30 8:OO 8:30 9:oo 9:30 1o:oo 10:30 11:oo 11:30 12:oo
1.9 2.4 2.3 2.5 2.0 1.5 2.7 2.8 4.3 3.4 2.7
2.7 3.1 3.0 3.4 3.0 2.4 8.6 3.8 5.2 4.3 3.5
1300 1270 1350 1290 1240 1220 1200 1270 1260 1240 1220
12:30 P. M. l:oo 1:30
2.5 2.1 2.9 2.2 3.1 3.8 2.3 3.4 1.7 3.3 4.1
3.2 2.9 3.7 3.2 3.5 4.0 2.9 3.6 2.7 3.7 4.2
1210 1200 1200 1240 1260 1220 1170 1190 1170 1150 1140
1.7 3.1 1.8
3.5 4.3 4.0
1100 1080 1050
12:30 A. M. 12:65 1:30 2:oo 2:30 3:OO 3:30 4:OO 4:30 5:OO
5:30 6:OO 6:30 7:OO 7:30 8:OO 8:30 A. 9:oo 9:30 10:30 11:oo 11:30 12:oo
1:oo A. 1:30 2:oo 2:30 3:OO 3:30 4:OO 4:30 5:OO 5:30 6:OO 6:30
M.
M.
2:oo
2:30 3:OO 3:30 4:OO 4:30 5:OO 5:30
6:OO P. M. 6:30 7:OO 7:50
..
..
1170 1170 1180 1160 1290 1190
iiio
...
... ...
... ... ... ... ... ... ... ... ... ... ... ... ... ...
... ... ... ... ...
... ... ... ...
... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ...
*..
...
*.. I
.
.
... ...
.... . .... ... ... ... ...
.8.0 . . .11.0 .. 8.7
10.4
. . . . 8.4 .. ... 5.0 i3.a ... 8.9 ii.i
11.6
... 6.0
1'i.k ... 7.4 ii.4 ... .5.8. . .ii.i ..
. . . . . . .. .. .. .. . .. . .. .. .. 1.27 .. .. .. .. .. .. .. .. .. . . . . . . . . . 66
112
. . . . . . . . . . . .
iii i:i& ::: ii' . . . . . . . . . . . . .........
. . . . . . . . . ii.5 . . . . . . . . . . . . iii i:iiz iii :::
.. .. .. .. .. .. .. .. .. . 55. . . . . . . ... ... ... .... ... .. ..129 . . .56. . .. .. .. .. .. .. .. .. .. . 57. .
. .. .. 1.27 . .. .. ......... ... ... ... .4.2 . . .14.0 . . .136 . ...... .. .. .. .. .. .. .. 92 .. .. .. 37 .. i6.k Y.'i . . . . . . . . . . . . . . . . .. .. 37 1 3 5 47 .9.8 . . .10.0 . . iib . . . . . . . . .. .. 66. 10.4 8.4 . . . . . . . . .. .. ... . . . . . . . . . 60 4.0 ii.0 .. .. .. . .. .. . .102. . . ... 16.2 3.2 .. 15.0 4.3 ii.2 ;.*I iii 1:iis ::: ii' . . . . . . 166 ... ... 6.4 ii.6 iii i:ii5 ::: 12.6 7.3 . . ii' .. .. .. .. .. .. . . . . . . . 129 ... iii i:iis . . . . . . 12.2 7.5
11.1 8.8 8.8 10.8
"'
.. .. .. .. .. .. .. .. .. . 63. . .. .. .. . .. .. . . 56 60 .. .. .. .. .. .. .. ... ... ..65.. .. . . . . . . 115 ... . . . . . . . . . . . .. .. .. .. .. .. . . . .. . .56.
166 163 166 164 172 168 168 166 168 172
ii.'s h;.i id.'6 '6.h
170 167 164 162 166 160 160 168 172 168 168 163
i6.i ... 6.0 1'4.0 ... 8.0 ii.8
134
. .. .. .. 9....b. ....
... ... ... . ... ... .. .. 91.. .. .. ... .. ..
161 160 165 168 158 160 160 164 154
... ... 161 157 ... 162 ... ..,
...
5.8 ii.8 ... .6.2. . i.2'..b.
152 167 169 158 156 158 156 163 169 163 165 153
...
. . . . 6.9 .. ... 8.8 i6.s
13.8
ii.*i
... 9.4
Y.'9
ii.'i
8.1
i6.i
...
1:iia
i6i
:::
. . . . . . . . . . . . .. .. .. .. .. . . . .. .. . . 63. .
. . . . . . . . .
68
. . . . . . .. .. .. .. .. .. .. .. .. .. .. .. 12.0 8.5 ... gg' ... iii i:iio 196 8.1 ii.*i .. .. .. .. .. .. . . . . . . ... 6.2 ii.8 ii,'i '6.k ... 8.2 i6.i
......
8.0 10.0 ... 8.0 ii.0 .8.8 . . . 9.2 .. ... 7.2 ii.k ... 4.5 i3.s
... 6.7 ...
ii.i
6.2 1'i.k .. .. .. .. .. ..
.. .. .. .. .. ..
. . . . . .
. . . . . .
99 ... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 60 . . . . . . 88.5 .....
ii4 1:iio
......
.. .. .. .. .. .. .. .. .. .. .. .. 55.5 iib i:ii7 'Qi... ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
l:iig .ii ... .. .. ... .. .. . . 80. . . ... .. ...
ii6
63
be dried in a direct-fired rotary dryer. Lignite will be used as the fuel €or both evaporation and drying. HARVESTING OF CRYSTALSAND TRANSPORTING TO PLANT.It is estimated that the cost of labor, power, and other operating costs of shovel and locomotive will not exceed 24 cents per ton of Glauber salt. About 2.5 tons of washed Glauber salt are required to produce 1 ton of salt cake; therefore, $0.24 X 2.5 = $0.60. PLANTLABOR. The plant will be operated with a three-man labor crew per 8-hour shift-one man on washer and feeder; one man on evaporator, settling tanks, and centrifugal; and one man on dryer and for general plant work, including shipping. Labor rate is assumed a t $0.60 per hour, or 3 X $0.60 = $1.80 per hour; cost per 24 hours = $43.20. One maintenance and repair man will be needed for 8 hours out of every 24 of plant operation; cost per 24 hours = $5.50. The plant foreman will be paid $250 per month, or $8.34 per 24 hours. The total plant labor cost per 24 hours is then $57.04; the total plant labor cost per ton of dried salt is $57.04/50 = $1.141. FUEL FOR EVAPORATION. Lignite with an average heating value of 6700 B. t. u. per pound can be delivered to thg plant from nearby mines a t $1.60 per ton. The experimental pilot-plant data have shown that about 0.6 ton of lignite is required for evaporation per ton of dried salt. Therefore $1.60 X 0.6 = $0.96. FUEL FOR DRYING FILTERED CRYSTALS I N ROTARYDRYER. It has been estimated, from data for the drying of other materials of about the same characteristics as sodium sulfate, that the a t e r e d crystals containing about 6 per cent water can be dried with 0.1 ton of lignite or $1.60 X 0.1 = $0.16. POWERREQUIREMENT FOR DRYm u PLANT.It is estimated that a plant of this capacity will require not more than 50 horsepower. It is anticipated that the power rate a t the proposed plant site will not exceed 1.5 cents per kilowatt-hour. Therefore (50 X 0.746 X 1.5 X 24)/50 = $0.268. REPAIRSAND M I S C E L L A N E o u s PLANTSUPPLIES. The estimate is $0.15 per ton of dried salt. TRANSPORTATION. Hauling the salt cake 4.5 miles from the d a n t t o the railroad siding a t G r e k r a by
64
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 1
structed for $50,000. The harvesting equipment will cost about $12,000. Operating capital requirement is assumed a t $30,000. Therefore interest on $92,000 a t 4.5 per cent is $92,000 X 0.045 or $4140; interest per ton of salt cake is $4140/18,000 or $0.23. Depreciation on $62,000 a t 10 per cent is $62,000 X 0.10 or $6200; depreciation per ton of salt cake is $6200/18,000 or $0.344. RECAPITULATION. The total cost of production per ton of dried salt is summarized as follows: Harvesting Labor Fuel for evaporation Fuel for drying Power Repairs and miscellaneous plant supplies Transportation Works general expense Heating and lighting Administrative and sales Cost exolusive of royalty, interest, and depreciation Royalty Interest Depreciation
$0.60 1.141 0.96 0.16 n -.--I 0.268 0.15
0.25 0.806 0.033 0.416
4.784 0.30 0.23 0.344
55.668
Total
Acknowledgment The authors desire to express their appreciation to the Greater North Dakota Association for financial assistance which made this study possible.
Literature Cited W. L.,and Caldwell, H. B., Trans. Am. Inst. Chem. Engrs., 16,2, 131 (1934). Douglass, E. W., and Anderson, C. O., Chem. & Met. Eng., 48, No.5, 135 (1941): Donnecke, H.W.,Douglass, E. W., and Anderson, C. O., U. S. Patents 2,086,902(July 13, 1937) and 2,169,759(May 23, 1939). Kobe, K. A,, and Hauge, C. W., Can. Chem. Met., 18, 177 (1934). Kobe, K. A,, Hauge, C. W., and Carlson, C. J., IND.ENG.CHEM.. 28, 589 (1936); Kobe, K. A., Conrad, F. H., and Jackson, E. W.,Ibid., 25, 984 (1933); Kobe, I(.A.,Chem. & Met. Eng., 41. 300 (1934); Kobe, K. A,, and Hauge, C. W.,Power. 77, 402
(1) Badger.
FIGURE 5. TYPICAL CRYSTAL SIZE WHEN SCALlNG (above) AND DID NOTOCCUR(below)
OCCURRED
(1933).
Lavine, Irvin, and Feinstein, H., Am. Inst. Mining Met. Engrs.. Contrib. 97 (Feb., 1936).
trucks will amount' to 80.25 per ton of salt cake. WORKSGENERALEXPENSE.The list includes the following items, per month: Superintendent, chemist, and accountant Yard labor, supplies, and miscellaneous Laboratory expense Liabilitv. comoensation. and insurance Property and group insurance General taxes Legal expense Office stationery and supplies Telephone and telegraph
WASHED
CRUDE
GLAUBER
SALT
5676 226 50 50 50
75 50
10
EVA P0 RAT LIQUOR T O LAKE
WATER
25
""V $I GASES
$1210
Therefore ($1210 X 12)/18,000 = $0.806. HEATING, LIQHTING, AND OTHEREXPENSES. Per ton of salt cake, these amount to $0.033. ADMINISTRATIVE AND SALES EXPEXSE. One stenographer, and one salesman who will spend about 100 days per year on the road and the rest of the time in the office. The cost per year is estimated a t $7500; per ton of salt cake, it is $7500/18,000 or $0.416. ROYALTY is estimated, per ton of salt cake, as $0.30. DEPRECIATION AND IXTEREST. The plant, including the building, can probably be con-
TO EXHAUSTER
FURNACE
AIR FUEL
SLURRY SXCRYSTALS
FIGURE 6. FLOWSHEETOF PROPOSED PLANT
OVER FLOV
