Countercurrent Extraction of Potassium and Magnesium Sulfates from Calcined Polv halite J
Continuous hot extraction of potassium sulfate and magnesium sulfate from calcined polyhalite, predicted on the basis of previous laboratory studies, has been successfully carried out in a countercurrent system handling up to 90 pounds of calcine per hour. Practically complete extraction of minus 10-mesh material has been obtained with a top liquor concentration of 10 parts of potassium sulfate per 100 parts of water. By accepting a decreased recovery of somewhat more than 90 per cent, a top liquor containing 11 parts of potassium sulfate per 100 parts of water has been produced. Efficient agitation to disperse the calcined polyhalite in the liquor near the top of the system, the maintenance of a temperature near the atmospheric boiling point throughout the system down to the final washing of the calcium sulfate residue by the incoming solution, and the presence in this solution of a low concentration of sodium chloride are desirable.
*+.
equivalent to considering the S E of t h e whole column as the top portion essential of a more e x t e n s i v e system, o-Derationsin yielded a partially e x t r a c t e d several processes previously outr e s i d u e , which on subsequent lined by this station (3-4, 7-11) batch e x t r a c t i o n produced a for the r e c o v e r y of potassium liquor equivalent to that fed to sulfate from polyhalite (K.,S04.the column. MgSO4.2CaSO4.2H20) is t h e The results o b t a i n e d in the treatment of the calcined mineral first series of t e s t s , i n w h i c h with hot water to dissolve potascharging rates of approximately sium s u l f a t e a n d magnesium 25 pounds of calcined polyhalite sulfate, leaving a solid residue of per hour were used, proved that calcium sulfate. This estracconcentrations of 9.5 to 9.6 parts tion might conceivably be carried of p o t a s s i u m sulfate per 100 out with batch, multistage, or parts of water could be attained, continuous c o u n t e r c u r r e n t together with the ultimate rehandling of materials. moval of 95 to 96 per cent of the L a b o r a t o r y studies of the potassium sulfate. The l a t e r batch ( 1 , 4, 7', 8) and multistage series of tests showed that con(2, 3 ) methods have indicated centrations in excess of 11 parts that concentrations of 10 to 11 could be attained, with some parts of potassium sulfate per sacrifice i n t h e p e r c e n t a g e 100 parts of water, t o g e t h e r recovery. with concentrations of magnesium sulfate corresponding apExtraction Equipment proximately to the 1:1 molar The equipment used i n t h e ratio of the two constituents, countercurrent ex t r a c t i o n of represent practical m a x i m u m calcined polyhalite comprised values for hot extractions conJOHN E. COIYLEY, F. FRAAS, essentially an extraction column d u c t e d a t 100" C. (212" F.). A S D EVERETT P. PARTRIDGE provided with accessory apparaThe ratio of solid to liquid retus for l i q u i d a n d solid feed, quired in the batch process to Nonmetallic Minerals Experiment Station, tanks to serve as traps for catchp r o d u c e these concentrations U. S. Bureau of Mines, Rutgers University, ing the partly e x t r a c t e d resiwas, however, high enough to due, a d i a p h r a g m p r e s s u r e introduce difficulties in handling. New Brunswick, N. J. pump to remove the extracted Since continuous countercurrent operation promised to decrease these difficulties and residue from the column and deliver it to the filter press with since the laboratory multistage experiments had indicated no provision for the return of the filtered solution to the system, fundamental objections to this mode of procedure, a series a plate-and-frame filter press, and an auxiliary pump to reof tests was made on an enlarged scale in the Chemical cover the drip from the filter press. The arrangement of Engineering Laboratory. The results of these tests presented equipment is represented diagrammatically in Figure 1. in condensed form in this paper demonstrate the feasibility The extraction column consisted of a series of sections of countercurrent extraction of calcined polyhalite. placed one above the other in a manner similar to a number Thirteen countercurrent extraction tests were made, a t of superimposed tray thickeners. The lower half of the temperatures approaching 100" C. a t the hottest portion of column consisted of 12-inch Shelby tubing and the upper the system, to produce a hot concentrated liquor and a residue portion of %inch tubing. The height of the column was aplargely freed of potassiuni and magnesium sulfates. Testa proximately 11 feet 4 inches. employing water alone or a dilute sodium chloride liquor POLYHALITE FEED. The problem of adding the calcined produced a final extracted residue very low in these constitupolyhalite to the concentrated solution a t the top of the ents. I n other tests the system was initially charged with a column presented some difficulty during the early tests. solution containing potassium sulfate, magnesium sulfate, Thorough dispersion of the calcined material as it is introand sodium chloride, and feed solutions also containing these duced is imperative to prevent caking and lumping. Initial constituents were used. This latter procedure, which was attempts to add the calcined polyhalite, with and without 1087
1088
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 27, NO. 9
agitation, directly to the top of the column invariably resulted During the period in which it was desired to collect the exin difficulty because of the eventual appearance of lumps. tracted residue, the traps were by-passed, and the extracted Later, efforts were made to disperse the material in a small polyhalite was pumped directly to the filter press. In all separate agitator provided with a stirrer and a circulating instances the lower portion of the apparatus was operated as pump for forcing the dispersed mixture into the column a t a closed system, and the solution after being freed of susone point and withdrawing from a higher level a fresh portion pended solids was returned to the column. Fresh feed solution of solution to mix with additional calcined polyhalite. This was added to a standpipe as shown in Figure 1 and entered the arrangement was fairly successful only a t the lower rates of column admixed with the filtered solution from the filter press. feed. To obtain satisfactory dispersion of the calcined mateThe filter press was of the ordinary plate-and-frame type rial a t higher rates of feed, the arrangement shown in Figure 1 constructed to permit closed or open discharge. In filling the was adopted. A system a part 07 the charging rate of 90 air was removed pounds of solids per t h r o u g h t h e drain hour eventually was cocks, but during a u s e d w i t h o u t diffir u n t h e p r e s s was culty. operated with closed The f e e d d e v i c e discharge. Before beshown consisted of a ing emptied, the press h e l i c a l screw feeder was blown with comwith a variable-speed pressed air. drive. This f e e d e r R E M O V AOF L Exdischarged into a TRACT LIQUOR.I n all small hopper attached the tests the concento a section of 4-inch trated extract liquor steel pipe welded a t a was obtained as over60" a n g l e t o t h e flow from the top of column so as to dethe column a t temliver the solids a t a peratures a p p r o x ipoint approximately mating 100" C. Be24 inches below the cause of the limited l i q u i d level. Thorsettling area in the ough agitation of the top of the column this mixture was assured l i q u o r usually conby a motor s t i r r e r tained more or less placed with the pros u s p e n d e d solids; peller near the point both incompletely dea t which the side tube composed fine parentered the column. ticles of calcined polyFIGURE1. DI.kGR.kX OF COUNTERCURRENT EXTRACTION SYSTEM A screen a t this point halite and secondary prevented any lumps solid phases formed from entering the column without being broken and minimized from solution were included, the quantity increasing with rate agitation inside the column proper. of feed. Removal of the suspended solids by a settling trap in SOLUTION FEED. As in the case of the solid feed, numerthe overflow line was incomplete, while filter screens in the top ous changes were made in the method of adding the solution of the column proved unsatisfactory owing to plugging. Evenduring the course of the investigation. In all instances the tually it was decided to collect the overflow solutions and apply solution feed entered a t the bottom of the column, and the suitable corrections for the quantities of potassium and magconcentrated overflow solution was removed a t the top. I n nesium sulfates being carried over in the suspended solids. the first test the concentrated solution was removed by means SAMPLING.During the extraction tests, samples of t h e of an automatic siphon adjusted to the proper capacity; this overflow liquor, of the underflow liquor, and of the mixture was replaced by a proportioning pump. Eventually the a t selected points in the column were taken a t definite interoverflow was regulated by controlling the solution feed; a vals. These were filtered immediately with particular precautions to obtain accurate and representative samples of proportioning pump was installed for this purpose but later was replaced by a calibrated rotameter which received the liquor and suspended solids for chemical and petrographic solution feed from a constant-head device attached to an analysis. The amount and composition of the suspended overhead supply tank. Hand feeding during the final stages solids in the overflow liquor were of special importance in properly correlating the extraction data. The solids filtered of the later runs controlled the ratio of solid and liquid feed from the solution were washed first with a small amount of within the narrow limits desired. REMOVAL OF EXTRACTED SOLIDS. Two tanks were placed 50 per cent alcohol, then with 95 per cent alcohol to remove completely all adhering extract solution. in the system to trap a portion of the partially extracted polyhalite during the later runs (Figure 1). A 50-gallon Preparation of Polyhalite steel drum provided with a removable flanged cover was used between the column and the pump-i. e., a t low pressure. The polyhalite used in all the countercurrent extraction The other trap was designed to withstand the pressures betests was taken from a carload obtained from the shaft of the tween the discharge end of the pump and the filter press. In U. S. Potash Company near Carlsbad, N. Mex. The avermany of the earlier runs in which rates of feed of the order of age composition of the entire lot is as follows: 25 pounds per hour were used, the solids were pumped through Mineral Constituent Per Cent Mineral Constituent Per C e n t the pressure trap and thence through the press during the 76.6 Magnesite 0.7 Polyhalite entire run. I n the later runs the two traps, in series with the 12.8 RlOa + SiOr 2.3 Halite Anhydrite 8 . 2 Total 100.8 filter press, removed practically all of the extracted solids.
SEPTEMBER, 1935
INDUSTRIAL AND ENGIl-EERISG CHEMISTRY
The different batches from the carload varied slightly in composition but in general showed the characteristic salmon color. The compositions of the various lots as calculated from the chemical analyses are indicated in Table I. Kearly all of the batches were given a preliminary wash to remove the bulk of the sodium chloride by a treatment similar to that developed by Davidson and Fraas ( 5 ) . Analyses of the washed polyhalite showed an excess of magnesium over the theoretical 1 : l ratio with potassium after calcination, probably due both to retained wash liquor and to the small amount of magnesite present in the original polyhalite. No attempt has been made to differentiate between these, the excess being reported as sulfate in Table I. The sodium chloride content of the calcined material ranged from 0.64 to 10.63 but in most instances was lower than 3 per cent. In many of the processes proposed for the extraction of potassium sulfate from polyhalite, a ratio of sodium chloride to potassium sulfate of 1:5 permits a satisfactory percentage recovery during subsequent evaporation and crystallization. Higher ratios have been avoided as much as possible in all the countercurrent extraction tests. A procedure that uses calcined polyhalite low in sodium chloride and introduces the maximum permissible amount of this constituent in the feed solution has certain advantages. The reaOons for this practice nil1 be discussed later. PARTICLE SIZE. AS may be noted in Table I, experiments were conducted on three different degrees of fineness. The majority of the tests were made on minus 10-mesh material, although three tests were made on minus 20-mesh and one on minus 30-mesh calcine. Previous experiments (2, 3) have shown that calcined, minus 10-mesh polyhalite yields its potassium sulfate readily enough to give high recoveries and satisfactory concentrations. One theoretical advantage of the countercurrent process over a batch process is the separation of the solids by classification or settling. Obviously, the coarser the material the more readily the liquid and solid may be separated. I n view of these facts, during the preparation of the polyhalite prior t o calcination, care was taken to keep the proportion of fines as low as possible. Washing to remove salt, grinding to specific screen size, calcination, arid the final passage through the helical scren- feeder, however, all tend to produce additional fines. Data on the increase of fines during calcination have been presented previously (2, 3 ) . Instead of removing the fines after calcination, which would necessitate separate treatment of this material or the discarding of an appreciable fraction of the calcined polyhalite, it seemed preferable to attempt extraction of the calcine
1089
as obtained. A typical example of the particle-size distribution of minus 10-mesh polyhalite after washing, grinding, calcination, and final passage through the screw feeder is given in the following table for the material used in run 12: Screen Mesh Per Cent 5.52 10 -10 20 29.02 10.64 -20 4- 28
+ +
Screen Mesh Per Cent -28 35 8.53 -35 65 10.95 -65 4- 100 4.52
++
Screen Mesh Per Cent -100 150 2.21 -150 30,82
+
Considerable care is essential in making a screen analysis of the calcined polyhalite, since prolonged treatment on the screens will increase materially the percentage of fine material. I n view of the apparent ease with which the percentage of fines in the calcined polyhalite is increased, feed or metering devices that tend to crush or grind the particles should be avoided. CALCISATION. The conditions under which polyhalite is calcined are extremely important ( I , 4). -411 poiyhalite used in the countercurrent extraction tests was calcined in a rotary gas-fired kiln, 6 inches X 11 feet in size. To eliminate any difficulties traceable to the calcination treatment, conditions were chosen which previously had been found satisfactory for batch and multistage laboratory extraction tests. With the exception of several of the preliminary tests, the extraction was made within 2 weeks after calcination. The average maximum temperatures, time of retention in the furnace, time above 300" C. (572" F.) and rate of calcination are given in Table I. Typical curves showing the average temperature distribution in the 6-inch rotary kiln during calcination and the variation of the maximum temperature with time are shown in Figure 2 .
General Test Procedure Of a total of thirteen countercurrent tests, seven were made a t a feed rate of approximately 25 pounds per hour of polyhalite, and six a t the higher rate of TO to 80 pounds per hour. Water or a dilute sodium chloride liquor was used as the extraction liquid in the first series, except in two tests in which a 3 per cent potassium sulfate liquor was employed. Liquors containing potassium sulfate, magnesium sulfate, and sodium chloride in concentrations corresponding to an intermediate stage in an extraction system were used in the later serieq. TESTS WITH Lorn R ~ T OF E FEED. An effort was made to build up the system to a steady state by prolonged operation in tests 1 t o 7 , but it was found that the low rates of feed mould require a much longer time than mas practicable to attain the desired condition. In these first tests the rate of rotation of the shaft carrying the cones and scraper blades was
TESTS POLYHALITE USEDIN COUNTERCCRRENT TABLEI. CALCIXED Run KO. Sominal size, screen mesh/in. Time from calcination to extn., days Analysis of calcine, per cent: K&Oa hlgSO4
NaCl Cas04 Calcd. compn., per cent: Polyhalite Anhydrite Halite &fgSOia
7 -10 7
8 -10 13
-10
8 mo.
6 -20 9
. . .. .5. ..5.8
26.32 19.29 5.21 49.38
27.15 20.07 1.30 51.05
27.67 20.46 0.74 49.95
27.02 19.58 3.51 49.71
85.06
85.65 8.24 5.21 1.10
88.35 8.62 1.30 1.30
90.04 6.70 0.74 1.34
87.93 7.48 3.51 0.90
1 -10
2 -30
3 -20 2 wk.
4 -20 7 mo.
24.52 18.80 10.53
26.90 20.07 2.84
27.69 20.01 0.80 52.11
26.14
79.79
87.54
90.11 8.83 0.80 0.87
....
....
....
10.53 1.85
....
.. . .
.2. . 8 4 .... ,,
.5. ..5. 8
....
5
-10
10 -10 10
11 -10 5
.-lo
13 -10 7
27.27 19.82 3.00 50.71
27.62 20.78 1.70 50.23
28.70 20.83 0 65 49.77
28.37 20.36 0.64 50.38
27.30 20.13 2.10 49.51
88.74 8.09 3.00 0.97
89.88 7.06 1.70 1.69
93.39 4.91 0.65 0.99
92.32 6.04 0.64 0.75
88.84 6.84 2.10 1.26
9 7
466 871 11.2 133.5 a b C
d
Includes any big0 derived from magnesite present in original crude polyhalite. Time above 300' C. (572O F.)instead of in furnace. Comprised lots 23 and 25 calcined 1.0 minute at 146 Ib./hr. and 8 . 3 minutes at 108.0 lb./hr., respectively. Comprised lots 27 and 32 calcined 7.7 minutes at 111.0 lli./hr. and 21.5 minutes at 69.0 lb./hr., respectively.
454 849 10.2 132.0
468 874 9.29 117.5
12 10
475 887
9.21
130.0
479 894 12.25 147.5
VOL. 27, NO. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
1090
I)
TIME ELAPSE, MINUTES ( C U R V E 0
15
30
60
90
Curves
120
I a2
150
:
Exp.
180
12
DISTANCE F R O M FEED END, I N C H E S
FIGURE2. TEMPERATURE DISTRIBUTION FOR CALCINATION OF POLYHALITE IN 6 x 132 INCHROTARY KILN (CURVES2 AKD 3) AXD TEMPERATURE VARIATION OF SELECTED POINT (CURVE1)
0
v
TIME,
HOURS
complete extraction unit. With the exception of the first two tests, either water or a dilute sodium chloride solution was used. Details of the various runs are given in Tables I1 and 111. These data show that the maximum concentrations of potassium sulfate were 9.40, 9.48, and 9.62 parts of water in tests 1, 2, and 6, respectively. The steady state had not been reached in any test, and the concentrations were increasing in every instance a t the termination of the run. The percentage extractions of potassium and magnesium sulfates based upon the extracted residue were fairly high in all the tests (Table 111) in spite of the fact that the concentrations reached were less than the theoretical maxima as indicated in Table 11. X typical example of the change in composition of the overflow liquor plotted from samples taken a t 30-minute intervals in experiment 6 is given in Figure 3. If these early tests had been continued long enough to establish a steady state, the concentration of the top liquor presumably would have approached the theoretical limit more closely, or else a greater amount of potassium sulfate would have appeared in the solid leaving the system. TESTSWITH HIGHERRATESOF FEED. ilfter study of the first seven tests, it was decided t o increase the rates of feed in subsequent runs to shorten the time required to reach a steady state. Adoption of the feed device shown in Figure 1 made it possible to charge as much as 90 pounds of calcined polyhalite per hour without difficulty. Curve B of Figure 4 shows the average temperature distribution in the column a t the higher rates of feed for run 8; curve A indicates the temperature variation a t the top of the column during the run. Run 8 represents the first attempt a t rates of feed greater than 31 pounds per hour. As in many of the previous tests, a dilute solution containing only 0.78 part of sodium chloride per 100 parts of water was used as the feed solution. The top concentrations of the overflow liquor were higher than in any previous test, and only slightly more potassium and magnesium sulfates remained in the extracted residue than in runs 5, 6, and 7. The time of retention of the solids in the column was only 31.5 minutes in run 8 and 25.7 minutes in run 9. Previous multistage laboratory tests (9)have shown that a much longer time is required for satisfactory extraction. I n run 9 a high ratio of calcined polyhalite to water was used to determine whether concentrations in excess of 11
IN CONCENTRATION OF OVERFLOWASD FIGURE3. CHANGES UNDERFLOW SOLUTIONS, RUN 6
Feed rate of 27.14 pounds of calcined polyhalite per hour
very close to 0.2 r. p. m. From the design of the column the theoretical time of retention of dry solids would have been approximately 3 hours. Calculations made on runs 5 and 6, based on the assumption that the amount of calcium sulfate in the column and in solution had reached a constant value, indicated periods of retention of 53 and 51 minutes, respectively. The approximate capacity of the column proper was about 360 pounds of solution. At the charging rates used in tests 1 to 7 the time of retention of solution in the column would have been between 6 and 7 hours. These conditions were unfavorable for efficient extraction, since the solids were retained for too short a period, tending toward incomplete extraction, and the solution much too long, favoring the crystallization of secondary solid compounds containing potassium. Many changes were made in the method of operation during the first seven tests, particularly with respect to the introduction of the calcined polyhalite. It is believed that the difficulty in this operation was much more pronounced in the small laboratory column than it would be in a large commercial installation. The object of these first tests was to obtain data on the treatment of calcined polyhalite, operating the column as a
T I M E , MINUTES, 0
60
I20
CURVE A) 300
640
I80
360
100
120
100
80
60
40
20
HEIGHT A B O V E D I S C H A R G E OUTLET, INCHES (E)
FIGURE4. AVERAGE TEMPERATURE AT VARIOUS POINTSIN COLUMN DURING ExTRACTION EXPERIMENT
SEPTEMBER, 1935
T.4BLE
11.
R.4TES O F CHARGE AND COSCENTRATIONS
R a t e of charge, lb./hr.: Calcined polyhalite
KzSO4 hfgSOaa
NaCl Cas04 Feed s o h . Water
hfgsoa NaCl Ratio, water t o polyhalite Concn. of top liquor, parts/100 parts Hz0: Equivalent t o 1 0 0 ~ extn.: o KzSOi hlgS0P Experimentally attained:
21.56 5.80 4.33 0.61
24.77 31.46 6.86 8.22 ... 4.96 0.20 1.76 . . . . . . 1 2 . 9 1 ... 41.10b 60.69b 44.84 7 8 . 4 8 38.79b 52.83 44.04 78.48 1.19 1.81 0 0.03 0 04 0.10 0 0 0.03 0.08 0.77 o 1.95 2.45 1.78 2.50
25.60 27.14 2 5 . 3 3 67.92 89.46 73.00 87.50 77.50 6.74 7.37 7.01 1 8 . 3 5 24.40 20.16 25.11 21.99 4.94 5.45 5.18 13.30 17.73 15.17 18.22 15.78 1.33 0.35 0.19 2.38 2.68 1.24 0.57 0.50 12.64 13.85 1 2 . 6 5 33.76 45.37 36.67 43.55 39.04 58.40 6 4 . 9 0 63.78 146.62 147.58 220.70 214.18 233.10 58.40 63.58 62.50 145.52 141.40 201.00 194.33 214.60 0 5 00 9.98 11.65 0 0 11.12 0 2.74 3.99 0 0.23 0 2.63 0 0 4.20 0.96 5.39 1.28 1.10 4.80 o 1.32 2.75 1.58 2.22 2.77 2.47 2.14 2.28 2.34
12.55 8.67
10.98 15.58 7 . 5 9 11.26
10.47 7.24
11.54 8.46
11.59 8.57
11.32 8.29
12.61 9.13
17.26 12.54
10.03 7.55
12.92 9.38
10.25 7.35
10.98 8.10
8.19 6.84
9.62 8.80
8.22 7.62
9.87 8.65
11.16 11.25
10.14 7.44
11.22 8.87
10.54 7.38
10.55 7.10
64.66 101.10 89.71 98.54 6-0 6-02 6-0 5-09 6-0 5-09
86.84 94.56 5-30 5-30 5-30
102.82 100.41 6-8 6-6 6-6
96.08 87.65 5-58 5-56 5-58
9.40 6.45
3
4
5
9.48 7.38
9.11 9.26
8.13 6.42
76.73 86.34 K2SOa 74.39 97.23 MgSOi 7-7 9-25 Total time of run, hr.-min. 6-36 9-20 Net time solid feed, hr.-rnin. 6-36 9-25 Net time s o h . feed, hr.-rnin. Based on total M g calculated ae MgS94. b Based on net weight of overflou solution obtained.
58.47 82.42 6-7 5-45 5-45
77.60 88.60 5-55 5-28 5-28
7
70.97 8 3 . 0 0 7 2 . 6 1 76.59 102.68 9 1 . 9 1 8-05 5-55 8-50 5-55 7-42 7-49 6-05 7-23 5-22
8
78.22 94.64 5-17 5-01 4-35
9
10
12
11
13 60.75 22.04 16.25 1.70 39.98 217.50 200.80 11.68 2.09 2.89 2.49
EXTRACTED RESIDUESWITH CALCUL.4TED PERCENT.4GE EXTRACTION IN COXTIXOUSCOUNTERCURRENT EXTRACTION TESTS 2
:I
4
5
6
9-25
6-7
5-55
5-55
1-44
103.6 62.8
89.0 52.1
108.0 66.0
112.0 69.5
1
....
.... . , ...
2.99 1.91 0 90 90 00
.., . . . . . . . .
94.3 93.9 104.97
,
,
,
,
,
. . . . . , ,
0.53 0.12 1 75
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . .
yo:
6
COMPOSITION OF
Time collected, hr.-min. 7-7 Net weight, lb.: 111.0 Wet 77.0 Dry Analy is of dried residue from columri, % : KnSOa 10.87 MgSOr 4.66 NaCl 0.40 .... Cas04 % extn. from solid in column:n KrSO4 , , , M g S 0 4 , total . . . MgSO4, mol. equivalent . . . Compn. of retained liquor in filter cake, parts/100 parts HzO: &SO4 MgSOn . . . NaCl .... Analysis of final residue, 92: K~SOI . . . NaCI CaSOa Over-all extn.,
TESTS
19.85 4.87 3.73 2.09
K2SOa hfaSOa
Rlgsoa
COXTISUOFS COUNTERCURRENT
2
% of max. concn. attained by top 1:quor:
TABLE111.
ATTAINEDIN
1
R u n No.
Run S o
1091
INDUSTRIAL AND ENGINEERING CHEMISTRY
. . . .
1.46 2.52
..,, ....
,. .... ,,
.... ,,,,
.... .,.,
7
8
2-05
2-47
98.0 66.5
79.5 50.5
145.5 101.5
9.09 3.57 0.18 76.64
4.23 2.76 1.12 80.00
3.65 2.62 1.37 72.81
7.60 3.60 0.53 73.27
82.55 93.80 99.50
93.82 91.79 101.63
95.61 96.31 103.04
89.15 92.79 97.31
2.47 1.80 0.23
3.09 2.03 2.03
2.64 2.09 1.89
3.90 1.81 0.63
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
10
11
12
13
1-52
2-01
2-11
243
2-21
149.0 117.0
157.0 122.0
168.50 123.75
.. ..
.. .. ,.
..
.. .. ..
114.5 84.6 19.29 2.53 1.08 67.13
22.38 4.26 0.80 60.78
20.60 4.90 0.87 63.04
23.80 4.50 0 62 6 3 60
54.57 95.06 103.53
42.65 88.80 93.27
48.82 85.113 88.46
39.69 86.89 92.74
5.56 2.55 2.38
6.22 3.85 2.18
6.28 2.89 2.25
6 20 2 60 1.45
0.39 0.43
4.43 2.88
1.51 1.48
... . ..
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . .
MgSOi .... . . . . . . . . . . . . . . . . . . a Corrected for retained liquor. 6 These values were further reduced t o 1.13 &SO4 and 1.53 ?dgSOn by 1-hour additional extraction a t 92' C. (198' F.). C &sed on total magnesia presenc calculated as MgSOI.
parts of potassium sulfate per 100 parts of water would result. The data obtained proved that concentrations of this order could be attained in 5 or 6 hours. The data obtained in runs 8 and 9 suggested the procedure for all subsequent tests; in these tests the column was operated to simulate only the upper or hot section of a more extensive countercurrent extraction system. Obviously such a mode of operation would permit the discharge from the column of a residue containing an appreciable quantity of soluble constituents, based on the assumption that additional extraction in the lower portion of the system ultimately would remove the bulk of the undissolved potassium and magnesium sulfates. This scheme would require that the feed solution charged to the column carry approximately the same amounts of potassium and magnesium sulfates as the residue being discharged from the column, together with the permissible amount of sodium chloride. Run 10 was the first test made according to this altered procedure x i t h the extraction system essentially as shown in Figure 1. Calcined polyhalite mas charged a t an average rate of 73.0 pounds per hour, while preheated feed solution containing 4.92 parts of potassium sulfate, 1.35 parts of
s3:33 99.10 88.750
3.76b 2.826
....
7
0
87.27
90.69 91.66
96.95 95.80
92.19 92.05
magnesium sulfate, and 2.66 parts of sodium chloride per 100 parts of water was added a t a rate of 220.7 pounds per hour, equivalent to 201.0 pounds per hour of water. These rates of feed correspond to theoretical maximum concentrations of 10.03 parts of potassium sulfate and 7.55 parts of magnesium sulfate in the overflow liquor. Actual values of 10.14 and 7.44, respectively, were attained, corresponding closely to complete extraction. During the last two hours of the extraction the solids issuing from the column were pumped directly to the filter press to be collected, dried, and reserved for final extraction treatment. Samples of solid suspended in the overflow solution and entrained in the underflow solution were also collected a t 30-minute intervals during the final test period. The filter-press cake after being dried had the composition indicated in Table 111. I n a continuous countercurrent extraction this residue would be subjected to additional treatment in a counterflow equipment, eventually receiving a wash with a solution free of potassium and magnesium sulfates. Owing to experimental difficulties, it was decided t o dry the residue, crush the hard lumps, and reduce to the 10mesh size of the original calcine, and then extract in an agita-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1092
tor provided with a means of maintaining the same temperatures as were found previously to exist a t the base of the column. Possible chemical changes incident to drying and regrinding probably would tend to make the extraction slightly more difficult than would be the case if the wet solids were treated as they issued from the column. The use of a single water extraction to which the permissible quantity of sodium chloride had been added would be less favorable treatment than a truly countercurrent procedure.
l
0, r
2 3 0
2 \
;? (L
4
a
i
-0G
+z w
U
z
0 u
5
c
TIME?, H O U R S
FIGURE5. CHANGES IN CONCENTRATION OF OVERFLOW AND UNDERFLOW SOLUTIONS, RLJN12 Feed rate of 77.5 pounds of calcined polyhalite per hour.
Based on the average composition of the overflow and underflow solids, the corresponding amount of calcined polyhalite represented could be calculated. The amount of solution then required to effect the final extraction could also be determined. Extraction of this filter-cake residue for 2 hours with the proper amount of water yielded a solution containing 4.79 parts of potassium sulfate, 0.82 part of magnesium sulfate, and 2.75 parts of sodium chloride per 100 parts of water, closely approximating the composition of the feed solution. The final residue contained only 0.39 per cent of potassium sulfate and 0.43 per cent of magnesium sulfate. The over-all extraction of potassium sulfate was 98.6 per cent based on the composition of the solution, and 99.1 per cent based on the composition of the solids. In run 11 an effort was made to obtain a higher concentration of potassium sulfate. The general procedure was the same as in run 10. Minus 10-mesh calcined polyhalite of the composition shown in Table I was added during the final test period a t a rate of 87.5 pounds per hour and a preheated solution containing 6.0 parts of potassium sulfate, 2.05 parts of magnesium sulfate, and 2.16 parts of sodium chloride per 100 parts of water was added a t a rate equivalent to 194.3 pounds per hour of water. The extracted residue was collected in the filter press during the last 2 hours and 11 minutes of the run which covered a total of 5 hours and 30 minutes. The rates of feed corresponded to theoretical top liquor concentrations of 12.92 parts of potassium sulfate and 9.38 parts of magnesium sulfate per 100 parts of water. The maximum concentrations reached were 11.22 and 8.87, respectively, corresponding to 86.8 per cent extraction of potassium sulfate and 94.6 per cent extraction of magnesium sulfate. The filter-cake residue was subjected to an additional treatment similar to that used in run 10. The extraction of the potassium sulfate from solids carried by the overflow solution and remaining in the filter-cake residue was made with a solution containing 360 pounds of water and 1.92
VOL. 27, NO. 9
parts of sodium chloride per 100 parts of water. Calculation of the amount of potassium sulfate to be extracted from these solids was obscured by the fact that the overflow solids reacted with the overflow solution to form syngenite as the mixture cooled. However, the amount of potassium sulfate remaining in the solids for the selected period was determined from the amount of calcium sulfate collected and from the average ratio of calcium sulfate to potassium sulfate in the solid samples taken from the overflow mixture a t the regular intervals. Chemical analyses showed that the overflow solids were higher in magnesium sulfate, and petrographic examination indicated a higher proportion of secondary polyhalite than was found in the filter-cake residue. A very close approximation was made by calculating the amount of potassium sulfate to be extracted and then calculating the percentage of the total that the filter cake represented. The equivalent percentage of the permissible leach water was then used to extract the filter-cake residue alone. Extraction a t 65" to 70" C. (149' to 133' F.) in a 100gallon steam-jacketed agitator yielded a solution containing 5.68 parts of potassium sulfate, 1.07 parts of magnesium sulfate, and 2.11 parts of sodium chloride per 100 parts of water after 2 hours of agitation. The unextracted residue contained 4.43 per cent of potassium sulfate and 2.88 per cent of magnesium sulfate, corresponding to an over-all removal of 90.7 and 91.7 per cent, respectively, of these constituents. Higher recoveries might have resulted if more solution feed had been used, probably without much change in the overflow concentrations. The preliminary procedure adopted in run 12 was planned to obviate the difficulties encountered in previous tests in bringing the system to the steady state. The column and accessory equipment initially were filled with a solution containing 6.0 parts of potassium sulfate, 2.0 parts of magnesium sulfate, and 2.0 parts of sodium chloride per 100 parts of 0 N
I
t o I-
c!z
3
01
0 \
c"U
3
z-
0 a 5
+ z
W 0 z 0
0
FIGURE6 . CONCENTRATIONS AT VARIOUS POINTSIN EXTRACTION COLUMN
THE
water. Calcined polyhalite was charged during the early part of the run a t a slightly higher rate than was used during the test period in order to build up the concentration in the upper part of the column. Minus 10-mesh calcined polyhalite of the composition shown in Table I was added a t an average rate of 77.5 pounds per hour. The feed solution, preheated t o 65-70' C. and containing 5.18 parts of potassium sulfate, 1.25 parts of magnesium sulfate, and 2.24 parts of sodium chloride per 100 parts of water was added a t an average rate of 233.1 pounds per hour, equivalent to 214.6 pounds per hour of water. These rates of feed would have yielded theoretical maximum concentrations of 10.25 parts
SEPTEMBER, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
of potassium sulfate and 7.35 parts of magnesiuni sulfate per 100 parts of water. The concentrations attained were 10.64 and 7.38, respectively, values which are actually over 100 per cent of the theoretical concentrations. This condition would result in a slight deficiency of soluble compounds in the extracted residue and subsequently in the solution to be produced therefrom t o replace the synthetic feed solution used. Figure 5 shows the composition of the liquors a t the top and bottom of the column during the test period of run 12. During the final test period of 2 hours and 33 minutes the overflow solution and the extracted residue were segregated. The filter-cake residue was collected, dried a t 105' C. (221' F.), crushed, and screened to pass the minus 10-mesh screen. Calculations were made to determine the proper proportions of water to use, and 118.5 pounds of the dried cake were extracted for 2 hours a t 65" to 70" C. in an agitator with 500 pounds of water containing 1.96 parts of sodium chloride per 100 parts of water. The concentration of the solution obtained was 4.53 parts of potassium sulfate, 0.99 parts of magnesium sulfate, and 2.13 parts of sodium chloride per 100 parts of water. The residue contained 1.51 per cent of potassium sulfate and 1.48 per cent of magnesium sulfate, equivalent to the extraction of 96.9 and 95.8 per cent, respectively, of these constituents. calculations based on the weight and composition of the extract liquors yielded check values of 96.6 and 97.4, respectively. Run 13 followed a procedure similar to that in 11 and 12, but calcined, minus 10-mesh polyhalite of slightly higher sodium chloride content, as indicated in Table I, was charged to the column during the test period a t the rate of 80.75 pounds per hour. Preheated feed solution containing 5.82 parts of potassium sulfate, 1.04 parts of magnesium sulfate, and 1.44 parts of sodium chloride per 100 parts of water was charged a t the rate of 217.5 pounds per hour, equivalent to 200.9 pounds per hour of water. Hand feeding during the final stages of the test insured the desired rates which, as shown in Table 11, corresponded to 10.98 parts of potassium sulfate per 100 parts of water in the overflow liquor. The average concentration during the test period of 2 hours and 21 minutes was 10.55 parts of potassium sulfate and 7.10 parts of magnesium sulfate per 100 parts of water, corresponding to extraction of 96.1 per cent of the former and 87.7 per cent of the latter. The low value for the magnesium sulfate was due only to the low concentration of this constituent in the feed solution, as the filter-press residue showed effective removal of magnesium sulfate. The filter-cake residue obtained during the final 2 hours and 21 minutes of run 13 was collected, dried a t 220" F., and crushed to minus 10 mesh. During the extraction of 119.5 poundsof thisresidueat65"to 7OoC.(149'to 158"F.)with445 pounds of water containing 1.33 parts of sodium chloride per 100 parts of water, samples of solutions were taken after 1 hour and samples of solution and solid residue after 2 and 3 hours. In 2 hours the solution contained 5.44 parts of potassium sulfate, 0.88 part of magnesium sulfate, and 1.47 parts of sodium chloride per 100 parts of water, while the residue still contained 3.76 per cent of potassium sulfate and 2.82 per cent of magnesium sulfate, corresponding to removal of 92.2 per cent of the former and 92.0 per cent of the latter from the original calcined polyhalite. During the final hour of extraction the temperature was raised t o 92" C. (198°F.). The solution concentrations increased to 6.22 parts of potassium sulfate, 0.96 parts of magnesium sulfate, and 1.51 parts of sodium chloride per 100 parts of water, indicating a slight loss of water due to evaporation. The residue, however, contained only 1.33 per cent of potassium sulfate and 1.53 per cent of magnesium sulfate, equivalent to removal of 97.8 and 95.9 per cent, respectively, of these constituents from the calcined polyhalite. Since the feed solution actually con-
1093
tained a lower concentration of potassium sulfate than the final extract solution obtained a t 92" C., not all of the latter could have been used. If a feed solution of higher concentration had been employed, higher top concentrations with slightly lower recoveries presumably would have been obtained.
Mechanism of Reactions during Calcination and Extraction -411 of the chemical and physical changes taking place during the calcination of polyhalite are not yet fully understood. Essentially, the calcination results in the dehydration of the hydrated mineral according t o the equation: heat XzS04~MgS04 2CaSO4.2H20-[KzS04,MgS04,2CaS04] 2H20
+
(I)
Possibly calcination produces a dehydrated mixture of intimately associated potassium, magnesium, and calcium sulfates, which under certain conditions may interact to form either new compounds or solid solutions. The degree of calcination to which the calcium sulfate particularly is subjected could easily affect its rate of solution and hence the extraction to form secondary compounds. As previously stated, to eliminate complications during extraction arising from improper calcination treatment conditions of calcination which had proved satisfactory in previous batch extraction tests were used in the experiments reported here. The chemical reactions taking place during the countercurrent extraction have been followed by chemical and petrographic examination, and are more clearly understood. The major reactions involved are:
+ +
[KzSO4,MgSO4,2CaS0~1HzO = K2S04 MgSO4 2CaSOa +H20 (2) solution solution solid soln. CaSOc HzO= K2S04CaS04.H~0 (3) solution solution syngenite K2S04 MgS04 2CaSO4 2H20 f solution solution solution KzS04~MgS04~2CaS04~2H~0 (4) polyhalite 5KzSO4CaSO4~HZ0H20 K2SO4.5CaS04,H203syngenite pentasalt 4KzSO4 Hz0 ( 5 ) KzS04CaS04.H20 &SO4 CaS04:zH20t HzO ( 6 ) syngenite solution solid
+ +
+ +
+
+ +
+
-
+
+
As the calcined polyhalite comes in contact with the solution in the column, the calcine dissolves rapidly, and the bulk of the potassium sulfate and magnesium sulfate and a limited amount of the calcium sulfate pass into solution. Any sodium chloride present in the calcined product also dissolves readily, while the other major impurity, anhydrite, remains practically inert. The form in which the calcium sulfate originally associated in the polyhalite molecule exists during the extraction a t temperatures approaching 100' C. (212" F.) is uncertain, but probably this constituent is in process of transformation to anhydrite, the stable phase. The optical properties do not correspond to gypsum, hemihydrate, or anhydrite, but are intermediate between hemihydrate and anhydrite. The amount of calcium sulfate in solution near the top of the column represents supersaturation with respect to all possible solid phases a t 100' C. The secondary reactions taking place in the top of the column a t temperatures near 100' C. were the same in the experiments made a t both the lower and higher rates of feed. Keedles of syngenite formed, according to Equation 3, on the calcined material being extracted and then descended in the column, most of them attached to the dissolving particles. Liberal quantities of secondary polyhalite also appeared from
1094
INDUSTRIAL AND ENGINEERING CHEMISTRY
solution, according to Equation 4, partly at the expense of the syngenite which is unstable under the conditions near the top of the column. In the earlier tests in which water, either alone or containing only sodium chloride, was used as the extracting solution, the syngenite and secondary polyhalite formed a t the more concentrated portions of the system but decomposed again as they descended into regions of lower concentrations. Practically all of these secondary compounds eventually were dissolved, and gypsum appeared in the lower portion of the system, particularly in the filter press in tests when the latter was a t 25" to 30" C. (77" t o 86"F.). In the later runs, 8 to 13, a slightly different condition existed. Temperatures were elevated at the base of the column and throughout the accessory settling tanks and filter press. The solution concentrations in the lower portion of the system were also greatly increased, as shown in Figure 6. This procedure made the whole column behave similarly to the top or hot portion in the early tests. Syngenite and polyhalite appeared in even greater quantities near the top. Fortunately, the long flat needles of syngenite formed on the Darticles of calcine being extracted and were thereby carried down the column. The calcium sulfate no longer appeared as gypsum, which would be unstable a t these higher temperatures and concentrations. Petrographic examination of the solids suspended in the overflow solution in all the tests indicated a predominance of secondary polyhalite over syngenite. Calculations based on the chemical analysis of these solids have confirmed this observation. The reverse was true of the extracted solids leaving the bottom of the column in the final runs, 8 to 13. The amount of syngenite in the central portion of the column was definitely greater in these tests than in the earlier ones, and it was present in approximately equivalent amount to the secondary polyhalite a t the bottom. In both the overflow solids and in the extracted residue leaving the column there was apparently a large amount of free calcium sulfate other than the anhydrite originally present in the calcined material. Identification of the form of this calcium sulfate has proved difficult. The most satisfactory deduction is that it is in a state of transition t o stable anhydrite. By far the most important observation relative to the secondary solid compounds in the extracted residue was the failure of pentasalt t o appear in more than negligible amounts. I n the absence of sodium chloride and magnesium sulfate, pentasalt is definitely a stable phase under the conditions obtaining in the bottom of the system. However, Hill (6) found it difficult to produce pentasalt a t 60" to 80" C. (140" to 176" F.) in the system potassium sulfate-calcium sulfatewater and the authors have had similar difficulty a t 100" C. (212" F.) in the system potassium sulfate-magnesium sulfatecalcium sulfate-water, provided the solid calcium sulfate present in the system is anhydrite. Gypsum, however, when added to solutions of potassium sulfate under these conditions, produces pentasalt readily. Higher percentage extractions and recoveries are assured if pentasalt is not encountered during the extraction process, since this double salt stubbornly yields its potassium sulfate a t 25' to 100" C. (77" to 212" F.) only on prolonged treatment and only to a very limited concentration.
Summary and Conclusions The results of the countercurrent experiments show that the treatment of calcined polyhalite to yield satisfactory concentrations and reasonably high percentage recoveries is feasible on a semi-plant scale. Minus 20-mesh and minus 30-mesh calcined polyhalite yield their soluble constituents more readily than minus 10-
VOL. 27, NO. 9
mesh, but the latter size may be treated satisfactorily. A larger proportion of fines is produced in suspension with the finer sizes. Care is essential t o insure complete and thorough dispersion of the calcined polyhalite as it is charged into the extraction system. Positive agitation of the reaction mixture near the point of addition of the calcined polyhalite is necessary t o prevent caking and lumping. The use of an efficient agitator, either operating separately or installed within a tray thickener, should prove satisfactory. Temperatures a t or as close to 100' C. (212' F.) as practical should be maintained in the upper portion of the extraction system where nearly complete extraction of the potassium and magnesium sulfate is obtained. The final portion of the extraction system may be operated a t temperatures ranging from 100' C. to room temperature. Petrographic and chemical data have proved that during the extraction procedure some of the potassium and magnesium sulfates dissolved from the calcine are removed from solution as syngenite and polyhalite. Continued treatment of these double salts with solutions of decreasing concentration ultimately results in their decomDosition so that high extractions and recoveries are possible. The most favorable conditions for extraction are those which allow the maximum permissible quantity of sodium chloride to be in solution a t all stages of the process. Indications are that sodium chloride retards the formation of secondary polyhalite and increases the concentrations of potassium sulfate a t the syngenite-gypsum equilibrium point. It is probable that the presence of magnesium sulfate does not materially affect this tendency. To obtain the full beneficial effectof the sodium chloride, polyhalite as low in sodium chloride as possible should be used and the salt should be introduced a t the bottom of the system in the feed water. The tests have proved that concentrations of 10 parts of potassium sulfate per 100 parts of water may be obtained with an extraction of 98 t o 99 per cent of the potassium sulfate, concentrations of 10.5 to 10.6 parts with an extraction of from 92 to 97 per cent, and concentrations of 11.0 to 11.2 parts with 90 t o 91 per cent extraction. While the present data do not prove conclusively that 98 to 99 per cent extraction can be attained at a concentration of 11 parts of potassium sulfate per 100 parts of water, it is considered a possibility under the most favorable conditions. The failure of pentasalt, R2S04.5CaS04.Hz0, to appear in more than negligible amounts a t any stage of the countercurrent treatment has improved materially the possibilities of extremely high recoveries approaching 100 per cent. The application of equipment now available on the market for countercurrent extraction processes, possibly augmented with a small amount of specially designed equipment, should offer a simple solution t o the continuous countercurrent extraction of calcined polyhalite for the removal of potassium and magnesium sulfates.
Acknowledgment The cooperation of A. Berk, Loyal Clarke, A. Gabriel, and
K. Fragen during the extraction tests is gratefuly acknowledged. Special credit is due to J. M. Davidson for the design of the extraction column, and to C. M. Davis and F. Spille for the mechanical construction and operation of the equipment.
Literature Cited (1) Clarke, L., Davidson, 3. >I., and Storch, H. H., Bur. Mines, Rept. Investigations 3061 (1931). ( 2 ) Conley, J. E., and Fraas, F., I b i d . , 3210 (1933). (3) Conley, J. E., and Fraas, F., IXD. ENG.CHEM.,25, 1002-9 (1933).
SEPTEMBER, 1935
INDUSTRIAL AKD ENGISEERING CHEMISTRY
(4) Conley, J. E., Fraas, F., and Davidson, J. RI., Bur. Mines, Rept. Investigations 3167 (1932). (5) Davidson, J. hf., and Fraas, F., I b i d . , 3237 (1934). ( 6 ) Hill, A. E., J . A m . Chem. Soc., 56, 1071-5 (1934). (7) Storch, H. H., 1x0. ESG.C'HEX., 22, 934-41 (1930). (8) Storch, H. H., and Clarke, L., Bur. Mines, R e p t . Investigations 3002 (1930). (9) Storch, H. H., and Fragen, N., I b i d . , 3116 (1931).
Protective Films on Ferrous Allov s FLORENCE FENWICK
United States Steel Corporation,
1095
(10) Storch, H. H., and Frapen, N., 1x0. EXG.CHEM.,23, 991-5 (1931). (11) Wroth, J. S., Bur. RIines, Bull. 316, 15-20 (1930). RECEIVED March 14, 1936. Presented before the meeting of the American Institute of Chemical Engineers, Viilmington, Del., 3Iay 13 t o 16, 1935. Published b y permission of the Director, U. S.Bureau o f Mines. (Xot subject t o copyright.)
Influence of Chloride Ion
upon Electromotive Behavior
Kearny, N. J.
T
HAT the ordinary structural metals can be safeguarded from corrosive influence only by the interposition of a barrier between metal and environment is now generally accepted. This barrier may be a film developed more or less spontaneously, as in the case of the stainless steels or of aluminum; or it may be an applied coating, whether of another metal or a paint or resin or of some kind of cement, which is itself resistant to corrosion in the environment in which the metal is to be used. It would be desirable to have a rapid, reliable method of comparison of t,he effectiveness of such films in order to replace, or at least to supplement, the only way now certain of yielding significant results-namely, to expose the various specimens for a period of months or years to the particular environment they will encounter in service. The method to be described offers possibilities in this direction; a t least it distinguishes, in a way which parallels general experience, between the films formed spontaneously on fairly sharply defined groups of ferrous alloys such as ordinary carbon steel, copper-bearing steel, and the stainless steels (the latter fall into subdivisions according to the heal, treatment to which the samples had been subjected). It is published in the hope that it, or its obvious modifications, will be tried by others 80 that there may be a wider range of evidence as to the usefulness of a measurement of the permeability of a film or coating to ions in solution as a possible measure of relative resistance to corrosion of a metal with such a film or coating in a similar environment. The method is essentially an electrometric titration wherein a chloride solution is slowly added to the solution in which is immersed the metal; this metal, with its film or coating, acts as one electrode, and the other is a suitable standard electrode appropriately connected. The sudden unmistakable change in the observed potential marks an abrupt change, not in the specific ion concentration of the solution in which the electrode is immersed, but in the character of the surface of the electrode itself The concentration of chloride ion in the solution when the sudden change (which may amount to as much
as 1 volt) occurs is reasonably reproducible for similar samples similarly prepared, changes with the mode of preparation of the a m , and differs systematically from one type of alloy (hence, of film) to another. These essentially reproducible differences seem to show that this general method offers a means of characterizing the film or coating on the metal, with respect to its relative permeability to chloride ions and possibly, therefore, to other ions, since chloride ion is one of the most active, as \Tell as the most prevalent, of all ions in corrosion processes.
Details of Method The metal specimen, usually in the form of strip 2 em. wide or of a cylindrical rod, is suspended in a 400-cc. beaker containing 200 cc. of a solution, usually a passivating solution of potassium dichromate, which is well stirred by a motor-driven stirrer. Electrical connection with the second electrode is made through a bridge tube, oontaining this same solution and so arranged as to prevent any transfer of chloride ions from the vessel; the vessel usually contains a saturated solution of potassium chloride into which dips one arm of the bridge tube and the arm of a silver-silver chloride, 0.1 M potassium chloride half-cell of the form described by Willard and Fenwick ( 3 ) . The potential of this cell, as chloride is slowly added from a buret to the beaker containing the variable electrode, is observed in tjhe usual way and recorded directly; a negative sign indicates that the variable electrode is more noble than the silversilver chloride electrode, and vice versa, in accordance with the convention as to sign adopted by Lewis ( 2 ) and his eo-workers (to refer the readings to the standard hydrogen electrode, add -0.287 volt to the stated values). The preparation of the metal specimens raises many questions which can be answered only by detailed investigation of how differences in mode of preparation affect the result of the electrode titration. The mode of preparation adopted vas either t o secure a fresh surface by fracture of a rod or t o scour the strip specimen with fine emery paper, followed by a wash with ether; the specimen was then coated with paraffin to obviate a metal-liquid-air interface when it, was immersed, leaving bare an area usually about 1 sq. cm. The result mas not affected by the method of securing a fresh surface used or by the size of the bare area, so long as it was not in contact with the air; if any part of the bare area was in contact with air, the results were quite irregular.
