AUGUST, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
The materials of equipment construction include plain steel, cast iron, chrome-nickel steels, nonferrous alloys, rubber, and wood. Although the theoretical solution contains sodium and potassium chlorides, the liquors actually contain magnesium sulfate and chloride, calcium sulfate, and some potassium sulfate. These add to the corrosive nature of the hot solutions. Electrolysis plays a large part in the affairs of a potash refinery, and the symptoms would be amusing were they not annoying. A steel valve interior will receive a bright copper coating, but when one looks for it a week later the coating has gone, perhaps to appear a hundred feet away on an agitator propeller. Natural gas, from eastern New Mexico fields, is burned under the boilers and about the plant. The chemical and designing engineer must give thought to the effect of altitude. It must be considered in many placesfor example, in psychrometric studies, calorimetry, boiling points, compressors, and exhauster design. The altitude of 3000 feet has appreciable effects in these respects. Weird problems were encountered by us when operations started. One week, several years ago, we were plagued with moths. They appeared literally by millions, and we even
W. A. GALE American Potash 8c Chemical Corporation, Trona, Calif.
The Searles Lake system of salts and brine is regarded as having nine components, three degrees of freedom, and six solid phases. Equilibrium diagrams are presented to show the behavior of major components upon isothermal evaporation and to illustrate the Trona process. At low temperatures the precipitation of the double salt glaserite [K3Na(S0J2] prevents a separation of potassium and sodium salts, whereas at high temperatures only sodium salts are precipitated. The hot concentrated liquor is removed and cooled rapidly to crystallize potassium chloride ; at the same time supersaturation is maintained with respect to borax, which is crystallized before returning the mother liquor to the cycle.
867
considered operating the refinery in the dark. A thoughtful foreman conceived the idea of putting out lights in the refinery and letting them burn in the power plant. When the moths were attracted there in droves, he concentrated them further by bright lights a t the boiler doors, then putting out all lights he opened the fire chamber doors and the moths poured in to their destruction. The traditions of potash have resulted in many popular misbeliefs regarding the product. The early and substantial trade of our country in refined wood ashes has left the impression in many quarters that all potash is caustic or corrosive in character. There is widespread belief that potash is largely used in munitions manufacture. Of course this conception started with the use of potassium nitrate in black powder. The idea was given further impetus by the publicity attendant upon the scarcity of potash during the late war. Our agricultural lands felt the pinch of scarcity, and the cotton plant is a greedy consumer of potash. Cotton is used to make nitrocellulose. So potash does help in the defense of our country, but the farmer stands between the potash industry and the present-day munitions factory. RECEIVED May 23, 1938.
HE phase rule is an expression of a simple fundamental relation existing between the number of components of any system in equilibrium, the number of phases and the number of conditions which we must arbitrarily fix in order to define the entire system. These conditions we call the “degrees of freedom” or variance of the system. (“Components” are taken to mean the least number of chemical substances necessary to express the composition of every phase present in the system; “phases” are the physically distinct, mechanically separable substances, each of homogeneous composition, which are present in a system; “conditions” are defined as the temperature, pressure, and concentration of the components.) The phase rule in its simplest form states that the sum of the number of phases P , plus F, the number of degrees of freedom, is always greater by 2 than C, the number of components making up the system, or
T
P+F=C+P
To many this would seem an unimportant and meaningless generalization in comparison with the more obvious laws of nature. However, during the formation of the earth’s crust, the separation of the various rock minerals from magma, and later the deposition of secondary minerals from water solutions, have all been governed by the principles underlying the phase rule. We find this rule a valuable guide in the study of any process of separating salts by fractional crystallization. In such a case it is first necessary to investigate the equilibrium conditions and phase relations of the system in question. This may take months or even years of patient laboratory work to obtain the desired solubility data and to identify the solid phases involved. Then we represent this information on
IXDUSTRIAL AYD ENGINEERING CHEhlISTRY
868
suitable charts and diagrams, from which we are able to predict what changes will take place in the system on bringing about any given variation in the conditions. For instance, the diagrams show what salts will crystallize out of a solution of any given concentration on evaporation or on cooling to any given extent, and we are also able to calculate
CO,
K2
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__
results and the midine principles of modern . physical chemis. try had no place. In the case of the Stassfurt deposits. which have resulted from the evaporation of sea water, the' main system is composed of water and the sulfates and chlorides of sodium, potassiLm, and magnesium; for purposes of study thesemay he regarded as a five-component system. In tlie case of Searles Lake, however, the ma,in system is composed of ~a-K-H~SO~-CO~-B,oscI-F--N,o~ When properly grouped together, these constituents form an eight-component system made up oi eight seven-component systems, twenty-four six-component systems, etc., making over two hundred systems of two components or more. These do,not include systems containing excess acid or excess alkali but only those made up of water and salts ranging in alkalinity from neutral solutions to solutions of the normal alkali salts (the carbonates and metaborates). THE composit,ion of Gearles Lake brine as shown in Table I is a considerable distance from the end point of crystallization of this eight-component system. The end point of crystallization of a system is that point in a systeminequilihrium, beyond which no further change occurs in the composition of the liquid phase upon continud isothermal evaporation. Before that point is reached in tlu's case, however, we have a phosphate compound separating from tile mother liquor. Therefore, in respect to any process of evaporation of the brine, we are forced to consider the lake as being at least a nine-component system. At presetit there are three degrees of freedoni-temperature, pressure, and the concentration of the unsaturated component, phosphate.
1. PoRriox or SYSTEM Na -K-SO~-C03-CI-€120 . FIQW SATURATED NaCl 20" C., Sxrowi~tiMOLECULAR PROPORTiONS Kz, co,, AND SO* THE
WITH OF
AT
PEESENT IN THE I d Q U i D
or to determine graphically the quautities which would separate. From such diagrams we can often design a combination of changes of conditions such as will produce a favorable division or rearrangement of the components between the different phnaes. Since by definition, phases may be separated by mechanical means, a theoretical process of operations may be devised to give the desired results. It then remains to study and 60 solve, if possible, the mechanical difficulties involved.
THE application of the phase rule to mnlticomponeiit systems is in reality a combination of physical chemistry and descriptive geometry, since the graphical representations of complex systems usually involve three-dimensional or space model diagrams. The purpose of this article is to present only a brief outline of equilibrium conditions in a few of the more important systems involved in the Trona process and to offer some idea of the magnitude of one of the many problems encountered in the commercial development of Searles Lake brine. The clnssical application of the phase rule to complex salt mixtures was made by van't Hoff (8)and his eo-workers in regard to the Stassfurt deposits of Germany. This work was carried out principally between 1895 and 1910, commencing some twenty years after the phase rule was first enunciated by Gihbs (8). This investigation of van't Hoff consisted of a thorougb study of the deposition of salts from sea water upon evaporation. It is hard to say how far such researches could have been carried by chemists unacquainted with the phase rule. They would have bad no guide to the apparently chaotic results obtained on evaporating mixed salt solutious. The meager reavlts obtained before the problem was taken up hy van% Hoff show that little progress could have been made by the older methods of experimentation in which the
FltinRE
PIIOTO%IICRO~~RAPII O P ARTIFICIALHhr-rxsi~r: CRYSTAL PRODUCED IN TAE LABORATOHY
2.
Magnzfiostion, about
loox.
In order to fulfill tlie requirements of the phase rule under these conditions, eight phases would have to be present: P+F--C+2 8+3 = 9 1 2
Therefore, in addition to the liquid and vapor phases, we should have six solid phases in equilibrium with the brine. The following six solid phases have been identified in the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
869
main crystal body of Searles Lake (water-soluble salts only) : halite, jSaC1; tincal (borax), Nat2B407.10HzO; trona, NazC 0 3 ~ N a H C 0 3 . 2 H ~glaserite, 0; KaT\la(S04)2;hanksite, 9NazSO4.2Na2CO3.KC1;and sulfohalite, 2Na2S04.NaC1.NaF. In addition to the above solid phases, other minerala, such as thenardite, burkeite as reported by Foshag ( I ) , etc., are to be found in isolated strata in the surrounding mud, where other sets of equilibrium conditions exist. -.
TABLEI. TYPICAL ANALYSIS OF SEARLES LAKEBRINE Total K Calcd. t o Remaining C1 Calcd. t o Total C02 Calcd. t o Total 908 Calcd. t o Total Bios Calcd. t o Total PaOs Calcd. t o Total F Calcd. t o Other minor constituents Total salts (approx.) Water Specific gravity
PH
KCI NaCl NaaCOs NazSO4 NaaBaO? NaaPOa NaF
4.70% 16.35 4.70
6.96
1.50 0.16 0.01 0.30
34.68
'
65.32 1.30
9.48
In the industrial development of Searles Lake brine for the recovery of potash and other products, however, it has been found that other lesser constituents concentrate and finally enter into the equilibrium reactions. Therefore, we have t o take into consideration a t least four more componentsnamely, h o d , S, Br, and Li-making thirteen components in all. The reader can figure out just how many simpler systems would be involved. The thorough investigation of such a highly complex system a t various temperatures would mean a lifetime of patient work by several investigators. However, by the study of many of the simpler systems and of small portions of the more complex ones, a fair understanding has been obtained of equilibrium conditions over the range of temperatures and concentrations encountered in the study of the deposit and in the Trona plant. Most of this information was published by Teeple (4). L E T us consider the five-component system comprising water and the sulfates, carbonates, and chlorides of sodium and potassium a t 20" C., using only that portion of the system which is saturated with sodium chloride. The composition of the lake brine, in so far as this system is concerned, is represented by point B of Figure 1. It is in
02 04
FIGURE 3. PORTION OF THE SYSTEM Na2-NaH-COsKnPOrB~04-C1-Hz0 SATURATED WITH NaCl AT 20 C. O
EVAPORATOR UNITNo. 3
equilibrium with hanksite, glaserite, and sodium chloride in addition to borax, sulfohalite, and trona; the latter phases will not be considered for the present. The triple salt hanksite (Figure 2) is rather remarkable since it has an extremely slow rate of crystallization. Even in the presence of the solid phase, supersaturated solutions of hanksite require many weeks and even months to reach equilibrium. Therefore, in regard to any rapid process of evaporation of the lake brine after removal from the deposit, we may neglect the existence of hanksite and assume the metastable point P (Figure 1) as being a monovariant point of stable equilibrium saturated with sodium chloride, sodium sulfate, glaserite, and burkeite (NazC03.2Na2S04), Then, upon isothermal evaporation of the brine, sodium chloride and g1aserit.e separate between B and G. At G the solution reaches saturation with burkeite. These salts continue to separate simultaneously or, in the Ianguage of the phase ruIe, paragenetically, until point D is reached, when the solution becomes saturated with NazCO3.7HLO. The path of crystallization then leaves the burkeite field and continues to point E, the end point of crystallization, saturated with sodium chloride, glaserite, Na2C03.7H20,and potassium chloride.
IiY ORDER to show what happens in regard to the borax content of Searles Lake brine upon isothermal evaporation at 20" C., it is necessary to consider a somewhat more complex system. Figure 3 shows that portion of the system composed of water and the normal sodium and acid sodium carbonates, borates, and phosphates, saturated with sodium chloride at 20" C. This diagram from Teeple (5) represents a triangular prism, in the three upper corners of which are the three normal alkali salts (NaQCOa, Na2Bz04, and Naa-
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PO4); in the lower corners are the three acid salts (XaHCOa, Ns2B407, and Na2HP04). Each prism face represents a reciprocal salt pair, so that the whole diagram constitutes a triple reciprocal salt pair, saturated with a seventh salt.
countercurrent fashion, so that the final concentration is done a t the highest temperature. During this evaporation, sodium chloride and burkeite are crystallized and removed continuously, while the com-
so4
K2 FIGURE 4.
PORTION OF THE SYSTEM Na-K-Sod-C03-C1-820 SATURATED WITH NaCl AT 100 C. O
The composition of Searles Lake brine, in so far as this system is concerned, is represented b y point B. It is saturated with both borax and the sesquicarbonate, trona. It is very close to saturation with the bicarbonate and contains a small s.mount of phosphate. Upon isothermal evaporation, the composition of the brine follows the path BC, while sodium chloride, borax, and trona crystallize out. At point C, the solution reaches saturation with a double salt Na2B2042Na3P04.36H20,and along the line CD this salt separates together with sodium chloride, borax, and trona. At point D another new double salt appears, NazB20~.2NaC1.4H20. The end point of crystallization is finally reached a t point E where sodium chloride, trona, NazC03.7H20, the borophosphate, and the borochloride are present. At the end point of crystallization of the nine-component lake system, we would therefore have the following solid phases present a t 20" C. : NaCI, KC1, K,Na(SO&, Na2C03.7H20, Na&03.NaHC03.2H20, NazB20~2Na3P04~36Hz0, NazB204.2NaC1.4Hz0, and a fluoride phase (composition undetermined). Thus, no complete separation of potassium salts from sodium salts is possible by isothermal evaporation a t 20" C . , which may be taken as typical of ordinary atmospheric temperatures. W E WILL now consider what happens upon evaporation of the brine a t higher temperatures-for example, 100" C. Figure 4 represents the system Na-K-SOd-CO3-Cl-HZO at this temperature. I n comparison with Figure 1, the burkeite field has expanded greatly, particularly a t the expense of the glaserite and KCl fields. Raw Searles Lake brine is represented by point B and the Trona plant mother liquor by M . A mixture of these two solutions in the proportions of about three parts of brine to one part of mother liquor is represented by point F. This mixture is fed to triple-effect evaporators, operating in
'
coj
K2
OF THE SYSTEM Na-KSOrCOa-C1-HzO FXGVRE 5. PORTION SATURATED WITH NaCl AT 35" C.
position of the liquor travels along the line FA, the path of crystallization of burkeite, until point A is reached. At this point the liquor reaches saturation with NazCOs.H20 which also precipitates with the burkeite and sodium chloride from A to C as the liquor approaches saturation with potassium chloride in the high-temperature effect. This hot concentrated liquor, free of suspended solids and containing about 19-20 per cent potassium chloride, is sent to the potash crystallizing house where it is cooled for the crystallization of potassium chloride. Figure 5 shows the same system a t 35" C. Point C, representing the composition of the hot concentrated liquor, is now near the center of the potassium chloride field, so that upon cooling to this temperature, only potassium chloride crystallizes out along the line C M . The potassium chloride PAN FEED
0 35
I
SRD EFEGT
.
.
50
.
2Nn
I
I
,
I
TEMPERATURE
FIGURE 6.
SOLUBILITY RELATIONS IN
.
EFFECT
a5
l
l
.
I
.
~
115
*C.
THE
TRONAPROCESS
AUGUST, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
871
is sepimated, m rashec seiIt to storagt’, the M is thI U S regex1 E:rated next cycle, :i so far as the potash content is concerned. Another way of illustrating what happens in this evaporation and cooling process is shown in Figure 6, the solid lines of which roughly represent the solubility polytherms of the single salts concerned. Thus as we concentrate the liquor in series from third through second to first effects, potassium chloride and borax become more soluble and the sodium tail salts less soluble. The approximate potassium chloride and borax contents of the pan feed and of the liquors in the three effects are also shown. Considerable quantities of sodium carbonate, sodium sulfate, and sodium chloride crystallize during evaporation and are removed to become raw material for the soda products plant. Upon rapid cooling of the concentrated liquor, potassium chloride only crystallizes out, the borax remaining supersaturated long enough to allow the separation of the crop of potassium chloride from the mother liquor. A crop ot borax is then caused to crystallize and is removed before returning the mother liquor to the evaporators with further quantities of raw brine.
dioxide,, due to the in.ards de(:ompsositionL of he, and this tends to shift the above reaction from left to right. However, the sodium carbonate is being precipitated in large quantities during evaporation, as will be noted from the equilibrium diagram (Figure 4). This precipitation occiirs both as burkeite and as the monohydrate. In fact, the quantity of sodium carbonate precipitated is practically equal to the tonnage of potassium chloride subsequently recovered. Therefore, by the law of mass action, the equilibrium is actually shifted in the other direction to such aq extent that from a practical standpoint it is possible in the cyclic process to recover essentially all of the boron oxide by crystallization from the liquor as the tetraborate, without having to resort to carbonation or other additional means of converting metaborate to tetraborate. I n order to make the reaction proceed from left to right, it would be necessary to remove more than 12 per cent of the total carbon dioxide of the brine in the vapor during evaporation, whereas measurements have shown that actually only about 1 to 1.5 per cent of the total carbon dioxide is lost in this way.
ANOTHER interesting point regarding the high-temperature evaporation process is the equilibrium between borates and carbonates according to the reaction:
(1) Foshag, W. F.,Am. Mineral., 20,50-6 (1936). (21 Gihbs, Willard, Trans. Conn. Acad., 1874-8. (3) Hoff, J. H. van’t, “Untersuohungen uber die Rildungsverhiiltnisse der oveanischen Salzsblagerungen, insbesondere des Stassfurter Salzlagers,” Leipsig, Akademische Verlagsgesellschaft, 1912. (4) Teeple, J. E., “Industrial Development of Searles Lake Brine,” A. C. 5. Monograph 49, New York. Chemical Catalog Co.,
3.
NaaBIO,
+ 2Na2COa + H20
2NasBzOd
+ 2NaHC08
Wells (6) showed that the instantaneous equilibrium conditions in Searles Lake brine are such that 71 per cent of the boron oxide is present in solution as metaborate and only 29 per cent as tetraborate. During evaporation in the Trona process there is a slight
Literature Cited
1929.
( 5 ) Ib?%:p. 181.
(6) Wells, Roger, IND.ENG.CHEM.,13,691 (1921). RECBIVED M a y 14, 1938.
