UTAH GEOWGICAL AND MINERAL SURVEY REPORT OF INVESTIGATION REPORT OF Ii\Vl·. STICA rIO;\; \;0. 1- 1 IDENTIFICATION AND DISCUSSION OF ;\~ I. \ \( SOL \R POND HARVEST SALIS, GREAT SALT L. ' kL . UTA 11 By J. WALLACE GWYNN published by UTAH GEOWGICAL AND MINERAL SURVEY a divhdon of DEPARTMENT OF NATURAL RESOURCES STATE OF UTAH REPORT OF INVESTIGATION NO.1 71 IDENTIFICATION AND DISCUSSION OF A\L\X SOLAR POND HARVEST SALTS, GREAT SALT L· kE. UTAll By J. WALLACE GWYNN DECEMBER 1981 REPORT OF INVESTIGATION NO. 171 IDENTIFICATION AND DISCUSSION OF AMAX SOLAR POND HARVEST SALTS, GREAT SALT LAKE, UTAH By J. Wallace Gwynn* INTRODUCTION During the late 1960's, after conducting pilot operations to select the best process for extracting magnesium from the waters of the Great Salt Lake, N. L. Industries began to build a magnesium extraction plant on the western shores of the lake. It is located at Rowley, Utah and was purchased by AMAX in 1980. The first step in the extraction process is the use of solar energy to evaporate and thus concentrate the lake brines; a 25,000 acre pond system was constructed in the Stansbury Basin, located west of Stansbury Island and south of Badger Island (Figure 1). This large pond system, though actually more com­ plex than shown in Figure 1, is essentially broken down into three ponds with­ in which the lake brine is concentrated in stages t under controlled cond i­ tions. As the concentration process takes place and the brines become saturated, sal ts are precipi tated. (If satur'ation is reached in pond one t and in pond two, sodium chloride is the principle salt to be precipitated during the warm summer months.) In the third pond, however, where the brine becomes highly concentrated before being pumped to deep storage ponds at the Rowley plant, S;,~,ts in addition to sodium chloride are precipitated. The most common of tt'~ese salts are kainite (MgS0 4 .KCl.3H20), epsomite (MgS0 4 .7H20), shoe­ nite (MgS04.K2S04.6H20) and possibly carnallite (MgCI 2 .KCl.6H20). Because there has been insufficient fresh water available every fall and winter to leach and/or flush the summer's accumulation of salts from the ponds, the sal ts have been allowed to accumulate as a thick blanket on the pond floor s , for about the 1 ast twelve year s. Some of the sal ts from pond three 'center' were harvested during the winter and spring of 1977 and 1981; only the salt deposited during a given summer is harvested, however. Leaching was conducted during 1975 and 1976 t and flushing was carried out in pond three (center) during 1977. A blue dye was introduced into the three segments of pond three in 1973-74 to enhance the evaporation rate of the brine. During the early part of May 1981, prior to flooding the three segments of pond three wi th pond two brine, 24 core samples of the sal t floor of pond three, from each segment, were taken to determine the chemical and mineral­ ogical makeup (Figure 2). The Utah Geological and Mineral Survey was provided a split of the salt from each core (composited into six inch intervals from top to bottom), for mineralogical identification, along with a copy of the chemical anal ysis of each composi te sample. A total of 135 samples \-lere provided. This report of investigation is based on an X-ray Diffraction (XRD) anal ysis of the sal t samples and on the chemical anal yses (Append ix 1), that were provided to UGMS by AMAX. * The author expresses his appreciation to Wal ter McCormick, Stan Johnson and others of AMAX for their help, and to Paul A. Sturm, of Utah Geological and Mineral Survey, for his technical assistance in the preparation of this report. CARRIN6rON GR£A T SA L T LA K£ I o ROWLEY '( STANSBURY \ T. 2 N. T. I N. IS LAN D ) i 3 1\ i / j \ ~ I ---+-------\ ~-~------.,~. ~-~~\...J I ~ I. I TEMPlE SPR. I ?i ~ 0 l MILES I I I I f'o.. U) oj I-eo co I f W.M.A. I I::;::d ~ ti 0::: ----=-..~--- ci tr-----__ L ! II ~-- __ J I Figure I - Map showing location of solar evaporation ponds. 2 PUMP STATION inflow from pond.-l.-- o g 3WI [U EAST (0 4~ ~~ o j'N3 3C4 3C~ 3E4 o 0~ 0) 3'N5 3C6 ~3~5 3E6 o (0 (0 (3) 3W7 3C8 CENTER 3C7 3£8 EXPLANATiON POND 3 o Sy-nbol indicates ho!a G in pond 3 west_ 3W6 Thickness of salt is 4 feet. Figure 2 - Schernatic diGgroln of pond i:)(ce sho'Ning core hole locations, g e n era I d ire c t ion 0 f b r i n e f low) and t hie k n e sS 0 f s a I t flo or. SUMMARY OF PONDING OPERATIONS As the evaporation season starts, pond one is filled wi th water from the Great Salt Lake by allowing it to flow in when the level of the lake is greater than that of the ponds or by pumping when the lake level is below. As the concentration and composi tion of the lake brines change, due to solar evaporation, the concentrating brine is moved by pumps from pond one to pond two and from pond two to pond three. These moves are made as the brine achieve certain concentrations or chemical makeup as shown in Table 1. Table 1 • Percent of each constituent. GSL Effluent Effluent Effluent Pond No. 3 Brine Pond No. 1 Pond No. 2 to Holding Pond Mg 0.4 2.0 4.8 7.5 K 0.3 1.5 3.6 0.8 Na 4.0 7.0 2.6 0.5 Cl 7.0 14.0 16.0 20.3 S04 1 .0 5.0 5.3 4.41 H2O 81 .3 70.5 67.7 66.5 Source: Toomey, 1980, p. 219. As the chemical composi tion of the brine is followed through the ponding system, it can be seen that the concentration and ratio of the ions change as evaporation proceeds. A detailed explanation of the chemical changes that occur in evaporating Great Salt Lake brines is discussed by Butts (1980). As the brine leaves pond two and enters pond three, other minerals, in addition to sodium chloride, have started to precipitate from solution. These minerals may include epsomite, kainite, and picromerite as seen on the 25°C phase diagram (Figure 3). As evaporation continues in pond three, kainite is the principle mineral to precipitate, however, epsomite precipitates readily, especially during the night as the temperature of the brine falls somewhat. Near the end of the evaporation season the concentration of the brine approaches about 7.5 percent magnesium (Table 1). The brine does not usually become concentrated enough to precipitate kieserite and carnallite; carnallite can precipitate under the proper temperature conditions, however. Bischofite saturation is not achieved under normal solar ponding conditions. As the end of the concentration season approaches during the latter part of Augu~t into early September, the "ripe" brine in pond three is drained and pumped to a large, segmented, deep storage pond at the Rowley plant site. If the solar ponds are to be harvested, leached or flushed, then these operations are started shortly after the brine has been drained and pumped from the ponds. If these operations are not to take place, the new deposi ts of salt remain within the ponds, adding to the accumulation of salt from previous years. 4 -.,r-~- Corna II ite Hexahydrite -¥-,J-~+-T--- Ka i nita Epsomi te --..J:;;=.=-J'\-"---r!rA:---~ Bloedite ---f-~,J- 4--.,t-:.I.o:---;.....::.~~-.,~-- Leo n i te Pi cromer i ta ---I---'--..,t--~~~...... (Schoenite) Thenordite -...,o-~- Sylvite EXPLANATION I. G. S. L. brine end Pond No. I eff I uent 2. Pond No. 2 effluen t 3.' Pond No. 3 eff luant - to holding pond Note: Brine compositions from Toomey, 1980, p. 219 Figure 3 5 TEMPERATURE DEPENDENT CHEMICAL CHANGES If the newl y deposi ted sal ts of pond three are not fl ushed, leached or harvested, they become exposed to the extremely low temperatures that are experienced at the pond ing si te during the winter months. If the sal ts are still in contact with their mother liquor, that is t the same brine from which they were precipitated, the salts will likely change in mineralogy. For example, kaini te, (see the 2SoC phase diagram or Figure 3) which is a stable salt phase at 2SoC or at typical summertime termperatures, is no longer stable at winter temperatures near OOC (see the OOC phase diagram, Figure 4). At OOC, schoenite (picromerite) is the stable phase, along with epsomite; carnallite mayor may not be found. Additional epsomite will likely precipitate from the residual brine as the temperatures drop. If the composi tion and concentration of the residual brine in pond three is right, mirabilite (Na2S04.10H20) will also precipitate from solution. This situation is especially prevalent in ponds one and two during the winter months. In any case, if the mother liquor of the pond salts is present during the winter months, a new or changed mineral assemblage can be expected to develop. If the newly deposited pond salts are sufficiently well drained or windrowed such that the sal ts are relatively free of their mother liquor, the c~ange of kainite into schoenite (picromerite) is not likely to occur. Nor will there be the additional precipitation of salts such as epsomite and mirabilite in these salts.