SALT-MAKING ON THE GREAT SALT LAKE THOMAS B. Bn~oaToN.UNIVERSITY OF UTAH,SALT LAKE CITY,UTAH
From the water of the Great Salt Lake common salt of various desired grades of purity is produced by simple steps of solar maporation, drying, screening, and grinding. The magnesium chloride and sodium sulfate present in the lake water are left in the bitterns or remmed by the drying-grinding process.
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Most of us take common salt for granted and do not stop to consider the magnitude of the industry which produces it. When, however, we regard salt as a preservative, food, and refrigerant, as well as the source of practically all sodium and chlorine compounds, we can begin to account for the more than eight million tons produced per year of late in the United States. The world production totals about thirty million tons, this amount being distributed among practically all the nations of the earth. Salt has been used and has exercised its influence throughout the entire history of the human race. Some of the world's oldest trade routes were established for traffic in it, and our forests and hills still carry trails made by animals going to and from so-called salt "licks." Salt is used to a greater or less degree by all people and whether it be used as a condiment or supplied chiefly in foods, we are told that it must make up from fifteen to eighteen pounds of the diet per year. I t is so important that where it is not widely scattered laws regulate its preparation and sale. A salt monopoly is extremely valuable as can be realized by studies of recent events in India and China. In the United States salt is widely distributed, with Michigan (31 per cent.) as the principal producer, followed by New York (26 per cent.), Ohio (17 per cent.), Kansas (10 per cent.), Louisiana (6 per cent.), California (5 per cent.), and Utah (1 per cent.). The remaining four per cent. is produced by the other states where the industry is not important. A study of government statistics for 1929 will show that 30 per cent. of the salt produced is classed as manufactured (evaporated), 45 per cent. is sold in brine and 25 per cent. is classed as rock salt. Evaporated salt is sold for approximately $7.00 per ton, rock salt $3.35 and salt in brine for $0.60 per ton, so that the total value of the 8,543,560 tons was $27,334,695, representing an average of $3.20 per ton. Of the 2,546,390 tons of salt classed as evaporated, 401,310 tons were produced by solar evaporation and of this 84,940 tons came from Utah. The manner of production of this Utah solar salt is to be described in this article. The Great Salt Lake, situated in about the middle of Northern Utah, is roughly seventy-five miles long by thirty miles wide and is the chief remnant of the very much larger Lake Bonneville which once covered a considerable area in the Great Basin. Lake Bonneville drained to the north into the 407
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Snake and thence into the Columbia River and Pacific Ocean. As climatic changes occurred the original lake receded, its steps well shown by terraces on the mountains around Salt Lake City, and now only the Great Salt Lake is left to remind us of the once mighty body of fresh water. The lake is relatively shallow, its average depth being 15 feet, and so the salt content varies considerably from year to year, increasing during dry years and decreasing following periods of greater rainfall. Thus the salinity, in terms of per cent., varies from fifteen to around twenty-five. Increased use of irrigating water from feeder streams and recent relatively dry seasons have caused the lake to be now near its lowest recorded level, with a consequent high salinity. The sodium chloride content of the Great Salt Lake has been estimated by Gilbert at 400,000,000 tons with sodium sulfate 30,000,000 tons. The sodium sulfate content of the water is so high that normally it becomes saturated at about O°F. and during continued spells of cold weather a sodium sulfate mush, often an inch or more thick, collects on the surface of the water. This Glauber's salt is carried onto the shore by the waves and left behind and it is often possible during cold winters to scrape together tons of it. Spring rains wash it back into the lake. The bottom of the lake, at least near the shore, is covered with a layer of oolitic limestone sand from a few inches to a foot or so thick. Under this limestone sand occurs a layer of Glauber's salt, sometimes solid, sometimes stratified with layers of sand. This Glauber's salt may be from an inch or two to two or three feet thick and is so hard that piling has to be steamed into place. After steaming in, the piling is in place to stay siuce the melted salt resolidifies. At present no use is being made of the sodium sulfate present in and under the water. The water of the Great Salt Lake is quite similar to sea water in composition though it is about seven times as concentrated. The calcium and magnesium contents are lower than in sea water so that, on the whole, the brine is a very desirable one for salt-making. Two plants are now active in the manufacture of salt from lake brines but siuce processes are similar in both, only the one of the Royal Crystal Salt Co. at Saltair, Utah, fifteen miles west of Salt Lake City, will be described. Brine a t the rate of 5000 gallons per minute for twenty-four hours per day during the evaporating season is pumped from the lake into a flume which carries it to a series of ponds for settling, conceutratiou, andcrystallization. The brine goes first to the settling ponds where it remains for five or six days to allow suspended matter to settle. It passes next into four concentrating ponds, each covering 250 acres. Here the brine is held for about three weeks and is allowed to evaporate until it is saturated and ready to deposit sodium chloride. The saturated brine flows by gravity to
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twenty crystallizing ponds, each of ten acres area. Here the water evaporates and salt crystals form but the bitterns run to waste at such a rate that the saturation point for sodium sulfate and magnesium salts is not reached. Clay dikes or embankments, twenty-two inches wide, held in place by hoards set on edge, separate the ponds. Pumping begins in April and continues until the middle of Septemher, with greatest activity between the middle of June and Septemher when the evaporation rate is highest. Flow through the ponds is continuous and is regulated so that only pure sodium chloride crystallizes out. Care must he exercised to drain off the bitterns before cold weather comes on in order to avoid precipitation of Glauher's salt which separates out a little above freezing temperatures. A permanent salt floor covers the bottom of each pond. This floor is from twelve t o fifteen inches thick and is maintained by allowing a little fresh brine to go to complete dryness early each summer. I t was formed by precipitating salt for several years without removing any of i t until after the desired salt thickness had been obtained. The salt crop each year is separated from this floor by the so-called "split" and can he harvested without disturbing the permanent floor. I n the late fall, after a pond has been thoroughly drained, ordinary plows, drawn by light tractors, are used t o separate the new salt crop from the permanent floor. With tractor-drawn scrapers and a conveyor, it is then stacked in large piles beside the railroad sidings along the ponds.
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which it is fed into automatic sacking and packaging equipment which put the salt into its market forms. A variety of grades of salt is produced by a process of grinding and screening. These may be enumerated as kiln-dried stock salt, used for cattle feed and curing hides; extra coarse, used for ice-cream making, for regenerating zeolite water softeners, and for stock feed; special stock salt, used for sheep feed and metallurgical purposes; hay salt, for curing hay and as stock feed; No. 2 meat salt, for curing meat, for curing small hides and for pickle making; No. 1 meat salt, for curing meats and pickles; table salt, for cheese making, baking, and general household purposes; butter salt, used by butter makers, bakeries, and for household purposes. A portion of the table salt is mixed with 0.2 per cent. by weight of potassium iodide to make "iodized" package table salt, used by many because of an iodine deficiency in the water and food supplies of much of the intermountain west. A considerable tonnage of salt is ground fine and pressed into blocks weighing fifty pounds each. These blocks are used as "cattle licks" and may be made of salt or may contain added sulfur. In some case iodides are added to the block salt and so a product is available to snit the fancies of each cattle or sheep raiser.
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I'RESSING BLOCKSALTTOR CATTLE LICKS
As the salt comes from the stock piles i t is relatively clear and shiny in appearance, with its chief impurity Glauber's salt. As it comes from the drier it has become somewhat dull in appearance but most of the sodium sulfate has been dehydrated and blown off. A strong air blast passes through the drier and a considerable part of the dust carried out is sodium sulfate. Along with this sodium sulfate are a few crystals of gypsum though this salt is relatively small in amount. When the dried salt is screened, the coarser crystals are found to he more pure than the finer ones and as the coarse crystals are progressively ground and screened, the coarse material on the screen approaches nearer and nearer 100 per cent. NaCI. The table, butter, and cheese salts are made from the last portions of the coarse crystals and so are of very high purity. A series of samples taken on the same day and analyzed by the author showed the compositions listed on page 414. Since gypsum is occasionally seen in the drier dust and sodium sulfate is near saturation in the mother liquor, it seemed justifiable to calculate all calcium as CaSOn and the rest of the SOI as NasS04. Doing this, a sample scraped direct from the drained pond but not weathered a t all showed:
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Water insoluble (mostly clay) CaSO, Na2S04 MgCI. NaCl (by difference)
Samples taken in the refinery, analyzed in the same way showed: 1. 2. 3. 4. 5. 6. 7. 8. 9.
Salt from stock pile Extra coarse kiln-dried Coarse kiln-dried Unscreened stock salt Hay salt No. 2 salt No. 1 salt 40 mesh-table salt Butter salt
99.3% NaCl 99.5 99.5 99.5 99.4 99.5 99.7 99.7 99.8
So, starting with a brine, nearly saturated with sodium chloride and relatively high in sulfate and magnesium ions, salt of a variety of grades is produced by solar evaporation, one heating, and grinding and screening, and is supplied to a considerable portion of the intermountain west, the temtory supplied being limited by freight charges.
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So while Utah produces but one per cent. of the nation's salt, the product is of high quality and the industry is an interesting example of the utilization of one of our natural resources. The process is of added interest because of its simplicity and directness in producing its finished products.
Bibliography CLARK, "Data of Geochemistry," 17. S Geol. Survey Bull. 770. PHALEN,"Salt Resources of the United States," U.S. Geol. Survcy Bull. 669. PHALEN, "Technology of Salt-Making in the United States," U. S. Bureau of Mines BuUetin 146. COONS,"Mineral Resources of the United States," U. S . Bureau of Miner, 1929, pp. 14740. STRATTON,"Utah's Salt Industry," Professional Engineer, 14, 13 (1929). AND DICE,"Increasing Purity of Common Salt," Ind. Eng. Chem., 23, 336 RR~GRTON (1931).
