How to Make Salt by Solar Evaporation

How to Make Salt by Solar Evaporation

Salt Making Processes

The production of salt in the United States divides itself at the outset into two distinct classes:

  1. The mining of rock salt and its purification and separation into marketable sizes, and
  2. the production of salt by evaporation from brines, bitterns, and other solutions.

The processes employed at the present time in the manufacture of salt by evaporation may be outlined as follows:

  1. Solar evaporation.
  2. Direct-heat evaporation (a) in open kettles; (b) in open pans.
  3. Steam evaporation, (a) in jacketed kettles, (b) in grainers.
  4. Vacuum-pan evaporation.

In this paper only the solar-evaporation processes practised in different parts of the United States will be described.

Solar Evaporation

In the Eastern States the solar-evaporation process is not generally employed for climatic reasons. In the Western States, particularly in California and Utah, the great bulk of the salt produced originates in the solar-evaporation process. Of the Eastern States producing salt, this process, so far as the writer is aware, is employed in New York State only.

The manufacture of salt by solar evaporation began in the vicinity of Syracuse, Onondaga county, in 1789. A natural brine 68° to 80° salometer, with 17 to 20 per cent, sodium chloride, was and is still used. The brine is stored in glacial gravels and was evidently formed by the circulation of ground waters, through adjacent beds of rock salt.

But a small part of the evaporated salt manufactured in New York is now made by the solar, process. Its manufacture is limited to Syracuse and vicinity, where it has survived from the early days of the industry. Though it has now lost some of its former importance, up to the year 1880, the time when the beds of rock salt in the western part of New York State began to be utilized, Onondaga county supplied the whole output of the State in rock salt. A large part of the total now returned for the county represents the salt in brine consumed in the manufacture of soda. The solar salt produced is mainly coarse salt and is used for practically the same purposes as rock salt. It is marketed through the Onondaga Coarse Salt Association.

The wells are located on lands once included within the Onondaga Indian reservation; and until recently the State supplied the brine to the individual plants, exacting a small tax on the product to cover costs of pumping and supervision. The lands and wells were sold in 1908 to private companies, namely, the Onondaga Pipe Line Co. and the Mutual Pipe Line Co. of Syracuse, the transfer being made for a nominal sum. This transfer definitely terminated the historic connection of the State with the salt business.

Solar salt is made from a natural brine—the only instance of its use in New York. The process, of course, is carried on only during the spring, summer, and fall months, usually from the middle of March to the middle of November, depending on the weather.

Method of Salt Manufacturing

Former Salt Making Method.—The manufacture of solar salt in New York was formerly carried on in shallow wooden vats or “covers,” provided with light movable roofs running on rollers. During the evaporation season the roofs were removed to one side, but replaced during rainy weather, and at the end of the salt-making season. There were ordinarily three sets of vats or “rooms,” known respectively as “deep rooms,” “lime rooms,” and “salt rooms.”

The deep rooms served for the reception of the brine from the well or pump house. When first pumped the brine was usually perfectly clear, but with the escape of gases (CO2), ferrous carbonate separated, which oxidized to the hydrated ferric oxides, and settled out as a yellow mud, leaving the clear brine.

The lime rooms were constructed nearer the ground than the deep rooms, the clear brine, being drawn from the higher deep rooms into the lower or lime rooms. Evaporation continued in the latter until the brine became saturated and salt crystals began to separate. During this process, particularly when the brine was near the point of complete saturation, gypsum began to separate. The brine, now fully saturated and known as “pickle,” went to the lowest set of rooms, known as the salt rooms. Here the salt and the remainder of the gypsum separated, sufficient pickle being added from the lime room from time to time to keep the salt crystals well covered. The term “lime room” in the above descriptions is somewhat misleading as no lime was used in the process. When sufficient salt collected in the salt rooms it was harvested. This took place generally three times during a season.

Present Salt Making Method.—The process described above was the original method of solar evaporation. Great improvements have been made in it, one of the greatest being the replacement of the deep rooms and lime rooms, by “aprons, ” whereby the evaporating surface and the yield of salt per cover (288 sq. ft.), have been greatly increased. The aprons are large troughs from 15 to 20 ft. wide, about 3 in. deep, and varying greatly in length. They are generally erected over the deep rooms serving as roofs, thus making store rooms for the saturated brines of the deep rooms. They are built on piles or posts, and are so constructed that brine or rain water falling on them flows toward two sets of holes provided with wooden plugs, the grade of flowage being 1 in. per 100 ft. One set of holes communicates with the deep rooms. The brine run on to the aprons often becomes saturated in one good drying day. It is then discharged into the deep room below and its place taken by a fresh portion of brine. When rain is expected the plugs over the cisterns, or deep rooms, are drawn and the brine which flows into them is thus saved from dilution. The plugs are then replaced and the other set removed, thus allowing the rainwater to flow to waste. When clear weather returns, the partially concentrated brine is returned to the aprons by pumps.

“The great advantage of this improvement lies in this—that the brine is concentrated and purified while being transported to the covers; the great length and width of the aprons, the shallowness of the layer of brine and its complete exposure to the sun, air, and wind greatly facilitate purification and evaporation while permitting the use of the entire number of covers for salt making, instead of having as formerly one-third of the total-number taken up for lime rooms and deep rooms.” In this way the production of solar salt during a given season has been greatly increased per cover. As the capital invested in covers is considerable, the economy of the process is readily seen. (Figs. 1 and 2.)

The Water of Great Salt Lake

Salt is obtained from the water of Great Salt Lake. According to different authorities who have analyzed the lake water, the chief differ-

yard of salina salt co.

loading-wagons-salina-coarse-salt-co

harvesting-salt-inland-crystal-salt-co

ences in the results are in the degree, but not the character, of the salinity. It is to be expected that the former would change with greater or less dilution, as the waters of the lake rise or fall, from local or other causes. Although the degree of salinity of the lake is variable, ranging approximately from four to seven times that of ocean water, the composition of the saline matter contained in it is much like that found in ocean water. The following analyses show the composition of the salts, contained in the water:

analyses-of-salts-in-water-from-great-salt-lake

Compared with the analyses of sea water, the sodium is higher, and the lime and magnesia, especially the former, are distinctly lower. Carbonates are nearly entirely absent, but are present to an appreciable extent in ocean water. There are other variations in composition between lake and ocean water, but in general they are of the same type. Additional analyses appear in a monograph by Gilbert. Gilbert states (p. 253) that the quantity of sodium chloride contained in the waters of the lake is about 400,000,000 tons and that the amount of sodium sulphate is about 30,000,000 tons. In the same volume Gilbert also considers the different surveys made of the lake, the depth of water, the oscillations of its surface, the changes and the causes of change in its area/the sources of its saline matter, together with the rate and period of its accumulation.

The Salt Industry

Salt has been made by the solar-evaporation process for many years along the shores of Great Salt Lake, the industry dating in fact from the early settlement of the region. A small amount of salt is reported as having been made recently at Promontory Point, Box Elder county; Withee, Weber county; Syracuse, Davis county; and at Garfield, Salt Lake county. Fifteen miles west of Salt Lake City, near Saltair, is the plant of the Inland Crystal Salt Co. The operations at the latter place are by far the most elaborate ever attempted along the lake shore. The process of obtaining salt near Saltair is described below.

The brine is pumped from the lake into a flume in which it is carried about 3 miles to the ponds, where concentration and crystallization are effected. The ponds are designated from the processes taking place in them as: Settling ponds, stock or evaporating ponds, and crystallizing or harvesting ponds. The ponds are separated from each other by clay embankments held in place by boards set on edge. These clay levees are 2 ft. wide and 22 in. high. Pumping begins in April and continues until the first of September, excepting during times of storm. Evaporation goes on approximately at the same rate as pumping, which is about 5,000 gal. per minute, averaging 16 hr. per day from the middle of June to the middle of September—the season of greatest dryness and hence of maximum evaporation. Pumping is regulated so as to maintain the level of the brine in the ponds.

The brine from the lake goes first to the settling ponds, where it is allowed to remain five to six days. Here all suspended-matter is deposited. The settling ponds are estimated to be about 75 acres in extent. From the settling ponds it goes to the stock ponds, which cover an area of about 1 sq. mile. Here it remains until concentration reaches complete saturation and salt is ready to deposit, tire length of time-depending upon the weather. The brine then goes to the harvesting ponds. During the summer of 1912, when the writer visited the region, 30 days elapsed between the beginning of pumping operations and the time the brine reached the harvesting ponds. The brine is conducted from one pond to the other by gravity, the flow being controlled by means of small gates.

In the harvesting or crystallizing ponds, as the name suggests, evaporation is carried to the point where all salt separates out. To insure a clean product, an underlayer, or salt floor, is allowed to crystallize out each year, unless it is left over from the operations of the preceding year. This underlayer serves as a clean floor upon which the subsequent crystallization takes place. When it remains from the previous year, as happens when the salt crop is larger than the market demands, the salt in it becomes more or less dirty. It is then used as stock salt though it is a first-class product in every respect. The salt which is marketed comes from the upper layer, the plane of demarcation between it and the floor being known as the “split.” The average crop is about 3 in. of salt. The minimum crop is about 2 in.— the maximum usually 4 in., but a crop of nearly 5 in. has been obtained. Three inches is considered a good crop.

During the course of the salt-making season the bittern formed in the harvesting ponds is drawn off twice, once during the middle of the season, and again at its end. In this way the salt is freed from the bulk of magnesium salts and sodium sulphate. Care must be exercised to draw off the bittern toward the season’s end, before cold weather begins, to insure against precipitation of sodium sulphate, which crystallizes out when the water reaches a temperature of 20° F.

When the bittern is drawn off for the last time and the salt is ready to be lifted, it is loosened by means of ordinary plows drawn by horses. (Fig. 3.) It is then stacked by means of scrapers, wheelbarrows, or hand cars run on tracks into large piles alongside the railroad which traverses the salt field. The salt is conveyed from the stacks to the cars, which are transported in turn by means of an oil-burning locomotive to the company’s mill, where the salt is further refined A portion of the yield may be sold without further treatment, for the different purposes for which rock salt is used.

Salt Making in California

Localities.—More than 97 per cent, of the output of salt in California originates from the evaporation of seawater along the coast. The main

 

view-of-salt-pondsproduction of this solar salt comes from along the east and west shores of San Fancisco bay, in Alameda county, and San Mateo county, respectively. In Alameda county the centers of production are near Alvarado, Mt. Eden, Russell, and Newark; in San Mateo county, solar salt is produced a short distance south of San Mateo and near Redwood City. Other places where salt is produced by solar evaporation are near Long Beach, Los Angeles county, and on San Diego bay, San Diego county. In all these places the operations are conducted along similar lines, but they differ considerably from each other in the details worked out by individual operators in charge. There is also considerable diversity in nomenclature connected with the processes in their various stages. In the following account it is aimed to give general descriptions of processes with certain details observed at representative plants in the different salt-making localities.

Salt-Making Season

The concentration of the water in San Francisco bay is variable, depending on the season of the year and other factors.

salt crystallized in a crystallizing pond

The dilution of the bay water is caused in part by the fresh water from the San Joaquin and Sacramento rivers and is increased after winters of heavy snow which is not melted away completely until the summer is well advanced. Brine is taken into the salt ponds from about the middle of May to the middle of October, but the strongest brines are obtained after July first, as the dry season lengthens and the inflow of fresh water from the rivers and streams is appreciably reduced. Generally the salt-making season is over by September 15, as the rainless season lasts usually from May to September. A season as long as 210 days without rain has been known, however, along San Francisco bay.

The season for 1911 may be taken as a typical one. During the summer of that year the evaporation and rainfall as observed at one plant were as follows:

rainfall-and-evaporation-in-inches

The water from the bay or ocean is not taken into the works continuously, but usually on two to six days each month, depending on the season when the tides are at their highest. Water is usually taken in at

gravity method of running brine from one pond to another oliver salt co.

the period of the new moon. Sometimes the high tides come twice a month, but once a month is the more usual. The periods of highest tides are ascertained from the tide book issued by the U. S. Coast and Geodetic Survey. During the periods of high tide, water is not running into a salt works continuously, but only from 1½ to 3 hr. during a given high tide. These are statements based on general practice, but it is understood that at some plants, where a slough runs through the intake pond, salt water may be taken into a given system at any time or every day. Thus, at Long Beach the sea water runs into a ditch at every high tide, that is, twice each day. Salt water may therefore be pumped continuously day or night from this ditch into the plant.

In southern California, naturally, the salt-making season is longer than it is near San Francisco. At San Diego it is estimated to be about seven months, namely, from May to November. On San Diego bay the salt water also may be taken in continuously during the salt-making season. This is accomplished by draining the tide ponds, whereby water may be taken in on every tide above a given height.

From 1908 to 1912, the mean average date when crystallization of salt began along San Francisco bay, was as follows: 1908, May 1; 1909, May 13; 1910, May 3; 1911, May 25; 1912, April 18. The reason for the early date in 1912 was on account of the very few late rains during that year. Records are closed, that is, evaporation readings are stopped, each November 1, when the days are so short and the weather so cold and uncertain that there is practically no evaporation.

Strength of Brine Used

The average density of seawater is 1.027, corresponding to a salinity of 3.72 per cent, regarded as pure salt. The three samples of bay water collected as they ran into plants on the shores of San Francisco bay and on San Diego bay gave the results indicated in the following table:

density-and-salinity-of-water-used-in-salt-making-process-on-the-california-coast

Thus it will be observed that the normal salt water entering the salt plants along San Francisco bay is less concentrated than ordinary sea water, while the opposite is true of the bay water near San Diego. These samples of bay water were collected: No. 23, Oct. 5; No. 28, Oct. 8; No. 33, Oct. 17; i.e., on days well advanced during the dry, or salt-making season. One of the reasons for the difference in density between the waters from San Francisco bay, and San Diego bay, is that farther south the warmer and dryer season lasts much longer. The evaporation therefore is more intense, hence at a given date during the dry season the southern waters would naturally be expected to show a higher degree of salinity. Other conditions which tend to reduce the salinity of the San Francisco bay water are the streams of fresh water which flow into it.

Ponds

The salt-making operations are conducted in what are known as ponds, designated from the operations conducted in them. Thus there are: (1) The storage, the intake, receiving or tide ponds into which the salt water is received from the bay, (2) the concentrating ponds, and (3) the crystallizing ponds. By some operators all the ponds between the tide ponds and the crystallizing ponds are known as secondary ponds. Technically they may be known as pickling ponds. The term “lime ponds” is sometimes applied to those in which the bulk of the gypsum crystallizes out.

The salt or ocean water comes into the works generally through a slough, passing thence into the intake, receiving, storage, or tide pond, as the first pond is variously called. This pond is provided with large flood gates with automatic adjustments to open them to allow the water to run in, but which close when the tide begins to ebb. The salt water remains in the intake pond a variable length of time, in the meanwhile evaporating. It is pumped out before the next salt water is taken in. It then goes forward through the different ponds becoming more concentrated, until it finally reaches the crystallizing ponds. It is run into these, latter when it reaches a strength of about 25° Be, or when salt begins to; form, usually to a depth of about 6 in. At some plants, when it reaches a strength of 29° Be, the bittern with some salt still in it runs over into, other ponds. Here it evaporates until a concentration of 32° Be, is reached, when all the salt is out of it (see table below). The mother liquor remaining is then run into the bay and goes to waste at many plants, but it seems that some use should be made of the magnesia and potash salts contained in it. At some plants a part of it at least is saved and: utilized as described later in this paper.

specific-gravity-of-bitterns-from-crystallizing-ponds

Area is one of the dominant factors in the industry. There are plants where the total acreage of ponds is as low as 500 acres, but others are reported to have more than 2,000. The ratio of the acreage of crystallizing ponds to that of the whole system was found at two important plants to be as 1 to 10, but the older the plant, the more acreage it has for crystallizing purposes, the reason being that the ground becomes thoroughly salted down with continued usage.

The ponds have mud floors. The salt in places is harvested to within 2 in. of the mud floors and removed to the main stock piles. The remaining 2 in. is shovelled up into smaller piles and this grade is used as ice cream and stock salt.

The ponds are separated from each other by low embankments known as dikes. The width and height of these dikes vary greatly. They may be as much as 3 ft. above the marsh level and 1 or 2 ft. above brine level. Some are very narrow, others, especially some of the outside dikes, are wide enough and sufficiently well constructed to serve as roadways over which teams and autos may be driven. The low narrow dikes or levees are not considered by all the salt makers as good practice as they require constant attention and repair owing to wash by the waves. Where the

slough-and-wind-mills

levees are too high the evaporating action of the wind is interfered with, but at San Diego there is not much wind. The dikes between the ponds were formerly flanked with sod, but this practice is now being done away with.

Methods of Pumping Brine

Pumping along San Francisco bay is done in part by Archimedes screws propelled by wind mills; also by means of paddle wheels. The average plant may have as many as 40 wind mills. Some plants use gasoline engines as an auxiliary power, especially in the fall when the trade winds die down. In August, the trade winds slacken or blow intermittently and therefore cannot be depended upon. The windmills are set solid to the northwest from which the trade winds conic and from their manner of construction they cannot turn around.

The wind mills are generally placed so as to pump from the outer or intake pond into the next pond. From the latter it is sought to have the brine move along by gravity, the flow from one pond to the next being controlled by small gates. Wind mills or other means may, however, be used for lifting in other parts of the system, when gravity is found to be inexpedient.

Harvesting Salt

The crop of salt of the preceding season is usually sold by July 1 of the following season. Enough salt is then lifted from time to time to supply the current demand until September 1, when harvesting is pushed as vigorously as possible. At many plants the first salt is lifted

method of lifting salt in harvesting pond

about June 1, and the operation continued until about the first of January or February of the following year if the crop is a big one and labor is rather scarce. Ordinarily, however, lifting is finished by December. The salt is lifted by hand into small tram cars each holding usually about 1 ton. The salt thus lifted is pinkish in hue and the mother liquors are likewise pinkish. A small locomotive propelled by gasoline or electric power picks up eight cars, more or less, and hauls them to the mill; or they may be hauled to the salt piles or stacks and dumped. There may be one huge stack near the mill itself from which salt is used according to the demand. The latter method perhaps may be the more economical if practicable, for it reduces the surface exposed to solution from rains and also the surface which has to be broken up in the subsequent utilization of the salt. This latter is an important consideration, for the surface hardens so firmly that it is broken up only with the greatest difficulty; and in some cases the cost of breaking up a particularly old crust on a small stack costs more than the salt in the stack is worth. Sometimes old and small stacks may be discarded completely. In some places after standing in the stock piles the salt hardens so firmly that crosscut saws have to be used. These saws cut it into 4-ft. lengths which are pried apart and then crushed in the subsequent milling operations.

general view of crystallizing pond and the laying of track

When the salt harvested is not taken directly to the mills, but to the stacks, it may be manipulated in various ways as outlined below. After reaching the works it may be dumped through a hopper on to a screw conveyor, passing thence to a bucket elevator, which lifts it to the main salt stack. Not every plant can use this method, because the ground has to be very firm to accommodate the enormous tonnage of one main stack, a condition not easily obtained in regions of marshy land. Sinking of the ground may occur and an appreciable part of the product be lost. An instance is known where 1 ft. of filling sank each year for a period of 20 years.

Instead of shovelling the salt into small cars to be hauled to the stacks, dumped and there elevated, a much simpler and comparatively inexpensive process may be used, in which the salt is shovelled by laborers, a line of whom extends the width of a pond, on to an endless belt 20 in. wide, running on rollers, which conveys the salt to one side of the pond. The power is furnished by a gasoline engine, but an electric motor might serve as well, or better. As the salt leaves the belt it is washed by a spray of salt water from the bay. It is next dragged up a perforated copper screen, during the ascent of which it is again washed with bay water. The endless drag chain is provided with forks to break up the larger lumps. The salt is then elevated to the stacks by a pan elevator run so that the open mouths of the pans are reversed to enable the excess of wash water to drain. By a washing device the main belt which conveys the salt from the pond is kept perfectly clean.

view-of-salt-pond

The method just described has many labor-saving advantages. As soon as a given area of salt has been removed from the crystallizing ponds the entire layout is moved forward on tracks provided for the purpose, and the salt pile may thus stretch out the length of the pond. The washing of the salt likewise is performed at a time when the adhering bittern is most readily and completely removed. The salt from the outside stock piles is brought up to the mill as needed in 5-ton cars and there subjected to the various operations such as crushing, etc., to fit it for the finer uses, or it is re-dissolved and prepared for the vacuum pans.

At Long Beach the salt is shovelled from the stock pile into small cars which are trammed by a rope tram to a hopper through which it passes, being ground in the process. It is next elevated by a bucket conveyor and dumped into a bin, which in this case is an ordinary freight car fitted up for the purpose. When the bin is filled the car is motored to the mill.

At San Diego the salt has to be picked as it lies in the crystallizing pond before it can be lifted with shovels. The greater firmness of the crystallized salt as compared with that along San Francisco bay, which makes picking necessary, is possibly due to the greater rapidity of crystallization. It is customary to begin harvesting just as soon as the bittern is run off the crystallizing ponds. This is due to the fact that the salt is easier to pick, shovel, and wash at this stage than at any other.

It is difficult to give the amount of salt which can be made during a given season, as the quantity is variable from year to year, depending upon the weather and other factors. Perhaps a thickness of 5 or 6 in. of salt per season, would be a fair approximation for the region along San

salt-pile-and-plant

Francisco bay, but in the southern part of the State owing to the longer and dryer season, it would be very much larger. At San Diego, for example, an average harvest is 6 in. of salt, but two such harvests are possible per season, in this region. Hence it is possible to make 12 in. of salt per year in this locality.

Waste Bittern

The bittern from the crystallizing ponds has never been saved at many of the plants in the past and at most of them it is even now running to waste. Since the importance of potash salts has come to be realized some of the operators have in mind the utilization of the potash content of the bitterns, but at most of them nothing definite with regard to such use has yet been evolved. At one plant a small part of the bittern is saved, refined and used for medicinal purposes; at another it has been used in the manufacture of “wood stone” and in the manufacture of magnesium oxychloride cement. The latter substance has been found very resistant to the passage of electricity, and hence has been applied in the manufacture of switch boards. It is, understood that the bittern has been used by the Santa Fe Railroad to lay the dust along its road bed, but experience has proved it to be too costly for this purpose, as it has to be renewed too frequently.

Milling

From the nature of the processes employed in the manufacture of solar salt, and more particularly from the differences between them and those employed in other parts of the United States, the first stages of the milling operations are different in California from the practice elsewhere. When the salt is lifted from the crystallizing pond, it is contaminated with considerable adhering pickle or bittern and dirt of various kinds. If it is not to be used at once it may undergo a preliminary washing after which it is conveyed to the stacks as already indicated under harvesting. If it is to be used immediately it is transported to the mill and there washed free from adhering impurities. This is accomplished in various ways. It may be dumped into a hopper and a current of hot brine from the pickling pond poured over it. This brine from the pickling pond represents bay water concentrated to complete saturation and from which no salt has been crystallized. For a concentrated solution it contains the least amount of mother-liquor salts. Its use results in the solution of a minimum amount of salt during the washing process. From the hopper the salt is lifted in an elevator, draining in the meanwhile. It is then passed through rolls where it is crushed into “half-ground salt” or “three-quarter ground salt.” It then goes forward to other vats, where there is an artificial brine made from salt and fresh water and in which consequently there are no mother-liquor salts. The salt on removal from these vats is stacked in heaps to drain. The coarse salt is then sacked for one branch of the trade. If it is to be subjected to further refining processes the salt goes from the vats to a centrifugal machine where adhering water is removed. It is then conveyed to dryers where it is thoroughly dried by steam heat. The dryers are elongated cylindrical affairs containing steam coils. They are provided with fans to pump warm air through them, which greatly assists in the drying operation. From the dryers the salt goes directly to the rolls where it is crushed. From the rolls it goes to the sifters where it is graded according to fineness, depending on the purposes for which it is to be used. It is then ready to be sacked for the trade. At different plants the operations differ greatly in details to suit individual tastes and those of the customers.

At some of the plants the solar salt in the stock piles is re-dissolved and the finer grades are made by the vacuum-pan process. When the vacuum-pan process is employed the manipulation of the salt at the mill may be quite different from that outlined above.

salt making by solar evaporation