Magnesium Compounds from Ocean Water
H. HENRY CHESNY Marine Chemicals Company, Ltd., South San Francisco, Calif.
HE presence of magnesium salts in ocean water has incited the imagination of many inventors to devise means of recovering these salts. The difficulties involved will be appreciated when we consider that a gallon of sea water contains only 0.01 pound of magnesium, and that the particle size of the magnesium hydroxide precipitated is less than 1 micron. The question is asked, why recover magnesia from sea water containing 0.1 per cent magnesium, when a material as abundant as dolomite contains over 12 per cent, magnesite contains nearly 28 per cent, and the less common brucite over 41 per cent? Again sources of magnesium are available in well brines, such as those of Michigan and Ohio, and in the bittern waters from the solar evaporation of sea water, which contain many times the magnesia content of ocean water. The production of magnesium compounds (Epsom salt, magnesium chloride, metallic magnesium) from well brines by the Dow Chemical Company, a t Midland, Mich., ranks as an example of ingenuity and technological perfection in the utilization of natural saline deposits. Nevertheless, the cost of drilling and maintaining deer,
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tance from rail transportation, and occurrence in the unreactive beta form make its economical utilization less attractive, except its possible use for refractories. As a result, dolomite has been used as the raw material for the production of magnesium basic carbonate, oxide, and hydroxide. The process consists of calcining the rock, slaking, and recarbonating with the carbon dioxide recovered from the calcination, to form sparingly soluble calcium carbonate and to throw the magnesia into solution as bicarbonate. After removal of the calcium by settling and filtration, the solution of magnesium bicarbonate is heated to boiling to precipitate magnesium basic carbonate which is filtered and dried. Calcination of this compound produces magneqium oxide; the hydroxide is obtained by slaking and redrying the oxide. Because of the low solubility of the bicarbonate, carbonation under pressure must be employed and the volumes of water handled in the carbonation and particularly in the boiling steps of the process are large. Consequently, the cost of production of magnesia by this process is much in excess of that using sea water as the raw material. Theoretically, the production of magnesium compounds from sea water is made possible by the fact that magnesium hydroxide is practically insoluble in water (0.34 pound per 1000 gallons a t 60” F. for the alpha and 0.078 for the beta modification). consequently, the addition of an alkali to a solution of a magnesium salt, no matter how dilute, will permit a nearly theoretical yield in magnesium hydroxide. From a practical point of view, the successful production of magnesia from sea water depends upon: 1. The means to soften the sea water cheaply. 2. The preparation of a purified lime slurry of proper reactivity and ability to maintain the chemical and physical characteristics of the slurry constant at all times. 3. The economical removal of the precipitated hydroxide from the large volume of water. 4. The inexpensive purification of hydrous precipitates. 5. The development of means t o filter the viscous slimes.
The application of research has solved these problems b y devising workable chemical and crystallographic processes and by developing special equipment capable of mechanically handling the precipitates. Contrary to the general impression, the cost of pumping the sea water is not a problem, amounting to only a fraction of a cent per thousand gallons a t South San Francisco.
Present Uses of Magnesium Compounds Magnesium and its compounds play an important role in everyday life, as some of its uses will reveal: The metal, one-third lighter than aluminum, has found an ever-increasing consumption in recent years. Its future among structural materials, particularly for airplanes, automobiles, railroad cars, electrical appliances, etc., is obvious. Its present uses are manifold and range from flash-light powder to stratosphere gondolas. The manufacture and uses of Epsom salt (MgS04.7HzO) and magnesium chloride (MgC136H20) are well known. The alkaline magnesium compounds comprise magnesium oxides, basic carbonates, and hydroxides. The largest market for the oxide, as manufactured a t present by calcining magnesite, lies in refractories for the steel industry. As “dead burned” magnesite, there are consumed from 2 to 4 pounds of magnesium oxide for every ton of steel produced. For high-temperature work the use of artificial periclase, containing 93 to 94 per cent magnesium oxide, appears to increase rapidly. Magnesium basic carbonate is best known as “85 per cent magnesia’’ insulation. The uses of powdered magnesium basic carbonate are widespread and are based on its low ap-
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parent density (6 to 12 pounds per cubic foot for the light grade), on its zdsorptive and adhering properties, or on its mild neutralizing action. Large users of the compound are the salt, rubber, and ink industries. I n the case of salt, magnesium basic carbonate is used to the extent of one per cent to prevent caking. Its remarkable covering power is illustrated when we consider that in coating salt with the dry magnesium carbonate, a pound of this material covers a surface of 15,000 square feet. The addition of one per cent of magnesium basic carbonate maintains the salt free-flowing by the dual action of separating adjacent salt crystals and by absorbing aamentary water. The rubber industry uses magnesium basic carbonate principally in the production of mechanical rubber goods. Automobile tires as a rule do not contain it except in the tire beads. In the production of printing inks, magnesium basic carbonate is employed mainly to hold the vehicle and to produce a dull finish. Pharmaceutical and toilet preparations consume large quantities of magnesium basic carbonate, oxides, and hydroxides, chiefly for the compounding of tooth powders and pastes, antacid powders and tablets, ointments, face powders, and a host of other uses. Magnesium citrate is produced by the reaction of a solution of citric acid and magnesium basic carbonate. Of particular interest in the production of magnesia from sea water is its use as milk of magnesia. This well-known household remedy, useful not only for the morning after but also for the night before, is consumed domestically a t a rate of 15,000 gallons per day. This figure may seem surprisingly large, but it amounts to only a pint bvttle for each family per year. Milk of magnesia consists of a suspension of hydrous magnesium hydroxide in water, containing 7 to 8.5 per cent solids. The suspension, originally produced by the reaction of Epsom salt and caustic soda, followed by purification to remove the sodium sulfate formed, is now obtained largely by the controlled hydration of special grades of magnesium oxide, followed by passage of the suspension through colloid mills to obtain the proper dispersion, or by the simple dilution of concentrates of hydrous magnesium hydroxide (HydroMagma paste), containing 30 per cent solids. Some of the older producers still adhere to the use of Epsom salt. The problem in producing a satisfactory milk of magnesia lies in the difficulty of producing a highly hydrated compound, which, due to its surrounding layers of water of hydration, becomes nonsettling. The use of sea water as raw material meets the requirements particularly well because of the great dilution a t which the hydroxide is precipitated initially. The new development in the production of magnesia preparations is magnesium hydroxide monohydrate which for the first time is made commercially available. This material consists of a dry powder which, mixed with water, produces particularly stable suspensions. Because of its low cost, convenience of use in the production of milk of magnesia, magnesia tablets, tooth pastes, etc., and complete freedom from residual impurities such as unconverted oxide or sodium sulfate, this compound should replace the older raw materials and methods.
Production from Ocean Water The process for the recovery of magnesium salts from sea water employed by this company is based on the well-known precipitation of magnesium hydroxide using lime as precipitant, according to the reactions: MgCL hIgS04
+ = Mg(OH)2 + CaC12 + Ca(OH)2 Ca(0H)S = Mg(OH)2 + CaSOa
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(Right) INSTALLATION OF Two PRESSURE FILTERS; ON EXTREME RIGHT WASHIKG TOWER
Since the lime, because of the low solubility of calcium hydroxide, must be used in the form of a slurry, a number of precautions must be taken to prepare this slurry. Insoluble impurities must be removed prior to reaction with the magnesia, and the reactivity of the lime must be governed to insure complete conversion, while permitting the precipitation of a magnesium hydroxide possessing a high settling rate. The salinity of sea water is relatively constant in all oceans. An analysis of Pacific Ocean water a t the plant intake shows the following analysis in grams per liter: NaCl MgClz MgSOa MgBn
27.319 4.176 1.668 0 076
Cas04 Ca(HC0a)n KzSOa BzOa
Si03 Iron and alumina, R203 Sp. g r . a t 20' C.
0.008
0.022 1.024
1.268 0.178
0.869 0.029
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Chlorination and Removal of Organic Matter Calcium bicarbonate is removed by treatment with slaked lime, forming sparingly soluble calcium carbonate. This step is carried out in a system of tanks provided with baffles, down tubes, and sludge pipes. The sludge removed consists of a mixture of calcium carbonate, silicates, and organic matter. The tendency of calcium carbonate to form supersaturated solutions is overcome by a novel process of crystallization which effectively removes this troublesome impurity. In order to give the water a final clarification, it is passed through percolation filters. The filter effluent, which is brilliantly clear, is moved by pumps operated by automatic proportioning devices to the main reaction tank. Burned lime for the process, which must meet close chemical and physical specifications, is shipped to the plant by rail. Special precautions are taken to prevent air slaking in transit and in storage. The calcium oxide is screened to remove oversize, and is then automatically proportioned and discharged to a continuous slaker. Since the manner of slaking controls the physical properties of the final products to a large extent, it was found necessary to develop special apparatus for this purpose. Again complete automatic operation is used. The only manual attention required is the unloading of the cars and the storing of the lime. The reaction between the calcium hydroxide and the dissolved magnesium salts in the sea water constitutes the most vital step in the process. Since impurities present in the lime slurry cannot be removed subsequent to the precipitation of the magnesium salts, special methods and equipment had to be devised to remove these impurities prior to reaction. The application of colloid chemistry and the study of the crystallographic behavior of calcium oxide and hydroxide now make it possible to produce a lime slurry possessing the desired chemical and physical characteristics. The lime slurry is screened finally to -200 mesh. The rejects from this screening operation, as well as those from the initial dryscreening of the lime, are used in the softening process of the sea water. The screened lime slurry is pumped to the main
precipitating tank. Again, the proportioning is carried out automatically, electrically operating valves serving for this purpose. The reaction between the lime slurry and the sterilized, softened sea water must be governed in such a manner as to form a highly hydrated magnesium hydroxide. Ordinarily, a t the degree of hydration required for the subsequent use of the material, the slurry formed possesses a settling rate of 2 to 3 inches per hour a t an initial solid concentrstion of 0.02 pound of magnesium hydroxide per gallon. The problem of concentration of the jelly-like precipitate from this extreme dilution was solved by a method, involving neither the addition of flocculating agents nor the control of the pH value. Process control makes it possible to obtain initial settling rates as high as 120 feet per hour, and a settling rate of 50 feet per hour is maintained a t all times. The slurry is concentrated to 0.5 pound per gallon in a Dorr thickener, 85 feet in diameter. The overflow from this tank consists essentially of sea water containing calcium instead of magnesium chloride and sulfate. The underflow consists of a solid phase of hydrous magnesium hydroxide and of dissolved as well as adsorbed sodium, potassium, and calcium chlorides, and calcium and magnesium sulfates, as well as minor quantities of other impurities originating from both the sea water and the lime.
Removal of Salts The problem of removing these salts is a serious one. Since the addition of fresh water does not permit the resettling of the washed slurry a t any commercially economical rate, a process of washing was devised consisting of countercurrently contacting the hydrous magnesia slurry and fresh water. This step is carried out in towers 4 feet in diameter and 24 feet high, in which the slurry is introduced a t the top from whence it flows through perforated monel metal plates in the form of multiplicity of threads through an upward moving body of fresh water. The threads of magnesium hydroxide slurry coalesce upon reaching the bottom, where a level of
SPECIAL DRIERFC PRODUCTIOX OF MAGNESIUM HYDROXIDE MONOHYDRATE
INDUSTRIAL AND ENGINEERING CHEMlSTRY
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381
slurry is maintained. During the downward travel of the slurry, soluble salts are removed by diffusional washing. The degree of hydration of the magnesia is so high that only 1 to 2 per cent of the solids is carried over with the wash water. Since the spent wash water is passed through the main settling tank, no loss occurs. There is practically no decrease in solid concentration of the washed slurry. By this process the bulk of the chlorides and also the greater part of the sulfates are removed. However, because of the high adsorptive power of hydrous magnesium hydroxide, the removal is not sufficiently complete for all products even when two towers are used in series. For very high-purity requirements and particularly for obtaining magnesium hydroxide concentrations above 0.5 pound per gallon, filtration must be used. Since .extensive tests with various types of standard filters and centrifuges failed to give results, research was commenced on the filtration of highly compressible slimes. This work culminated in the development of a pressure filter of special design of which two installations have been in continuous operation a t South San Francisco for three and two years, respectively.
Filtration The filter consists essentially of a steel tank in which a number of tubes are arranged vertically, the tubes being connected by a manifold. The feed is forced into the tank by means of a high volume-low pressure and a low volume-high pressure pump operating in parallel. At operating pressures of 80 to 125 pounds per square inch a thick cake builds up on the filter cloth sleeves covering the tubes. The filtrate leaves the tubes a t the top through t,he manifold. A small portion of the fitrate, or wash water, is pumped into a high-pressure surge tank, compressing a pocket of air into the dome of this tank. At the end of the filtering cycle, which may vary from 3 to as high as 15 minutes, the feed is shut off, the hydraulic pressure in the filter tank is released, and the water contained in the surge tank is allowed t o rush suddenly to the inside of the tubes. The latter are built of steel bars, arranged circularly, thus providing grooves between which the loose filter cloth forms deep vertical indentations. Consequently, the filter cake is formed in the shape of wedge-shaped strips or bars of a length equal to that of the tube, a width corresponding to the spacing between the steel bars, and a depth depending upon the vertical folds in the cloth. Since the cloth is stretched tight vertically but is left loose horizontally, the filter membrane under exterior pressure (filtering) is star shaped, but under interior pressure (discharging) it is circular. As a result of this arrangement, the strongly adhering, gummy filter cake, at the time of discharge, contacts the cloth a t the point which corresponds to the apex of the cross section of the strip of cake during the filtering cycle. The contact area between cake and cloth, therefore, is but a few per cent of the filter area, and consequently the cake falls off easily. Because of the possibility of using relatively high filtering pressures and insuring a thorough cleaning of the pores of cloth every few minutes, filtering rates are realized far in excess of those obtainable with standard equipment. The operation of the filter by means of an electrical cycling clock, solenoid, and diaphragm valves, is entirely automatic. The only attention required is in changing filter cloths, which have a life of about 6-8 weeks, and to oil the motors. Subsequent to filtration, the magnesium hydroxide may be processed directly or may be rediluted for the production of magnesium basic carbonate. This compound may be obtained in a number of different,
SPECIAL R o T a R Y
KILN
hydrated, double salts of magnesium carbonate and magnesium hydroxide, depending upon certain variables in the process. Chief among these are temperature, rate of carbonation, partial pressure of carbon dioxide, concentration of the solid and liquid phases, the ratio of height to diameter of the reaction vessel, the rate and final temperature in the heating step, the amount of agitation after heating, the retention time between the heating and drying steps, and finally the rate of drying. Needless to say, the study of these variables involved considerable experimental work in laboratory, semi-plant, and full-scale operation. Although the knowledge gained is still very sketchy, it was possible to use the data to develop seven different magnesium basic carbonates (a total of thirteen commercial grades and types are manufactured a t South San Francisco) to meet varying conditions in the consuming industries, thus increasing the use of magnesium compounds. The ability to produce different types of material, each specially suited to the particular requirements of the various consumers, is a good answer to the question, does research pay dividends, since only two grades of carbonate were known previously in this old industry.
Production of Basic Carbonates The process employed a t South San Francisco for the production of magnesium basic carbonate comprises the following steps : standardization of the magnesium hydroxide slurry, passage of carbon-dioxide-bearing gases through the slurry, heating to drive off part of the carbon dioxide, intermediate storage, filtration, and drying. The process goes through the following chemical stages: The absorption of carbon dioxide by the dissolved magnesium hydroxide forms normal magnesium carbonate trihydrate. At this point two solid phases are present, hydroxide and normal carbonate. Finally, when all solid hydroxide has
changed to normal carbonatc, heating of the slurry releases some of the carbon dioxide to forrri the double salt. The process is cumplicated by the appearance of magnesium bicarbonate in the liquid phase. The reactions in the different stages are as follows:
Obviously, a low partial pressure of carbon dioxide and a slon rate of absorption will favor reaction 1 while a high rate of absorption d l force the formation of bicarbonate in solution according to reaction 2. Since tlie nornial carbonate is desimd, the end of the reaction can be determined easily by ihe abrupt drop in the pII of the solution from that of completed reaction 3. Heating of t.he normal carboiiate causes tho evolut.ion of carbon dioxide and water to form the Iiasii: carbonate. Tlrr following table s11o~r.sthe composition of compounds involved in the productioir of the basic carbonate arid of five of t,he basic salts:
I'hysically, Clic Ohdllge from the lrexaguoal needles of tlrc iiorirral i:arbonate to the crystals of the basic salt. is acconljranied 11ya rapid thickening of the slurry, due to the sudden increase iii the adsorptive propert.ies (surface) of the solids in changing from the relatively large, liexagonal iiecdlcs of the lionrial carbonate to the slimy mass of the basic product. In filtering the basic salt, cakes 2 inches thick and inore can be pulled, containing but 12.5 per cent solids. Sdandard equipment is used in these pro( dryirig is carried out on atniwpheric rirmn Peol)les spray drier. I n t h i s drier the magnesium basic carbonate is a,toniized by means of a slotted disk turning a t a peripheral speed of 25,000 feet IJer nhinut,c. Tho slurry is thus sprayed directly into the hot gases (at 1050" P.) obtained by the combustion of natural gas. The dried powder is reiwvcd from the mixture of gas, corrling air, and steam by cotton bag filters, to be packaged. By variation of the condit,ions under which the process ps are carried out, it is possible to vary the chemical and particularly tlie physical characteristics of the magnesium basic cartionate. Thus, the adsorptive index, an important variable for tlie use of magnesium basic carbonate for production of inks, paints, varnishes, etc., can be varied from 125 to 235 (in ternis of eo. water required completely to wet 100 grams of solid). Similarly, variations in processing conditions permit the pruduction of a number of sizes and shapes of magnesiunr basic carbonate crystals. Particle sizes from 0.1 to 20 inicrons CRLI he prodiiced. Tire sliajie of the crystals can be controlled to give snbstantially spherical, elongated, UT platelike crystals. Apparent densities can be varied from 5 to 20 pounds per cubic foot, rvitlioilt the use of grinding or rolling to obtain the heavier grades.
Magnesium Hydroxide and Oxides Magnesium hydroxide powder is produced directly from the purified slurries by spray-drying. In this case the spray-drying operation pennits the production of a very fluffy hydroxide.
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INDUSTRIAL AiYD ENGINEERING CHEMISTRY
Magnesium hydroxide, however, does not possess the extreme low apparent density of the basic carbonates. The hydroxide is produced in three grades of different purity. Magnesium oxide, with an apparent density of 28 pounds per cubic foot, is produced a t 1950' F. by continuous calcination of the hydroxide in an externally fired rotary kiln of special design. The shell is constructed of a steel containing 25 per cent chromium, 20 per cent nickel, and less than 0.25 per cent carbon. The feeding, discharging, and accurate temperature control are automatic. This kiln does not require any attention other than removing and sealing the full barrels of the product. Light magnesium oxides, with apparent densities ranging from 7 to 10 pounds per cubic foot, are produced by calcination of the basic carbonate either in a kiln or by direct introduction of the carbonate into a gas flame and collection of the oxide from the cooled combustion gases. Hydro-Magma paste, a concentrate of hydrous magnesium hydroxide containing 30 per cent solids, is produced by refiltering the purified and concentrated hydroxide slurries. The plastic material is shipped in galvanized steel drums for use in production of milk of magnesia, magnesia tablets, and dental creams. An entirely new development is the production of HydroMagma powder [magnesium hydroxide monohydrate, Mg(OH)z.H20]in dry,. powdered form. This compound, not previously reported in the literature, is obtained by dehydration of a concentrated slurry of the hydrous hydroxide under carefully controlled conditions of temperature and humidity. Since the nonsettling characteristics of magnesium hydroxide suspensions are due to the successively looser bonds between a multiplicity of layers of water of hydration surrounding the hydroxide particles and the particle itself, it seemed reasonable to suppose that the outer layers could be removed without impairing the hydrous properties of the substance.
389
Since the break in the dehydration curve, corresponding to the monohydrate, is rather slight, delicate process control is required. The hydrous hydroxide passes through three distinct physical stages during its dewatering process. Starting with a plastic hydroxide paste, a gummy substance of varying water content is formed, probably correspcnding to more or less complete abstraction of all free water, leaving the water of hydration only. This substance, which is dark in color, changes rapidly to the monohydrate. This compound is pure white. Drying equipment of special design and construction is used in the production of the monohydrate, consisting of a wire mesh belt onto which the concentrated feed is pasted. Superheated steam a t an initial temperature of 900' F. is employed as the drying medium. The new product possesses the property of forming stable suspensions when mixed with water, thus permitting the production of milk of magnesia from a dry powder merely by suspending the solid in water. The properties of the suspension formed equal and exceed those made by direct precipitation of a soluble magnesium salt with an alkali. The inherent advantages of the product in the manufacture of pharmaceutical products over magnesium oxide, over precipitation from Epsom salt and caustic soda and over magnesium hydroxide paste are obvious when we consider the ease of handling, storage, shipping, and immediate availability as hydrous hydroxide of the new product. Owing t o its process of manufacture by precipitation and dewatering, there is no possibility of incomplete hydration as in case of magnesium oxide, nor does the monohydrate present difficulties in remoying sulfate or excess alkali as in case of precipitation from Epsom salt and caustic soda.
Plant Operation Owing to the extent to m-hich automatic process control has been developed for the South San Francisco operation,
CRYSTALLIZING
CONCElVTRATIOR
SCREENING
SLAKINQ
MIXING
PILIIIRCATILW
WASHING
WATER TC rAR15US PROCESS 5 T E P S H
M4 R Ilr'CO-DD-U.
S
0
P
MRINC 0-8-U. S .Po M A R INCO-A-U .S- P.
FLOW SHEET FOR PRODUCTION OF MAGNESIUM COMPOUNDS FROM OCEAN WATER
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INDUSTRIAL AND ENGINEERING CHEMISTRY
this plant, producing twenty-two products, is operated by only three men per shift. On account of the amount of expansion and experimental work which is carried out a t all times, a fairly large mechanical crew is employed. Of the operating personnel, one man per shift is required for the sacking of the magnesium carbonate. The other two men divide their time in supervising the operation of the equipment, and in making operating adjustments. Although a continuous process for the carbonating step has been worked out, the present method of semi-batch carbonation is still in use and is the only discontinuous operation in the process. A considerable portion of the time of one man is needed, therefore, in nonautomatic filling, carbonation, heating, discharging, and testing a t this point. The steps involving pumping, softening, crystallizing, and filtering of the sea water, the preparation of the lime slurry, the reaction of sea water and lime, the concentration, washing, and filtering, all operate without any interruptions for nearly the full year. The production of the thirteen grades of basic carbonates, three grades of hydroxide, three oxides, hydroxide paste, Hydro-Magma powder, and milk of magnesia, from the purified primary hydroxide slurry is interrupted only to change from one product or grade to the other, depending upon shipments. Since the bulk of the production of all compounds manufactured must meet, and for reason of competition often exceed, the purity requirements of the United States Pharmacopeia, close process control and laboratory supervision are essential. .
Future Uses for Products The successful production of inexpensive magnesium salts opens large fields for future applications. Chief among these should be the manufacture of refractories to meet the requirements of alloy steel production by their high temperature resistance, long life, and freedom from spalling. Because of the high degree of purity and uniformity of the magnesia produced from sea water, this source should become a factor in the production of metallic magnesium a t a time when the demand for a light structural material outgrows the present supply of well brines. The use of magnesium hydroxides as neutralizer in place of caustic soda shows important advantages in that the magnesia as a powder is easily handled and proportioned, is harmless as far as human contact is concerned, forms highly soluble products of neutralization and can be added in excess without exceeding the neutral point. Most important, magnesium hydroxide can be sold, if manufactured on a larger scale, a t a cost considerably less than caustic soda, based upon its neutralizing action. Under consideration of com-
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mercial purities, 0.78 ton of magnesium hydroxide is equivalent to 1 ton of caustic soda in active alkali content. The use of a special type of magnesium oxide has proved experimentally to be far more efficient than decolorizing clays for the oil industry. The sweetening of gasoline (removal of mercaptans) has been carried out with magnesium hydroxide to advantage over caustic soda and litharge. Similarly, the compound presents the ideal means for the removal of hydrogen sulfide from gases. I n the production of alcohols by pyrolysis of gaseous hydrocarbons, magnesium hydroxide permits higher yields from the alkyl sulfuric acids, owing to its action as “controlled” alkali. Other large potential ’uses for inexpensive magnesium products are found in the paper, distilling, paint, and miscellaneous chemical industries. An additional future use of magnesia should be mentioned in passing. According to views expressed by medical authorities, there has been an alarming increase in certain diseases of the human race during the past decade, such as cancer, infantile paralysis, chronic stomach ailments, certain nervous disorders, and others. It has been pointed out further that this has gone hand in hand with a steady decrease in the amount of magnesium salts available for assimilation by the human body. The cause of the magnesia deficiency lies in the greatly increased refinement of our foodstuffs and drinking water. For example, the old type flpur contained, because of the inclusion of hulls carrying the mineral, a great deal more magnesia than the present-day white flour. Salt, both rock salt and that obtained by solar evaporation of sea water, was relatively rich in magnesium salts until modern improvements eliminated it. The use of virgin soil for the growing of vegetables and staples has decreased to the vanishing point, thus eliminating another former source of magnesium salts for the human body, since modern methods of fertilization include only potash and ammonium salts. Considerable medical research on the effect of magnesium salts on the human system has been carried out, both in this country and abroad, and although it seems too early to draw final conclusions, little doubt is left as to the fact that the deprivation of magnesia results in as serious consequences to humans as that of calcium and phosphorus. Consequently, the time when magnesium salts must be included in the field of fertilizers is not too far distant. There can be but little doubt that the ultimate source of magnesium compounds for large-scale consumption will be their production from sea water. NOTE. The process and equipment described is covered by United States and foreign patents and patent applications. R ~ C E I V February ED 15, 1936.
SECTION OF RESDARCH LABORATORY, MARINECHEMICALS COMPANY, LTD.
