INDUSTRIAL A S D ELVGIA7EERIAVGCHEAVISTRY
694
I-01. 32. T o . 7
The Magnesium Industry’ John A. GannZ THEDow CHEMICAL COMPAKY, MIDLAND,MICH.
M
AGSESICM metal is celebrating its one hundredth birthday this year. I t s commercial production and utilization, however, are the products of the last quarter of a century. Although the mineral industry has been responsible for the development of most of our engineering metals, the rapid promotion of magnesium must be considered an outstanding achievement of modern chemical engineering. Magnesium metal does not occur in the native state, but its compounds are very widely and profusely distributed throughout the world. It constitutes approximately 2.1 per cent of the earth’s crust, which makes it the eighth most abund m t element or sixth most abundant metal (5, 6). A more accurate perspective of the availability of magnesium is obtained if we study the Situation from the standpoint of the metallic versus non-metallic elements and then finally focus our attention on the relative amounts of the more important engineering metals (Table I). Magnesium is thus seen to be about seventy times as plentiful as nickel, two hundred times as abundant as copper, and of the engineering metals is surpassed in quantity by two metals only-aluminum and iron. The more important magnesium-bearing minerals are the oxide, carbonate, and haloid salts, either as such or in the form of double salts and chemically complex ores. Magnesium salts are likewise found in many mineral waters and in the ocean. At the present time Michigan salt brines constitute the important raw material for magnesium production in this country. All of these substances are chemical compounds that are extremely stable in so far as oxidation, weathering, and other naturally occurring chemical reactions are concerned. These facts, coupled Jvith the chemical and electrochemical behavior of magnesium, are in good agreement with the location of this metal in the electrocheinical series and in the periodic classification of elements, and help to account for some of the technical difficulties encountered in the production of the free metal.
electrolysis of a fused bath of anhydrous magnesium chloride contained in a small porcelain crucible. Subsequent modifications of his method consisted in the electrolysis of anhydrous mixtures of magnesium chloride and an alkali chloride, and in the electrolysis of the dehydrated naturally occurring mineral carnallit e. T a b l e I-Occurrence
ELEMEKT
AI Fe Ca Na K Mg
Ti P C H Mn S
c1
All others
46.7 27.6 8.0 5.0 3.6 2.7 2.5 2.1 0.7 0.2 0.1 0.1 0.1 0.1 0.1 0.4
HEAVYMETALS
NOK-METALS
0 Si Others
46 7 27 6 0 7
75.0
LIGHT METALS
so
70
% 0 Si
of E l e m e n t s in E a r t h ’ s C r u s t
Fe
Ti Mn Ni cu Zn Others
70
5.0 0 7 0.1 0 03 0 01 0 006 0.1-
AI
8 0 2 6 3 7 2 5 2.1 0.1
lCi a K Mg Others
19.0
6.0
RELATIVEAMOUNTS OF ENGIKEERING METALS
100.0
Aluminum Iron Magnesium Nickel Copper
800 500 200 3 1
Other methods ( 1 ) have been investigated and some operated on a commercial scale. They include a variety of chemical reduction processes as well as the electrolysis of various aqueous solutions, fused double sulfides of magnesium and alkali metals, and also molten fluoride baths containing dissolred magnesium oxide. Magnesium had a very fluctuating career until the present German industry was established a few years before the beginning of the World War. Great secrecy has been maintained concerning their methods of operation and production
History
The history of magnesium compounds dates back to 169& when a n English physician, Grew, discovered the medicinal properties peculiar to a salt (Epsom salt) in the mineral spring waters a t Epsom, England. The medicinal value of the oxide was discovered shortly after this, but its chemical composition was not completely established until 1808, when Davy showed that it was the oxide of a new metal which he called “magnium,” a name later changed to “magnesium.” The earliest attempts to isolate metallic magnesium, both by chemical and electrochemical methods, are attributed to Davy, but he succeeded in obtaining an amalgam only, rather than the pure metal. I n 1830 the first coherent substantially pure metal was prepared by Bussy ($), who fused anhydrous magnesium chloride with potassium. About thirty-five years later Deville and Caron ( 7 ) improved this method and produced the metal on a manufacturing scale. They introduced distillation as a method of separating the magnesium from the impurities present. I n 1852 Bunsen ( 2 ) laid the foundations of the present magnesium industry when he produced the metal by the 1 Received M a y 10, 1930 Presented before t h e meeting of t h e American I n s t i t u t e of Chemical Engineers, Detroit, Mich , June 4 t o 6, 1930. 2 Metallurgist, T h e Dow Chemica! Co., Midland, Mich.
YEAR
Figure 1 - M a g n e s i u m
S t a t i s t i c s , 1915-1929
statistics. The American industry was firmly established during the early part of the war because of our inability to import the German product. By 1917 there were five producers, but when government war requirements for magnesium ceased the number of producers decreased. The rapid growth of our domestic industry, particularly during the last few years, is shown in Figure 1 and Table 11, using data from the annual reports of the Bureau of Mines (8).
ISDUSTRIAL B S D E S G I S E E R I S G CHEMlSTRY
July, 1930
695
7 A f cobo/
L
c
Figure 2-General
Products Flow S h e e t , Dow Chemical Company
Foreign prciduction is estimated to be about ten times as large a- that in the I-nrted States. Yearly Statistics for United S t a t e s 4r. ISGOT CossrnwCONSUMP- PRICEP E R YZAR TIOZI TEAR TIOS POUND Poicnds Pounds 1915 87,500 % 00 1923 125,000 s1.25 1916 1924 1.10 76,400 4 13 128,000 1917 115,813 2 07 1925 245,000 0.86 1918 %84,1S8 1 81 0 80 1926 322,650 1919 127,465 1 83 192i 0.68 366,400 1920 1 60 1928 483,000 0 82 I921 45,000 1929 0 .57a 908,351a 1 30 1922 60.000 1 60 ii Manufacturer's unpuhlished figures Table 11-Magnesium
AT. IXGOT PRICE PER POUSD
Manufacture
Published statements and patent literature lead us to believe that metallic magnesium is universally produced by the electrolysis of dehydrated magnesium chloride. Several methods for preparing this raw material are possiblenamely, (1) inter-reaction between magnesium oxide, carbonaceous material, and chlorine gas; (2) dehydration of lIgC12.GH20 in the presence of ammonium chloride or a n alkali chloride; (3) partial dehydration of hIgC1?.6HzO in air and final drying in a hydrochloric acid atmosphere. The first inethod appears to be the process preferred by the Germans, while the last one is einployed in this country. The Do\$-Chemical Company's process for the production of metallic magnesium is unique from several standpoints. Its basic raw material is a salt brine and not a solid ore. The process permit. the separation of hundreds of tons per day of a mixture of the chemically allied salts-magnesium chloride, calcium chloride, and sodium chloride-in addition to the liberation of liromine. The entire cycle of operations requires the use of very fen- chemicals, which reduces analytical control to a minimum. These brine constituents, when separated froin the niagiiesiuni chloride, become the essential raw materials for other branches of our local chemical industry. Finally. the metal made is the purest grade of magnesium coniinercially available. An appreciation of our closely interlocking chain of chemi-
cal reactions, of the diversification of products, and of horn inagnesium fits into the picture can be obtained from a study of Figure 2, which depicts the flow sheet for about one-third of the chemicals produced. I t gives a few illustrations of hciw the products from the various steps are cornhined with other chemicals to finally yield more valuable materials. It' id interesting to note the variety of complex high-molecularweight dyes, pharmaceuticals, and insecticides that niay be synthesized from a salt brine and a few of the siinpler materials such as sulfur, carbon, lime, white arsenic. henzene. ainmonia, and carbon dioxide. On the other hand, the production of magnesium proceeds in the opposite direction, since in this case 1IgC:l2.6H2O,a salt with a relatively high inolecular weight, is decomposed to yield a low-molecularn-eight metal. The essential steps in the 1irocluction of magnesiuni are represented in Figure 3 . A natural brine is pumped from wells 1200 to 1400 feet deep. I t contains approximately 14 per cent sodiuiii chloride, 9 per cent calcium chloride, 3 per cent magnesium chloride, and 0.15 per cent hoinine. After the bromine is removed, the brine is treated with a inagnts' "lull1 hydrate slurry t'o precipitate the iron and other impurities which are separated in contiiiuous thickeners and sedimentation tanks. The decanted liquor is then evaporated until the sodium chloride has crystallized. Exhaust steam from the power plants is used for this purpose. The salt is reniored on rotary filters and used for the production of chlorine and caustic soda. which in turn are consumed in the product,ion of more valuable chemicals. The magnesium and calcium chlorides in the rotary-filter mother liquor are separated from each other by fractional crystallization. This is made possible by the fact that, under properly controllcd composition and temperature conditions, the crystals separating from a complex salt solution may have a different coniposition from the solids reniaiiiiiig in the mother liquor. In practice this is accomplished by coiicentrating a solution with a 1: 3 weight ratio of magnesium chloride to calcium chloride, whereupon crystals of the double salt "Tachpdrite" are formed. The composition of this salt is represented by the forniula 2~IgCly.Ca('1?.12H1t:). It has a 2 : l ratio of MgC12 to CaCl?. but its cryst,als ar? in
696
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
VOl. 22, No. 7
Bromino
.f
Fe(OHI8 and Impurities
equilibrium with a mother liquor having a RlgC12 to CaClz ratio of 1 : 10. The crystals and liquor are separated in false-bottom tanks. This CaClz-rich solution and wash water from the Tachydrite are reworked elsewhere to give calcium chloride and Epsom salts. The Tachydrite crystals are then dissolved in hot water and the solution is transferred to a series of crystallizers where substantially pure MgC12.6Hz0 separates, leaving a mother liquor containing MgClz and CaCl2 in a ratio of approximately 1:l. This solution, together with the hIgCl2.6Hz0wash water, contains approximately half of the magnesium chloride originally present in the brine and is returned to the process for reworking as indicated in the flow sheet. I n order to facilitate further treatment, the MgC12.6H20crystals are melted in their water of crystallization and the fused mass is flaked on rotating steel drums. The final steps in preparing feed for the electrolytic cell consist in a series of dehydration operations to remove the 6 molecules of water of crystallization which correspond to 53 per cent of the weight of the hydrated flaked chloride. This work has shown the existence of a complete series of hydrated salts containing, respectively, 6, 4, 2, and 1 molecules of water. Air-drying on the countercurrent principle is practical to a composition corresponding approximately to MgC12.2Hz0,provided the temperature is carefully controlled to prevent incipient fusion. The last 2 molecules of water are removed by heating to still higher temperatures in an atmosphere of hydrochloric acid gas] which is necessary to prevent hydrolysis and the formation of magnesium oxide. The electrolytic decomposition of the dehydrated magnesium chloride is carried out in large, rectangular, cast-steel pots capable of holding several tons of molten cell bath. The steel pot serves as the cathode, while the anode consists of graphite bars. The electrolytic process is a continuous operation, the metal being removed daily. The cell bath is maintained at approximately a constant level, by either a continuous or intermittent feed of dehydrated magnesium chloride. Sodium chloride is added from time to time to reduce the melting point and increase the conductivity of the bath. Exterior heat supplied by a series of stoker-fired furnaces helps to maintain the proper cell temperature and
reduces power consumption. Figure 4 shows a general view of one bank of cells and Figure 5 the corresponding series of stokers. The magnesium metal formed is lighter than the cell bath, and therefore rises and floats on the surface of the same, but does not burn because of the protecting action of a thin film of the molten salt bath. On the other hand, the sludge which forms during the normal operation of the cell is heavy and immediately sinks. This sludge is largely due to the small percentage of magnesium oxide present in the cell feed. This automatic separation of metal and sludge, the high purity of the cell feed (less than 0.01 per cent heavy metal impurities capable of affecting the quality of the metal), the freedom from contamination due to chemical attack of the magnesium on the cell parts, and finally, the washing and purifying action of the cell bath itself-all combine to yield a metal of such purity that subsequent refining is unnecessary. The average analysis of this magnesium direct from the cells reveals a purity of 99.9 per cent, a figure that at times rises to 99.95 per cent. The minute traces present consist of silicon, iron, aluminum, and manganese. Foundry Practice
The melting of magnesium alloys presents no particular difficulties provided a few fundamental factors are kept in mind. Molten magnesium reacts readily with the oxygen and nitrogen of the air as well as with moisture. Any successful melting operation must therefore] exclude these substances, a condition preferably met by the use of a flux, particularly when this same flux likewise prevents non-metallic contamination and permits a continuous melting and casting process. The recommended flux consists of an anhydrous mixture of magnesium and sodium chlorides quite similar in composition to the bath used in the electrolytic cell. Its gravity can be decreased by the addition of potassium chloride or sodium chloride and increased by the addition of barium chloride. The gravitymust be so regulated that it just floats the metal, while its viscosity and surface tension must be such as to cause the formation of a continuous enveloping film. This condition is diagrammatically illustrated ih Figure 6. ]
INDUSTRIAL A N D ENGINEERINQ CHEMISTRY
July, 1930
The flux h a no chemical action on the magnesium, but readily removes nonmetallic impuridies by wetting the same and causing them to coalesce into a dense, heavy sludge which sinks underneath the clear flux layer. This washing action is so complete that even the t,hin film of oxidc and accuniulat,ed dirt on shavings and machine-shop scrap may be completely removed; and since iron contamination from the cast-sieel inelting pot and other eqnipment does not. owur, the magnesium industry is not confronted with the problem of secondary versus virgin metal. The pnrification process does not require special equipment, hut is condnct,ed in tho regular melting pot. Intimat,e contact of fliix, metal, and impurities is obt,ained by puddling with a ladle. As soon as agitation ceases the metal and flux scparate into layers. One of the unique features from a metallurgical standpoint is tlie relatively large amount of flux employed. This is economically sound, however, since the purification eficiency is thereby increased and since the flnx may be used repeatedly. In the caiting operation nietal is dipped from the pots by t,he use of ladles equipped with skimming lips and underfeed spouts. (Figure 4) Ladles for small castings are handoperated, while larger ladles are opcrat,ed with cranes or chain hoists. Clean metal for casting is secured by parting the proteet,ing flux film with thc skimmer lip prior to filling the ladle. The flux film is t,lien rcformcrl by stroking the sllrfaat?of the melt with t.he hoitom of the ladle in case the film did not form automatically as the ladle was lifted. All the various casting procedures used with other metals maybe siiccess€iillyemploycd in the case of magnesium alloys. Sand castings are particularly sound and free from internal porosity. The cast metal possesses high strength and duct,ility and is very resistant to shock. M q i m u m properties are obtained by suitable heat breatment.
697
seams and joints are remarkably clean and free from gas holes and inclusions. The strength of the welds is fully equal t,o that of t,he cast metal. Magiiesium is the easiest of all met,als to mac.hine. It takes a fine, smooth finish and the work can he held to exact dimensions. Both the speed and dept,h of cut can be increased over usual shop practice. As a rough comparison, magnevium can be rnachincd about three times as rapidly as cast iron. Ordinary carbon-steel tools may he satisfactorily employed. ?io eutting conipounil is necessary; in fact, its presence interferes with efFicient scrap reclamation. Corrosion Resistance and Protection
The resistance bo chemical attack is a factor in which the chemical engineer is particolarly interest&. Magnesium is very stable in contact with caust.ic alkali and normal carbonat,es. Most, aqueous acids and salt solut,ioris at.tack the metal wit,h the evolution of hydrogen, but notable except,ions occur in the case of hydrofluoric acid, acid phosphates, chromates, bichromates, and even diliite solutions of chromic acid. In some of these cases a slight at,tack takes place with t,he formation of a protective film that prevents further action. In Germany niagnesiuin has been used for t,he construction of diaphragms and cell parts in clectrolyt,ic equipment used with fluoride baths (9). Magnesium is as permanent and corrosion-resistant as the other common engineering metals when suhjecfed to atmospheric exposure. In fact, magnesium-alloy castings that have been in our plant at,mosphere for over two years are still in exeellcnt condit,ion. Tlie bright metallic surface is soon dulled owing to the formation of an oxide film, which gradually darkens hiit does not Rake off under these wcather-
Fabrication
Magnesium a h y s ham been developed which can be rncchanically worked, thereby causing rnarkerl irnprovement,s in physical properties. Rolling, extrusion, drawing, and forging proce ' * arc conducted along the same general principles usedwith otlicr metals, although t,he details of the different operations have to be rcgulaterl to meet certain fundamental characteristics of this metal. Working is best done at 400-800" F.. in which teniprrature range maximum malleability is obtnined. A limited amount of cold working is possible on prei.iously worked and recrystallized metal. Forgings can lie made with a hammer or a press, but the latter is preferable as more time is allowd for ilie metal t.o take its new shape. The reduction in cross section should be 5G per cent or more to insure masimum property improliement. Rolled sheet. can be made in thicknesses down to 0.005 inch. The sheet metal can he worked to most shapes provided suitable annealing steps are introdriccd between working operations. Extruded billets make excellent forging and rolling stock. A recent important advance in the fabrication of magnesium is the perfecting of successful welding methods. Electric spot and seam welds are readily made with standard equipment,. Acetylene welding requires a suitable flux, hiit inert atmosphere and special technic are not necessary. Vlnxes composed essentially of sodium, potassium, and lithium chlorides such as are used in welding aluminum alloys have been found satisfactory. Best results are apparently obtained with a fused and ground flux. The welded
Figure 4-Bank
of Celle for Production of Magnealum
ing conditioiis. Whcre corrosion does occur in long-time exposure test,s, the slight loss in strength is equivalent to tlie loss in area, but there is no loss in ductility or strength t,hat can be attributed to int,ercrystalline corrosion. This is a very essential factor where stability is cf prime importance, &s in aircraft work. The corrosion resistance of magnesium has been greatly improved during the last few years. The electronegative heavy metals have been reduced t,o practically nil by manufacturing refinements. A more complete appreciation of the purifying action of the flux has led t.0 a better control of
INDUSTRIAL .4ND ENGINEERING CHEMISTXY
698
foundry practice and an improvemeut~in tho resulting cast.ings. Thi: corrosion resistance of magnesium alloys has been st,ill iurther improved hy tlie introduction of manganese which raises the liydrogen overvoltage. The amount of manganese that can be introduced is quite small and varies with the arnount of other alloying metals present,. Approximately 2 per cent can be successfully alloyed with purc magnesium, while 0.3 to 0.5 per cent manganese is the cguilibrium coni:entrat,ion in alloys coirt.aiiiing 4 to 8 per cent alurniniim. Fortunately, this low perceritage of manganese is very effcctire as a corrosion inliihit,w.
FiWre 5- .Stokers Used in ROdUction of Mabnesium
Where service conditions are part.icularly severe, the met,al should be protected with a lacquer, paint, or enaincl that is resistant. to t.he pltrticular corrodant under consideration. In order to secure maximum adhesion of the protecting material to t.he magnesium parts, the metal siirface should I)
