Mineral Facts and Problems: 1960 Edition Page: 484
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MINERAL FACTS AND PROBLEMS, ANNIVERSARY EDITION
ciency is 75 to 80 pecent. Power requirements
are approximately 9 kw.-hr. per pound of mag-
nesium produced. The magnesium chloride
cell feed continuously enters the cell to main-
tain the correct bath composition and level
within the cell.
When the magnesium metal is liberated at
the cathode, it rises to the surface of the bath
where it is guided by inverted troughs to the
metal wells in the front of the cell. This mol-
ten metal having a purity of 99.8 percent or
more, is dipped from the storage wells into
pig forms. The hot chlorine given off at the
anode is collected under the tightly closed re-
fractory cell cover and piped to the hydro-
chloric acid plant (6).2
In the silicothermic process metallic mag-
nesium is produced in 2 operations: 1. Cal-
cined dolomite and 75-percent-grade ferrosili-
con, in ratio of 5 to 1, are mixed, ground,
heated and briquetted. 2. The briquettes are
charged into tubular retorts, which are heated
and evacuated. The magnesium oxide in the
calcined dolomite is reduced by the silicon,
producing magnesium vapor which is con-
densed as crystals on a removable condenser
that projects from the furnace, and is easily
removed. The crystals of 99.98 percent mag-
nesium in the shape of crowns or muffs, are
melted and ladled into casting forms.
Other methods of production of magnesium
were tried before the close of World War II
and abandoned soon thereafter.
SAFETY
Before adequate safety measures were
adopted, disastrous fires and explosions have
resulted in plants that accumulated magne-
sium dust and drosses, and permitted them to
become wetted or exposed to fire. During
World War II, the Bureau of Mines pro-
vided advice to magnesium producers and
consumers for the safe disposal of these waste
materials. The care with which magnesium
alloy scrap was handled in modern foundries
and fabrication plants was described in detail
in 1956 (13). Dust collectors were described,
and dust disposal methods were given.
In 1954, when magnesium-thorium alloys
were developed, it became necessary for studies
to be made of radiation safety in the plants
producing and fabricating these alloys, even
though radioactivity was comparatively low.
Nominal thorium content was 3 percent or less.
The Dow Chemical Co. published a bulletin
relating its experience with the alloys thus far
and stating that external radiation is not a
2 Italicized numbers in parentheses refer to items in the
bibliography at end of chapter.serious problem when melting and fabricating
these alloys. The following precautions should
be observed: (1) Local exhaust ventilation of
areas where the alloys are melted and milled;
(2) check regularly air sampling of these
areas to determine extent of radiation; and
(3) provide adequate local exhaust at the
point of welding, to control exposure to radia-
tion.
MAGNESIUM ALLOYING
Magnesium does not possess sufficient
strength in its pure state for many structural
uses. Other metals are added to the primary
metal to produce alloys with improved prop-
erties.
By the close of 1958, 18 standard commer-
cial alloys were in use. The alloying constitu-
ents were aluminum, manganese, zinc, zir-
conium, thorium, and rare earth metals.
Standards have been set for these alloys by
the American Society for Testing Materials,
as shown in table 3.
TABLE 3.-Alloy element content of principal magnesium
alloys
[Percent]
Alloy 1 Al Mn Zn Re 2 Zr Th
AM100A----------10 0.13 0.20 -
AZ81A------------- 8 .15 .90 ..... ... ....
AZ31B .... 3 .45 1
AZ61A ......... 6. 50 .30 1 - - --
AZ63A ..6 .25 3
AZ91A .. 9 .20 .60....................
AZ91C ----------- 8.70 .20 .70
AZ92A------------- 9 .15 2
EK30A ...--------------------------------3 0. 55
EZ33A.----------------- -------- 3 3 .70
HK31A .70 3
M1A_-------------------- 1.50 .
HM21A-------------___--------_.-_-------------.55-
ZEHM21A ....60 1.20 .202
ZH62A ------------------------- 5.70 ..... .70 1.80
ZK20A-------------------------- 2.30 ------ .55
ZK51A---------.---------------- 4.60 ....... .70
1 Standard ASTM designation system. Letters representing alloy
elements: A-Aluminum; E-Rare earths; H-Thorium; K-Zirconium;
M-Manganese; Z-Zinc. A serial letter is arbitrarily assigned (after
the numerals) in alphabetical sequence, starting with A and serves to
differentiate otherwise identical designations.
2 Rare Earths: In general, this term applies to the 15 metallic elements
having atomic numbers 57 to 71. Essentially; the rare earths in Misch
Metal, which is usually used in preparing magnesium-rare earth alloys,
are cerium, 50 percent; lanthanum, 40 percent; and didymium, 10 per-
cent.
Aluminum is added to magnesium to in-
crease its strength. When manganese is
added to magnesium alloys, it increases their
resistance to salt-water corrosion. The addi-
tion of rare earth metals to magnesium re-
sults in alloys of improved creep strength at
elevated temperatures.
Zirconium added to magnesium and its al-
loys results in grain refinement and improved
mechanical properties.484
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United States. Bureau of Mines. Mineral Facts and Problems: 1960 Edition, report, 1960; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc38790/m1/492/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.