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Chapter 5
WINNING OF TITANIUM METAL SPONGE
Scheduled new (1982) domestic production of vacuum-distilled,
Kroll-process sponge and of additional high-purity elec trolytic
crystals, coupled with recent expansion of existing leached-Kroll and
leached-Hunter sponge production, promise a near-term abundant supply of
suitable sponge for existing and new applications. The diversity of
production methods, the commissioning of new plants and of plant
expansions, and the incremental process and equipment improvements that
are occurring are considered as positive factors in assuring future U.S.
titanium supply. U.S. titanium production technology and economics could
profit, however, from the development of methods for more effective
removal of volatiles f ram sponge, f or recovery of anhydrous magnesium
chloride from leach brine, and for the direct preparation of
high-quality, low-cost titanium powder for powder metallurgy.
The four current (1982) and two prospective U.S. sponge producers
represent a spectrum of processes, operating scales, and modernization
levels. The modernization level also varies for different operations
within the same plant. ~ Recommendations f or incentives to induce
modernization of outmoded facilities are presented in Chapter 12. ~
Commercial, experimental, and theoretical methods for the production
of titanium metal were comprehensively reviewed in a 1974 NMAB report.
When that report was published, magnesium reduction of TiC14 (Kroll
process) was the major process used in the United States, Japan, the
Soviet Union, and China. Sodium reduction of TiC14 (Hunter process)
was the other process used in the United States and Japan, the only
process used in England, and the process used in one of China's five
plants. Electrowinning from TiC14 in a fused salt bath was being
t ested on a pilot-plant scale. Few of the many other proposed processes
reviewed in the 1974 NMAB report had advanced beyond a laboratory stage,
and most were considered by the authoring NMAB panel to be unlikely to
progres s to product ion soon.
The f inding s of the 19 74 NMAB report related to co~nmerc ial and
certain other processes are summarized and up-dated in this chapter.
Since the report was written, several major events have occurred: the
first U e S e vacuum-distilled Kroll sponge production, since du Pant ceased
production in 1962, occurred in 1980 when Teledyne Wah Chang Albany
converted from zirconium to titanium production; titanium metal
production by electrowinning was initiated in 1980 on a full-scale,
production-module basis by the D-H Titanium Company; and construction of
an advanced technology, Krol1-type plant was begun by International
41
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Titanium, Inc., (ITI) at Moses Lake, Washington. ITI expects to produce
sponge of higher quality than the older domestic Kroll-type plants using
acid leaching and at a rate of 5.5 million lbs of metal per year by
mi d-198 2 . ~
Of the world ~ s total 1980 sponge production of about 85 ~ 000 tons ~
approximately 7 2 ~ 000 tons (85 percent ) were made by the Kroll proces s .
Of the 26,000 tons of U.S. sponge produced in 1980, about 17,000 tons (65
percent ~ were made by the Kroll process.
U . S . Product ion of Ti tanium Sponge
U. S . titanium sponge capacity is discussed in Chapter 8. The four
current producers (in order of startup and of size of production) are
TIMET, RMI Company, OREMET, and Teledyne Wah Chang Albany. Two
additional prospective producers are the D-H Titanium Company and ITI.
Several others are being discussed. One, Albany Titanium, Inc., has
announced plans f or a small Kroll-type sponge plant f or approximately
500, 000 lbs per year starting in 1982.
General descriptions of the Kroll, Hunter, and electrolytic
processes, followed by some operating details of each U.S. titanium
production process are given below.
Magnesium Reduction of TiC14 (Kroll Process)
In the basic Kroll process, molten magnesium metal and gaseous
TiC14 react in a sealed steel pot at a temperature of 800 to 900°C with
formation of solid metal titanium in sponge form and molten MgC12.
Theoretically, a ton of titanium metal and 7,950 lbs of MgC12 would be
produced by the reaction of 2,029 lbs of magnesium with 7,921 lbs of
TiC14. In practice, an excess of magnesium (10 to 15 percent) is used
to assure the complete utilization of TiCl4. With the escalation in
the cost of magnesium, plants that leach and impound MgC12 are forced
by economics to minimize the amount of stoichiometrically exces s
magnesium. This i s done at the ri sk of leaving TiC12 in the sponge
titanium that can ead to low-density inclusions in the ingot. The
exothermic reaction takes place over abou~ a 24-hour or longer period
with the pot in a furna~e to provide temperature control supplemental to
that obtained by regut_~ing the rate at which liquid TiC14 is sprayed
into the pot. Since the reaction is highly exothermic, an excessive
TiC14 feed rate .'uld damage the pot.
Magnesium metal is introduced to the pot in solid or liquid form.
When a solid charge is used, the lid of the pot is welded to the
cylindrical body after the ingots have been stacked in the pot. Air then
is evacuated through valves in the lid, and the pot is purged with argon
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or helium and placed in a furnace to melt the magnesium before starting a
measured flow of TiC14. When molten magnesium is used, it is pumped
into the argon- or helium-filled pot in the furnace before starting the
f low of TiC14.
Since molten magnesium floats on the surface of the molten MgC12
that forms, it remains accessible to the TiC14 feed. Molten MgC12 is
tapped from the pot at intervals of 3 to 4 hours or longer and is charged
to electrolytic cells for conversion to magnesium metal (for reuse in the
reduction pots) and chlorine. The chlorine produced is used by the
producer to prepare TiC14 by reaction with rutile concentrate or is
returned to the TiC14 supplier or sold on the open market.
When the scheduled amount of TiC14 has been reacted, a f inal tap i s
made of MgC12 along with any of the excess magnesium that drains from
the pot. Residual MgC12 and magnesium metal entrained in the titanium
sp onge are separated from the sponge by one of the following procedures:
1. Removal of MgC12 and magnesium by vacuum distillation from a
heated pot at a temperature of 900°C and a vacuum of below 100
microns of mercury. The operation may take 48 hours or more
depending on the capabili ty of the vacuum equipment . Both MgC12
and magnesium metal are recovered by condensation. After cooling,
the weld metal holding the lid to the pot is ground away and the
pot is opened . The sponge is bored, chipped, and extracted on a
removable cradle or is pressed from the pot. Pressing the sponge
from the pot requires that the pot be composed of three sections;
the top and bottom sections are removed and a ram is used to force
the sponge from the center section of the pot. Vacuum
distillation, originally used by the U.S. Bureau of Mines, and by
du Pont from 1948 to 1962, is employed by Teledyne Wah Chang
Albany and by ITI at Moses Lake, Washington. Vacuum distillation
is standard procedure in Japan and the Soviet Union.
Sweeping the heated pot with helium gas to reduce volatile MgC12
and magnesium to a low level and subsequently recovering these by
condensation. After cooling, the pot is opened and the titanium
sponge is removed, sheared, crushed, and then leached in acidified
solution to remove remaining MgC12 and magnesium. This practice
is used by OREMET in Albany, Oregon.
3. Opening the cooled pot in a "dry" chamber. The dry atmosphere is
necessary to avoid reaction of retained salts in the sponge with
moisture in the air. Titanium sponge, and admixed MgC12 and
magnesium, are bored out of the pot, crushed, and leached in a
buffered acid solution. This practice is used by TIME: T in
Henderson, Nevada.
Vacuum distillation is capable of yielding sponge that contains less
magnesium, magnesium chloride, and other volatiles than does acid
leaching of the sponge. Distillation is more energy-intensive than acid
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leaching (although this can be minimized by using the heat of reaction
for distillation) and requires more expensive reactors (to withstand
vacuum at high temperature). The low-chloride sponge also is more
difficult to crush and grind. Conversely, it produces metal of generally
better quality than acid leaching, recovers more magnesium and magnesium
chloride for re-use, and avoids the problems and expense of disposing of
an acid solution containing magnesium chloride. Generally, leaching was
the economic choice in the 1950s when magnesium and energy were
inexpensive, but in the 1980s, vacuum distillation is the preferred
route. This important aspect is discussed in more detail in a subsequent
section of this chapter. The major features of the main titanium
producers using the Kroll process are described immediately below.
IDMET DIVISION OF TMCA
The TrMET plant at Henderson, Nevada, owned by NL Industries and
Allegheny International, has been producing sponge by the Kroll process
since the early 1950s. It s current sponge capacity is about 15 ~ 000 tpy
As a fully integrated producer, from ore to titanium ingot and finished
items, TIMET makes its TiC14 by fluidized-bed chlorination of imported
rutile or rutile substitutes. Most of the plant 's magnesium cells date
from the World War II basic magnesium plant at the Henderson site, but -
re placement program i s under way .
TIMET loads its mild-steel reactor pot with magnesium ingot and
maintains a helium atmosphere in the pot during the introduction of
TiC14 into the reactor and during reduction and cooling. The pot is
opened and the titanium sponge is bored out in a dry room. Af ter
crushing and leaching with HC1/HNO3 solution that is buffered with
citric acid, the sponge is dried and stored until needed for conversion
to ingot
a
About an inch of sponge is left as a lining in the cylindrical pot to
absorb the trace of iron dissolved from the steel pot by the molten
magnesium. The lining sponge is recovered after about 60 cycles when the
pot is scrapped. TIMET generally sells this high-iron sponge to
manufacturers of ferrotitanium for use in steel alloys. The softest and
best quality sponge occurs farthest from the reactor wall and is cut
selectively from the pot when highest purity is desired.
Each pot produces about 1 ton of sponge in a 3-day cycle. Titanium
is harvested from about 44 pots each day. No salts are recovered from
the leach solution; it is stored and evaporated in tailings pond s.
Make-up requirements are about 0.3 ton of magnesium and 1 ton of chlorine
per ton of sponge produced.
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OREME:T
The OREMET plant at Albany, Oregon, publicly owned but with the Armco
Steel Corporation as the major stockholder, has been producing sponge by
an advanced Kroll process since 19 66 . It s current sponge capacity is
about 4, 500 tpy. The plant is fully integrated from sponge deco f inished
items, except that it does not now prepare its own TiC14 (although it
formerly did so and is considering it once again), and chlorination
facilities are still available on site. TiC14 ~ s purchased from Gulf
and Western, Ashtabula, Ohio, and delivered in 55-ton tank cars.
Chlorine from OREMET's electrolytic magnesium recovery cells is sold on
the open market.
OREMET uses 304 type stainless steel reduction reac tors of a
horizontal cylindrical design that produce 14,000-lb batches of sponge.
Reactors are loaded with molten magnesium through valves in the welded
reactor. An inert atmosphere is maintained during the loading and
reduction of TiC14. Most of the MgC12 and magnesium not drained from
the reactor af ter the last tap are removed by a helium sweep of the
heated reactor. The reactor then is cooled, and the head is removed by
cutting. A steel "ridded cradle the same length as the reactor vessel
facilitates removal of the sponge in the form of a single, 7-ton, roughly
semi-cylindrical shape. This sponge shape is sheared, crushed, and
leached to remove remaining chlorides and magnesia. The results ng
sponge is screened to remove fines, dried, and stored until needed for
conversion to ingots.
TELEDYNE WAR CHANG ALBANY (TWCA)
TWCA had been producing zirconium by a Kroll-type reduction of
zirconium tetrachloride since the late 1950s. Because of a depressed
market for zirconium and a shortage of titanium, idle equipment was
converted in 1980 to produce titanium sponge at a rate of 1,000 to 1,500
tpy. TWCA titanium sponge is vacuum distilled and therefore has
significantly lower volatiles relative to acid-leached sponge. TWCA the
second U.S. producer of vacuum-distilled titanium sponge; du Font was
first, in 1948.
INTERNATIONAL TITANIUM, INC .
This new Kroll process titanium sponge producer will be the third
U.S. user of vacuum distillation. Construction of the ITI plant at Moses
Lake, Washington, started in July 1981. Ownership is by Ishizuki
Research Institute and Mitsui Company, Ltd., of Japan and other Japanese
and American business interests. Sponge production began in early 1982
at a rate of 2, 750 tpy, and if market conditions warrant, the facility
could readily increase production to 3, 300 tpy in 1983. ITI has no
current plans to produce ingot.
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The fluid-bed process for chlorination of rutile is used by ITI
f allowed by purification of the TiC14 produced by solids condensation
and fractional distillation. Sectional vertical reactors produce a 5-ton
batch of sponge, but it is possibile that they will later be expanded to
accommodate 7-ton batches. Magnesium and magnesium chloride not drained
from the pot are removed by vacuum distillation and recovered. Sponge is
removed from the sectional reactor by pressing it out of the reactor
cylinder.
Magnesium and chlorine for recycle in the plant are produced f rom
recovered magnesium chloride in a newly designed and enclo sed Ishi zuki
magnesium cell. The energy requirement per lb of magnesium is expected
to be only 4 .6 kWh compared to between 7 and 8.5 kWh in conventional
magnesium cells. Other advantages predicted for the new cell are longer
life of components, less release of chlorine, higher purity chlorine, and
improved yield of magnesium and chlorine. On the basis of sponge quality
and production costs, ITI plainly expects its plant to be
c as t-compe t it ive with old, depre ci ated U . S . sponge p lent s and
quality-competitive with new Japanese sponge plant s.
Sodium Reduction of TiC14 (Hunter Process)
As initially practiced, the Hunter process reacted TiC14 wi th
elemental sodium under an inert gas atmosphere in a sealed steel pot at
temperature of about 900°C. Ti tanium sponge and molten sodium chloride
were formed. Subsequently, a two-stage reduction procedure was adopted.
In s Cage one, TiC14 and enough liquid sodium to reduce the TiCI4 t o
TiC12 are f ed continously to a stirred and continuously discharging
reactor. In the second stage, the flowable mixture containing TiC12
and molten salt formed in the f irst stage is transferred to a batch
reactor pot that contains molten sodium for completion of the reduction
to titanium sponge. An inert gas atmosphere is maintained in both
reactor stages. The second-stage reactor is positioned in a furnace for
temperature control to supplement the exothermic reduction reactions.
Theoretically, 1 ton of titanium metal and 9,762 lbs of sodium
chloride would be produced by the reaction of 3,841 lbs of sodium winch
7, 9 21 lbs of TiC14 . In practice, an excess of TiC14 i s used to avoid
the presence of free sodium in the reaction products because it poses a
fire and explosion hazard. Since sodium and titanium subhalides are
soluble in molten sodium chloride, it is necessary to retain the
reactants in the sealed pot until the reaction has been completed, the
pot has been cooled, and the welded head removed. The mixture of sponge
titanium and sodium chloride i s chipped from the reactor, crushed to
about 3/8-inch lumps, and leached in dilute hydrochloric acid solution to
dissolve the salt. The resultant leach brine may be reprocessed in the
sodium-chlorine plant to make pure sodium chloride for electrolysis. The
washed sponge is dried, screened to remove f ines, and pressed i nto compact s
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for vacuum arc melting. Unlike the Kroll process that tends to form
sponge high in iron near the reactor walls, sponge throughout the Hunter
reactors is of uniform grade and low in iron content.
The 1974 NMAB report alluded to the desirability of doing the second
stage, as well as the first stage, of the Hunter reaction in a continuous
manner as was investigated by RMI, but such a technology has not been
established to date. There also was conjecture about using the
first-stage Hunter reaction to prepare TiC12 feed for a titanium
electrowinning cell. New developments on this have not been reported.
RMI COMPANY
Titanium sponge has been produced since 1957 at Ashtabula, Ohio, by
RMI and its predecessors via the Hunter process. RMI is owned jointly by
U.S. Steel Corporation and National Distillers and Chemical Corporation.
As an integrated producer from sponge to finished items, RMI buys TiC14
and currently makes sponge at a rate of 9,500 tpy. TiC14 is purchased
from the nearby Gulf and Western pigment plant, and chlorine, from RMI's
sodium-chlorine cells is returned to Gulf and Western for reuse.
Because the second-stage reactor (sinter pot), at completion of the
reaction between sodium and TiC12, contains about five times as much
salt as sponge, the batch size of the resulting sponge is smaller than
when the same size vessels are used for magnesium reaction with TiC14.
Jack-ha~ners are used to remove the mixture of sponge and salt ~ spelt
that then ~ s crushed, ground, and leached in a continuous system.
As much as 10 percent of the leached sponge is removable as fines
that have a potential market in powder metallurgy applications. Material
as coarse as 60 mesh (0.0098 in. or 250~) has been sold for powder.
Difficulty has been encountered in removing the final traces of chlorides
from leached powder. The presence of chlorides interferes with the
welding of titanium shapes made by powder metallurgy as noted in some
detail in Chapter 11.
Direct Electrowinning of Titanium Sponge
Both the Kroll and Hunter sponge processes are indirect
electrowinning procedures that rely on electrolytic production of
magnesium or sodium for reduction of TiC14. The design and operation
of test cells for direct electrowinning of titanium from TiC14 fed to a
fused salt bath was reviewed in the 1974 NMAB report. Although titanium
sponge (intermeshed crystals) of excellent quality was produced in
pilot-plant cells with a daily titanium capacity of up to 190 lbs, major
design and operating problems were apparent. Among these were:
1. The difficulty of preparing durable diaphragms with limited
permeability and low electrical resistance to divide the cell into
anolyte and catholyte compartments.
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2.
The need to prevent the TiC14 cell f eed from exiting with the
C12 produced at the anode and the limited solubility of TiC14
in the fused chloride salts that requires provision for its promp t
reduction to soluble TiC12 and TiC13.
3. The need to control the average valence of dissolved titanium at
about 2.1 to obtain deposition of premium-grade metal.
4. The need to maintain an inert atmosphere in the cell even when
withdrawing the cathode sponge deposits and inserting new cathodes.
5. The need to minimize fused salt entrainment (dragout) and loss of
dissolved subhalides.
1}H TITANIUM COMPANY
[A recent news item (American Metal Market, December 30, 1982) states
that the expense for completing the joint program and the present
economic climate have forced the dissolution of the D-H Titanium
Company. With the breakup each company is to proceed with developing its
own technologies; Dow will continue research and development work on the
electrolytic process, and Howmet will proceed in the metals fabrication
area. Available data on the D-H process are included here to show where
progress has been made in a process that may in time give an alternative
for producing high grade titanium sponge. ~
With its entry in 1981 into semicommerc~al sponge production at
Freeport, Texas, D-H Titanium, in a close working relationship with the
HOW MET group, became the fourth U.S. integrated producer from sponge to
f inished items. Cell design, operating procedure, metal quality,
proposed production, and economic projections have been described (Cobel
et al. 1980~. Assessments by D-H Titanium, based on its projected
capital and operating costs, are that new electrolytic plants would cost
less to build than new Kroll or Hunter sponge plants and that the
projected operating cost for the D-H titanium electrolytic process is
equal to or below that of fully depreciated Kro11 or Hunter operations.
The semicommercial plant was expected to produce about 100 tons of metal
in 1981. Depending on satisfactory operation, verification of projected
operating costs and the availability of an adequate market, operation at
a rate of up to 5,000 tpy in 1985 was forecast by D-H Titanium.
A major cell improvement is the D-H Titanium design and fabrication
of a metal screen diaphragm that is electroless-plated with cobalt or
nickel to give the required electrical and flow characteristics. The
TiC14 feed is reduced to TiC12 at a separate feed cathode within the
cell. The electrolyte is a eutectic mixture of LiC1 and KC1 containing
about 2 percent TiC12. The preferred operating temperature is about
500°C
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attenti on lately, at least there is an absence of research reports in the
technical press. Included in those processes were gaseous and plasma
reduction, iodide decomposition, calcium and aluminum reduction,
disproportionation of TiC13 and TiC12, and carbothermic reduction.
Starting materials for reduction included halides, carbides, nitride s,
oxides, and sulfides. The use of TiC14 as the dominant feed for
winning titanium metal currently is not challenged.
Transportation of TiC14
Only one of the current domestic titanium sponge producers--TIMET at
Henderson, Nevada--prepares it s own TiC14. The others purchase TiC14
from Gulf and Western's (G&W) pigment plant at Ashtabula, Ohio. The
panel was informed that G&W further purifies its pigment-grade TiC14
before shipping it to sponge metal manufacturers. TiC14 is shipped by
rail in steel tank cars that carry about 55 tons of TiC14, equivalent
to about 14 tons of titanium metal. Cars for transporting chlorine are
suitable f or TiC14. Availability of tank cars solely for TiC14
transport would have to be considered in a national emergency.
Energy Use in Manufacturing Titanium Sponge
The mining, processing, and transportation of rutile and i ts
conversion to sponge titanium metal has been estimated by Battelle
Columbus Laboratories ( 1975) to require an energy expenditure of about
423 million Btu per ton of metal in the Kroll-sponge-leach process and
370 million Btu in the Hunter process. Of these amounts, about 225
million Btu (21,500 kWh) is assigned to the electrow~nning of magnesium
and chlorine in the Kroll process and 245 million Btu (23,400 kWh) to
electrowinning of sodium and chlorine in the Ilunter process. Battelle
assumed that 10,500 Btu is required to generate and deliver 1 kWh of
electricity. The D-lI electrowinni ng process was reported to require
about 16,000 kWh (168 million Btu) for electrolysis per net ton of sponge
(G. Cobel, presentation to the panel, 1980) .
Energy requirements for make-up (to cover processing losses)
magnesium and sodium also were estimated by Battelle (1975) as 110
million and 38 million Btu, respectively. Energy requirements per ton of
sponge for chlorine make-up was estimated as 20 million Btu for
Kroll-sponge leach and 21 million Btu for the Hunter process. No
comparable requirement for make-up reductant metals or chlorine has been
reported for the D-H process. Published information to date does not
permit a comprehensive estimate of total energy use in the D-H process,
but direct current required for electrowinning appears to be only about
half that required for the Kroll and Hunter processes. If it is assumed
that other D-H process steps have energy requirements roughly equivalent
to analogous Kroll and Hunter process steps, total D-H process energy use
would be about 250 million Btu per ton. This is about 60 to 65 percent
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Metal-winning cathodes are individually pulled, stripped, and
replaced in the cell, in an argon atmosphere, by a self-positioning and
automatically operated mechanical device. A sealed, argon-shielded
hopper containing the titanium crystals and entrained electrolyte is
cooled before being opened to discharge its contents. Crystalline metal
and dragout salts are crushed to a 3/8-inch size and leached in dilute
HC1 solution. The solution then is processed to recover LiC1 and KC1 for
return to the cell. Dragout of electrolyte varies with the titanium
crystal size and ranges from about 1/2 lb per lb of titanium for coarse
crystals to about 1 lb per lb of fine titanium crystals. The leached and
washed metal is dried and passed over a magnetic separator, and metal
fines are removed by screening to about 80 mesh (0.007 in. or 177~).
D-H Titanium metal is extremely low in 02, N2, C, Fe, H2, and
C12, and it has a Brinell hardness of 60 to 90.
TDMET Electrowinning Cell
TDMET has operated pilot-plant electrowinning cells since 1956.
Later models produced 800 to 900 lbs of titanium metal in one cathode
deposit. The cell uses a central metal basket cathode with several
internal vertical rod cathodes away from the wall. Graphite anodes are
peripheral to the basket. Operation is cyclic. TiC14 initially is fed
at a low rate into the center of the basket to form a titanium crystal
lattice on the inside of the basket walls. This porous sidewall deposit
serves as a diaphragm to keep the lower chloride inside the basket while
allowing chloride ions to migrate to the anodes. TIMET also developed a
mechanical system for withdrawing the large cathode deposits into an
inert-gas-filled chamber, installing a new cathode, and reclaiming the
inert gas for reuse. No expansion of electrolytic metal production has
been announced by TIMET.
U.S. Bureau of Mines Electrowinning Cell
This cell (not described in the 1974 NMAB report) used a ceramic
diaphragm around the graphite anode as part of a replaceable anode
assembly. A separate cathode feed tube introduced the TiC14 below the
electrolyte level in the cell. Frequent failure of the diaphragm proved
troublesome. High-purity metal with a Brinell hardness of only 70 was
made consistently although a small percentage of minus 60-mesh crystals
in the deposit was much harder (Leone et al. 1967~.
Other Sponge-Winning Technologies
The numerous alternative procedures for preparing titanium crystals,
sponge, powder, or alloys (particularly TiA13) discussed in the 1974
NMAB report as having been examined or as being of conceptual interest in
the future have received little or no additional research and development
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o f the total Kroll and Hunter Btu requirements and about the same as the
244 million Btu (now quoted by the industry at about 180 million)
requirement for aluminum metal made by the Hall process reported by
Battelle (1975~.
Examples of energy use of other metals ~ Battelle Columbus
Laboratories 1975) are 358 million Btu per net ton for magnesium, 112 for
copper, and 65 for zinc. Prices of these metals, and of aluminum, range
from 61.25 to t0.41 per lb. Assuming an average energy cost of $2.00 per
million Btu, the energy component of the price of these metals ranges
from 32 percent for aluminum to 13 percent for copper. In contrast, with
titanium sponge selling for 67.22 per lb, the energy component of the
price is only 6 percent for Knoll metal, 5 percent for Hunter metal, and
3 to 5 percent f or D-H metal.
Appraisal of Titanium Sponge Production Technology
Intricate trade-offs are involved in making a choice between using
vacuum distillation, inert gas sweep with leaching, or leaching alone as
a finishing operation for Kroll-process sponge. Distillation produces
sponge that is lowest in magnesium, magnesium chloride, and hydrogen. It
also is most efficient in recovering magnesium and magnesium chloride for
re-use. However, it is energy-intensive and ties up expensive retorts
over a prolonged time cycle, and there reportedly is some difficulty in
crushing and grinding the low-chloride sponge.
The inert gas sweep followed by leaching reportedly produces sponge
somewhat higher in volatiles than does distillation. Also, it recovers
less magnesium and magnesium chloride and requires the disposal of a
small quantity of waste bri net However, it is less energy-1 ntensive than
distillation because it uses longer batches and the retort cycle time is
shorter.
The all-leaching process yields sponge that is somewhat higher in
volatiles than the sweep-leach procedure. The all-leaching process has
no retort cycle t ime f or removal of chlorides and would appear to require
the least energy. This is counteracted by its high loss of magnesium and
magnesium chloride that translates to a high energy requirement for
make-up magnesium and chlorine. There also is the disposal of a large
quantity of waste brine to be considered. Moreover, the process also may
be more vulnerable to nitride inclusions that require triple melting to
minimize lo~density inclusions in ingots.
Chlorides, reductant metals, and hydrogen remaining in leached sponge
can be removed during vacuum arc melting. TIBET, as an integrated
producer, has installed enough vacuum capacity and pup protection to
cope with a high level of volatiles during arc melting. In fact, TIMET
considers its f irst melt operation as part of sponge ref ining.
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Nonlntegrated producers generally have melting furnaces designed for
low~volatile foreign sponge. Such furnaces can melt high-volatile sponge
but at a reduced rate and with added vacuum pumping and maintenance
requirements. (This subject is discussed in greater detail in the next
section of this chapter. ~
Launching of the D-H Titanium Company's electrowinning demonstration
plant at a scale of up to 1 million lbs per year is a major investment in
new technology. However, full production success and the economic
competitiveness of direct electrowinning of titanium are still to be
established. Substantial add-on sponge production by U.S. Kroll and
Hunter plants in the past two years can be taken as confirmation of the
operators' expressed beliefs that the competitiveness of Kroll and Hunter
technology is not threatened by direct electrowinning, at least in terms
of costs for expansion of existing plants. Establishment of completely
new (greenfield) plants, however, may well be based on new or improved
technologies such as vacuum distillation or electrolytic processes.
A trend toward increasing the batch size in the Kroll process serves
to improve metal quality by shrinking the ratio of pot surface to sponge
weight and reduces handling costs as at OREMET. Kroll-type metal was
first produced domestically in large batches with vacuum distillation in
1981 at Teledyne Wah Chang Albany and a similar operation began early in
1982 at ITI's Moses Lake, Washington plant. More efficient production of
titanium, with a possible price reduction, should stimulate the use of
titanium in traditional as well as newer sectors as noted in Chapter 10.
U.S. sponge production technology could profit from development of
new concepts for: improved leaching or other means of purification of
the sponge; recovery of anhydrous magnesium chloride from sponge leach
brine; and direct preparation of (or conversion of sponge to)
high-quality, low-cost, titanium powder for powder metallurgy. A
continuous sodium-reduction process might be a possible route to
production of powder.
Ti tanium Sponge Quality and Specif ications
Technically, the ideal specification for raw titanium would be 100
percent titanium. This would allow maximum scrap use plus master alloy
additions and melting to any desired ingot chemistry. The iodide process
could produce 99.9 percent or better titanium but at uneconomical cost.
Each of the currently economical processes--Kroll, Hunter, and,
hopefully, electrolytic--has its own characteristic impurities. These
can be categorized into two groups: nonvolatiles like oxygen and iron
and those that boil off during melting, notably sodium and sodium
chloride in Hunter sponge and magnesium and magnesium chloride in the
Kroll product. Each of the processes (Kroll, Hunter, and electrolytic)
involves growing the titanium dendrites in molten chloride baths. The
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f ree portions of these baths subsequently can be drained and then
volatilized under partial pressure, leached, or vacuum disti lied
completely away f ram the titanium. However, traces of the chloride
baths, which alsp may contain dissolved, stoichiometrically excess sodium
or magnesium, invariably are trapped among the dendrite branches of the
titanium crystals as they grow and interlock. These traces thereby
become hermetically encapsulated to the extent that neither draining,
helium sweeping, leaching, or even vacuum distillation can remove them
completely. These irremovable residues then cause significant problems
in subsequent vacuum arc melting and in the arc welding of consolidated
powder metallurgy products (as described in the direct-powder
consolidation processes covered in Chapter 11~.
Even though neither leaching nor vacuum distillation can remove all
volatiles, vacuum distillation does remove considerably more than does
leaching. This significantly simplifies and improves subsequent vacuum
arc melting. Worldwide, therefore, vacuum distillation has become the
preferred by-product removal process. It first was piloted by the U.S.
Bureau of Mines and then commercialized by du Pont in 1948 and by the
Japanese and Soviets in the 1950s and 1960s. Teledyne Wah Chang Albany
was the second U.S. manufacturer of vacuum distilled titanium sponge and
ITI now also vacuum distills all its sponge production.
Electrolytic titanium crystals apparently encapsulate less
chloride~etal bath traces than either the Kroll or Hunter processe s .
With further development, leached electrolytic titanium may rival the
low-volatile, vacuum-distilled product . All of the volatiles boil of f
during vacuum melting and, therefore, are not present to affect the
properties of mill products made from ingots. Their presence, however,
does adversely affect the melting process both in the casting of ingots
and in the making of arc welds in powder metallurgy products.
In the vacuum arc melting of sponge to ingots, melting ef f iciency is
reduced by the evolution of the volatiles. Volatiles also condense on
the mold and mar the ingot surface, increase furnace maintenance, and
require that much larger vacuum pumping systems be used. Nevertheless,
using pioneer facilities built in the 1950s, it was found economical to
employ vacuum melting with steam ejectors to back up large vacuum
dif fusion pumps as the f inal refining step for acid-leached sponge. lrhe
refining was an integral part of the melting and casting of the ingot.
Such re f ining-melting f urnaces served well to start the industry .
The process, however, wastes magnesium and chlorine and requires their
safe ~ and increasingly expensive) disposal. Escalating costs of
magnesi um and thermal energy also have shifted the economic advantage to
larger reactors that conserve and apply the heat of reaction to vacuum
distillation and economical recovery of magnesium and magnesium chloride.
A further consideration of national concern is that the independent
U.S. melters (who do not manufacture but rather purchase their sponge as
noted in Chapter 3 ~ are not equipped to melt high-volatile sponge. The
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independents, therefore, greatly prefer and have largely confined their
purchases to Japanese and Soviet low-volatile, vacuum-distilled sponge.
Since the independent U.S. melters have almost one-third of the nation's
titanium melting capacity, and since the U.S. National Stockpile contains
only high-volatile sponge, the U.S. titanium industry is not properly
positioned with respect to melting for a national emergency. (Stockpile
specifications are discussed in this regard in Chapter 6. ~
The melting that removes essentially all volatiles f ram even
high-volatile titanium sponge functions similarly when consolidated forms
made from titanium sponge powder are arc welded--the chlorides disrupt
the welding arc. This adverse feature has blocked the commercialization
of tonnage titanium powder metallurgy for the past two decades (see Chapter ll)
There are two public U.S. specifications for sponge titanium; American
Society for Testing Materials (ASTM) B-299, and the U.S. National
Stockpile Purchasing Specification P-97-R6 (the former is being updated).
The salient features of the sponge grades defined in these specifications
and correlations between grades and types are shown by the data of
Table 3. Some of the features of Japanese and Soviet sponge are compared
with U.S. specification data in Table 4. Detailed excerpts from ASTM
B-299 and Stockpile Specification P-97-R6 are given in Appendix H.
The U. S. National Stockpile Specification shows that there is both
premium-grade (lB-O) and standard-grade (lA-O) magnesium-reduced,
vacuum-distilled metal. Compared with the standard grade, the premium
grade is lower in iron (0.05 versus 0.12 percent), oxygen (0.07 versus
0.10 percent), and Brinell hardness (100 versus 120). Compared with
magnesium-reduced, leached sponge, the standard-grade distilled sponge is
lower in carbon (0.02 versus 0.025 percent), magnesium (0.08 versus 0.50
percent), chlorine (0.12 versus 0.20 percent), and hydrogen (0.005 versus
0.03 percent) but is nigher in iron (0.12 versus 0.10 percent). Compared
with sodium-reduced leached sponge, the standard-grade distilled sponge is
lower in chlorine (0.12 versus 0.20 percent) end hydrogen (0.005 versus
0.05 percent) but higher in nitrogen (0.015 versus 0.010 percent) and iron
(0.12 versus 0~05 percent).
The differences in oxygen and iron contents and in Brinell hardness
between the premium and standard grades in Kroll-process sponge are
believed to be a function of the location of the sponge in the reactor
pot. As has been noted, the interior sponge is lowest in iron, oxygen,
and Brinell hardness. It is understood that Soviet, Japanese, and U.S.
manufacturers use center-region sponge for premium-grade sponge.
Sodium-reduced sponge characteristically is considerably lower in iron
than magnesium-reduced sponge. This is principally due to iron in the
electrowon magnesium charged into the reduction part. The magnesium
usually contains 0.03 to 0.04 percent iron that transfers to the titanium
sponge.
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TABLE 3 Nomenclature and Features of Selected Sponge Titanium Grades Defined
in ASl~l B-299 and National Stockpile Purchase Specification P-97-R6
Items Nomenclature and Features
Grade lA-O LA-O lA-O lB-O
P -9 7-R6 Type A 8 C A
B-29 9 Type MI)-120 ML-120 SL-120 M1)-120
Produc Lion f eature s,
reductant and finishing
operation
Mg reduced Mg reduced
and vacuum and acid
dis tilled leached or
inert gas
sweeping
Na reduced Mg reduced
and acid and vacuum
leached distilled
Selec ted im
puri ties content Fe 0 .12 0 .10 0.0 5 0. 05
max. weight O 0.10 0.10 0.10 0.07
perc ent
Volatile, max.
weight percent Mg 0.08 0.50 0.08
Na -- -- 0.19 -
C1 0.12 0.20 0.20 0.12
H 0.005 0.03 0.05 0.005
H2O 0.02 0.02 0.02 0.02
Total 0.225 0.75 0.46 0.225
Nominal titanium
content, weight percent 99.3 99.1 99.3 99.3
The ASTM specification is slightly more liberal with respect to
impurities. Except for industrial use requiring optimum corrosion
resistance or for rotating parts in aerospace engines, most of the sponge
meeting the ASTM specification is suitable for a wide range of
applications provided it is properly melted, alloyed, and processed.
Premium-grade sponge, including interior-location, magnesium-reduced
sponge and sodium-reduced sponge, best meets the requirement for low iron
content essential for corrosion resistance.
Although ingot and fabrication shops that do not produce their own
sponge prefer to buy low-volatile foreign sponge that is easier to melt in
times of sponge shortage, domestic high-volatile sponge has been melted in
these shops with some penalty in melting rate and added equipment
maintenance . Sodium-reduced leached sponge i s the easiest to grind f or
powder metallurgy applications; however, its relatively high volatiles
content limits its use in some powder applications.
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TABLE 4 Comparison of Selected Grades of Japanese and Soviet Sponge
Titanium with U.S. Specification Data
It ems
Specif ication No.
Conforms to:
P -9 7-R6: Or ade lB-O lA-O
Type A A
B-299 'Type MD-120
Production f eatures
Se lected
Impurities FE
Content O
(weight percent ~ N
Nomenclature and Feature ~
,
Japanese U. S. Specif ication Soviet
B-299 and P-97-R6 MRTU-14
lB-O lA-O*
A A
MD-120 MD-120 MD-120
Magnesium reduced and vacuum distilled
0.02 -0.05 0.12 max. 0.05 max. 0.07 max.
0.04 -0.05 0.10 max. 0.07 max. 0.04 max.
0.005-0.008 0.015 max. 0.015 max. 0.02 max.
Chlorine as a
volatile
(weight percent) 0.07 - 0.09 0.12 max. 0.12 max. 0.08 max.
Average hardness as
once melted. (BHN
or max. BHN) 97 - 99 120
~ 100 (100)
Nominal Ti content
(weight percent) 99.8 99.3 99.3 NA
*Conforms to P-97-R6, Grade lB-O, Type A, in most categories.
As indicated by the lack of specification coverage, titanium produced
by the electrolytic process is not included in either of the public
specifications; indeed, there is a dearth of public experience in the
matter of defining the quality and characteristics of electrolytically won
titanium sponge. If the production of such material becomes sizable, the
qualification testing that leads to specifications would be conducted.
Such testing almost certainly would show the suitability of the material
for the wide spectrum of uses where thermal-chemically reduced sponge
grades now are applied (unpublished preliminary evaluations by D-H
titanium already have indicated this to be the case).
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The sponge titanium used in the United States to make ingot is covered
under the specifications whether it is of domestic or foreign origin and
has been ranked according to the degree of melting difficulty in terms of
the quantity of volatiles that boil off during melting. The sponge grades
with the fewest volatiles generally are the least difficult to melt. The
ranking in order of least to most difficult to melt is listed in Table 5.
TABLE 5 Ranking of Sponge Titanium in Terms of Ease of Melting
.
Sponge Description Ranking _ ASTM Grade
~ _
Vacuum distilled, Soviet Least difficult
Vacuum distilled, Japanese
Vacuum distilled, Chinese
Inert gas sweep, U.S. Producer
Leached sponge, British
Leached sponge, U.S. Producer
Leached sponge, U. S. Producer
1 1
Most difficult
MD-120
MD-120
MD-120
ML-120
SL-120
SL-120
ML-120
Specifications are most important in the ultimate utilization of
materials, especially those like titanium i n which trace impuri ties often
have important effects on processing characteristics and on the physical
properties of end products. The effects of volatiles in titanium sponge
on melting and welding were discussed above. The effects of trace and
alloying elements on the properties of titanium metal are described in
Chapters 7 and 8. Chapter 6 discusses, among other aspects, the key role
of specif ications with respect to the U. S. National Stockpile of titanium.
REFERENCES
Battelle Columbus Laboratories. 1975. Interim Report on Energy Use
Patterns in Metallurgical Processing. Columbus, Ohio.
May Cobel, G., J. Fisher and L. Snyder. May 1980. Electrowinning of
titanium from titanium tetrachloride. Paper presented at the 4th
International Conference on Titanium, Kyoto, Japan.
Leone, O . Q., J. Knudson and D . Couch. 1967 . High purity titanium
electrowinning from titanium tetrachloride. JOM Vol. 20, 18-23.
National Materials Advisory Board Committee on Direct Reduction
Processes for the Production of Titanium Metal. 1974. Report
NMAB-304 , Washington, D. C.: National Academy of Science s.
Tukomoto, S., E. Tanaka and K. Agisu. 1975. The deposition of titanium
metal by fusion electrolysis . JOM, Vol. 28, 18-22 .
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Representative terms from entire chapter:
vacuum distillation
