75    SELECTIVE PRECIPITATION OF TITANIUM FROM AN
HF/HN03 MIXED ACID SOLUTION
Steven P. Rivers, Chemical Engineer
U.S. Bureau of Mines
Rolla Research Center
Rolla, Missouri 65401
INTRODUCTION
The U.S. Bureau of Mines (USBM) has been conducting research on the stainless steel industry's HF- .
HNO3 pickling solutions for a number of years.1 As environmental regulations become more stringent
on the disposal of spent or waste solutions, research has focused on minimizing acid consumption,
recovery of a valuable metal, and recycling of the mixed acid. As the research progressed, it was
expanded to look at the solutions that were being used in the Ti milling and pickling industry.
Titanium, because of its desirable traits of high strength-to-weight ratio, its resistance to corrosion,
and its light weight, makes a valuable metal for the aerospace industry and other critical applications.
A domestic source of Ti that is being lost is the Ti in solution in these mixed acids. Typically, chemical
milling or pickling processes use a mixed-acid bath to remove unwanted material from a part or a
sheet. A typical industrial mixed acid pickling solution would be 4.5 M HNO3 and 1.13 M HF.2"3 In.
time, the bath would load up with Ti and become F" deficient, slowing down the dissolution rate.
Nitric and hydrofluoric acid would then be added periodically to extend the bath life. USBM has
found that a spent Ti-6A1-4V alloy solution would contain as much as, in g/L, 40 to 60 Ti, 2 to 3 Al,
1.5 to 2 V, 0.5 to 2 Fe, < 0.2 Cr, 20 to 100 total F", and 280 to 425 total NOf. The bath would then be
neutralized and discarded, losing the Ti and the costly HF-HNO3 acid.
Conventional technology, such as the Kroll and Hunter Process, used to recover Ti from rutile (Ti02),
is a noncontinuous, multistep, energy-intensive process.4 USBM has been working with the Ti industry
for years in developing processes to extract Ti from low-grade ores, develop powder metallurgy, and
reclaim or recycle Ti scrap. Research conducted at USBM Centers in Albany, OR, and Salt Lake City,
UT, have dealt with the recovery of Ti from double salts of Ca and K, respectively. In research at Albany
Research Center (ALRC), ilmenite was leached with HF to produce a solution containing Ti and Fe.5
After removing the Fe by solvent extraction, a purified solution containing TiF4 remained. Calcium
carbonate (CaCOj) was added to the purified solution and heated to precipitate calcium fluorotitanate
(CaTiFj). This fluorotitanate was reduced in an induction slag furnace with Ca as the reductant to
produce metallic Ti.6 Research at Salt Lake City Research Center focused on generating a Ti powder by
the reduction of TiF3 and double salts of K2TiF6 and K2TiF5 with K.7 With the previous USBM research
in mind, it was decided to see if a fluorotitanate could be selectively precipitated from the mixed acid
solution to recover the Ti as a double salt and regenerate the mixed acid.
DISCUSSION AND EXPERIMENTAL RESULTS
Work began using an actual spent industrial-mixed acid solution containing, in g/L, 44 Ti, 18 Fe,
97 F~, and 785 N03". Preliminary experiments attempted to form an NH4, Ca, or Na fluorotitanate
precipitate by adjusting the pH of the solution to 2. The results of the initial tests are included in Table
I. Attempts to precipitate a fluorotitanate with either NH4 and Ca hydroxide to adjust the pH,
resulted in low-Ti extraction and the selectivity between the Ti and Fe was low. Using NaOH to adjust
the mixed acid solution to a pH of 2 produced a white precipitate, which contained 16.6 wt % Ti and
2.7 wt % Fe. This represents extraction of 52.5% of the Ti and 10.6% of the Fe contained in the
mixed acid solution.
Experiments continued in an effort to enhance the precipitation technique used on the industrial
solution. Calculations showed that the fluorine content in the spent solution was insufficient for the
stoichiometric formation of Na2TiF6; therefore, NaF was added to the solution before adjusting the
pH to provide sufficient F". The experiments were performed with 50 mL aliquots of the solution at
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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