Title of Invention

A PROCESS FOR RECOVERY OF FLUORIDE CHEMICALS FROM THE LEACH LIQUOR GENERATED IN THE REFINING OF LOW-GRADE MOLYBDENITE CONCENTRATE

Abstract The present invention relates to a process for recovery of sodium silicofluoride and cryolite from low-grade molybdenite concentrate. This invention particularly relates to a process for selective recovery of sodium silicofluoride and cryolite from the solutions containing hydrofluosilicic acid, hydrofluoaluminic acid, hydrofluoferric acid, copper and nickel generated during refining of low-grade molybdenite concentrate.
Full Text The present invention relates to a process for the recovery of fluoride chemicals from the
leach liquor generated in the refining of low-grade molybdenite concentrate
This invention particularly relates to a process for selective recovery of sodium
silicofluoride and cryolite from the solutions containing hydrofluosilicic acid,
hydrofluoaluminic acid, hydrofluoferric acid, copper and nickel generated during refining
of low-grade molybdenite concentrate
The only cryolite mine at luigtut, Greenland, was mined until 1990. Cryolite, apart from
ite use in smelting of aluminium, is used in the glass and ceramic industries, as an
insecticide on grape vines, as a binder for grinding wheels, and as a catalyst for the
polymerisation of alkenes. The world production of cryolite is of the order of 4xl05 t
year"' and likely to grow further. It is mainly produced from fluorspar, which involves the
neutralisation of hydrofluoric acid, produced from fluorspar with soda ash and hydrated
alumina in the proportions occuring in cryolite (Singh, G. and Kar, B.C., NML Tech. J.,
1965, Vol. - 7, No. - 3, pp.21-24)
Reference may be made to a US patent [No. 4,178,347, Dec. 1979] wherein a process for
the simaltaneous production of phosphoric acid and sodium silicofluoride by a wet
process is developed. The drawbacks are (i) use of both sulphuric acid as well as sodium
salts for manufacturing process and (ii) large number of processing steps involved.
Another US patent [No. 4,765,822, Aug. 1988 may be cited] wherein fluoride containing
gases are converted into fluosilicic acid or fluosilicate salts and subsequently it is treated
with alkaline substance which forms fluoride salt. Sodium hydroxide or sodium carbonate
are the preferred alkaline substances when sodium cryolite or aluminium fluoride are
produced. Drawbacks of the process are the large number of processing steps and its
limited applicability with respect to molybdenite enrichment process which is the subject
matter of the present invention.
Several other methods have been developed to produce cryolite and sodium silicofluoride
from different sources. Some of these methods include the recovery of fluorine from
industrial gases such as (i) effluent gas from phosphate processes, (ii) fluorine gas of the
electrolysis bath of aluminium shops etc. The recovery of fluorine from phosphate rock
suitable for production of cryolite has been limited by technological and economic
problems (Hill, W. L. and Jacob, K.D., Mining Eng. 6, pp. 994-1000 October 1954). In
this process hydrofluosilicic gas is first condensed to liquid and reacted with hydrated
alumina, the hydrofluoaluminic gas is then neutralised to produce cryolite.
Hexafluosilicic acid is mostly available as a by-product of the phosphate-based fertilizer
industry from the action of sulphuric acid on phosphate rock (fluorapatite) containing
fluorides and silica, or silicates (Industrial Inorganic Chemicals : Production and uses,
Edited by R. Thompson, pp. 199 - 229, Pub. : The Royal Society of Chemistry, 1995).
The hydrofluosilicic acid (H2SiFe) is neutralised by soda to prepare
metallurgical/chemical grade sodium silicofluoride(Na2SiF6). Its importance lies in
berylium industry, fluoridization of municipal water supply for dental health (Indian
Mineral Year Book 2000), laundry sour, polishing/etching additive for glass
manufacturing, preparation of pure SiF4 etc.
In order to enrich the molybdenum content of the molybdenite concentrate, attempts were
made to develop the process to remove gangue minerals by hydrometallurgical technique
using hydrochloric acid and hydrofluoric acid. Reference may be made to (Mankhand,
T.R. and Prasad, P.M., Met. Trans. B, 13B, 1982, pp. 275-282) wherein molybdenite
concentrate containing 41% Mo is enriched to a level of over 57%. The leach liquor so
generated is a waste solution and contains valuable fluoride chemicals such as HsAlFe,
HaSiFe, HsFeFa and unreacted HF along with small quantities of Cu and Ni. Reference
may be also may be made to (Saha A.K., Srinivasan S.R. and Akerker D.D. NML
Technical Journal, Vol. 27, Aug. & Nov., 1985, pp. 47-56) wherein low-grade
molybdenite containing molybdenum 40.87% was enriched to high grade molybdenite
containing molybdenum about 54% and thereby generating leach liquor containing
hydrofluosilicic acid. Therefore, the recovery of fluoride chemicals will obviate the
pollution problem associated with the molybdenum enrichment process and also improve
the process economics. This will supplement the supply of cryolite and sodium
silicofluoride in view of large consumption of cryolite (3500 tonnes/year) whereas
production of sodium silicofluoride in the country is about 6000 tonnes/year. Recently
studies on precipitation of sodium silicofluoride and cryolite were carried from theindividual fluoride solutions. Reference may be made to [Nani Babu, M.Tech. Thesis, Mat. Science, BHU, Varanasi 1999] wherein upto pH 4.0, cryolite is precipitated and upto pH 5.0, sodium silicofluoride is precipitated from the respective individual solutions. But, separation of the said fluoride chemicals from leach liquor is not attempted. The development is all the more important in view of the fact that no report appears to be available on recovery of fluoride chemicals in the processing of molybdenum concentrate.
The main object of the present invention is to provide a process for recovery of fluoride chemicals from the leach liquor generated in the refining of low-grade molybdenite concentrate.
The another object of the present invention is to provide a process for preferential precipitation of sodium silicofluoride over sodium aluminofluoride (cryolite) from the waste solution containing various amounts of hydrofluosilicic acid, hydrofluoaluminic acid, hydrofluoferric acid, copper and nickel obtained as waste solution during refining of low-grade molybdenite concentrate.
Accordingly the present invention is to provide a process for recovery of sodium silicofluoride and cryolite from low-grade molybdenite concentrate which comprises:
a) adding 8-12M of hydrochloric acid to low-grade molybdenite concentrate containing hydrofluosilicic acid, hydrofluoaluminic acid hydrofluoferric acid, copper and nickel,
b) adding 0.5-5 M of sodium salt solution to the above said resultant solution, at a pH level ranging between 0.5-1.5 followed by stirring for a period of 1-2 hrs to precipitate the desired sodium silicofluoride,
c) filtering, washing and drying the above said precipitate at a temperature in the range of 100 - 150°C for a period of 2-4 hrs,
d) increasing the pH level of raffinate in the range of 1 - 4 by adding 0.5-5 M of sodium salt solution to the above said resultant raffinate solution and precipitating the

sodium aluminofluoride, at a temperature in the range of 30-70 °C under stirring for a period of 1-2 hrs. e) filtering, washing and drying the precipitate at a temperature in the range of 100-
150°C for 2-4 hrs. In an embodiment of the present invention the leach liquor generated in the refining of low-grade molybdenite concentrate has the following composition
Hydrofluosilicic acid : 0.1- 0.32M
Hydrofluoaluminic acid : 0.02 - 0.12M
Hydrofluoferric acid : 0 - 2M
Copper : 0 - 0.02M
Nickel : 0 - 0.03M
In still another embodiment the sodium salt used is selected from commercial grade sodium carbonate and sodium hydroxide.
Novelty of the present invention is recovery of sodium silicofluoride and sodium aluminofluoride(cryolite) from the complex solutions. Another novel feature of this invention is the selective recovery of high purity sodium silicofluoride from solution containing hydrofluosilicic acid, hydrofluoaluminic acid, copper and nickel. The inventive step of the invention is the selective precipitation of sodium silicofluoride at a specific pH by the neutralisation of hydrofluosilicic acid with the alkali. Also the formation of sodium chloride by the reaction of sodium carbonate with hydrochloric acid help in the precipitation of sodium silicofluoride as shown in Reaction (2) and (3) which aids in crystallisation of the precipitation. Another inventive step is the selective recovery of cryolite from the solution containing Cu and Ni.
H2SiF6 + Na2CO3 ► Na2SiF6 _ CO2 + H2O (1)
Na2CO3 + HCl ► 2NaCl + H2O + CO2 (2)
H2SiF6 + 2NaCl ► Na2SiF6 + HCl (3)

With no loss of copper and nickel during the precipitation of sodium silicofluoride and sodium aluminofluoride, both (Cu and Ni) on separation and recovery will improve the process economics.
The following examples are given by way of illustration and should not be construed to limit the scope of invention.
Example -1
100 ml of the leach liquor containing 0.06M H3A1F6 and 0.32M H2SiF6, 120ml of 1M Na2C03 is added. The desired pH is maintained with the addition of requisite quantity of cone. HC1 and monitored by pH meter. Results of the typical experiments for selective precipitation are given in Table 1. Selective precipitation of sodium silicofluoride is obtained for equilibrium pH at 1.35. The precipitate is filtered, washed and dried in an electric oven kept at 110°C for more than 2 hours.They are characterized by XRD phase identification. Similar result is observed if sodium hydroxide is used as precipitant.
Table 1
(Table Removed)
Example 2
100 ml leach liquor containing 0.06M H3A1F6, 0.32M H2SiF6, with 1M H3FeF6 in absence or in presence of 0.009M Cu, and 0.013M Ni are housed in the water bath maintained at 30°C. Selective precipitation is attempted using 1M Na2CO3 as the precipitant. The recovery of sodium silicofluoride is found to be more than 80% at 1.35 pH. Precipitates are dried in oven at 110°C for more than two hours and are characterised for the phase identification by XRD. Results of selective precipitation of sodium silicofluoride from the leach liquor are summarized in Table 2. Recovery of pure sodium silicofluoride
without any contamination with cryolite shows selective precipitation of Na2SiF6.
Table 2
(Table Removed)

Example - 3
To the 50 ml leach liquor containing 0.32M H2SiF6 and 0.06M H3A1F6, 0.009M Cu,
0.013M Ni etc. 3M Na2C03 is added at 30°C. Experiments are conducted to arrive at the
quantity of excess precipitant required for achieving the precipitation without diluting the
raffmate. Corresponding quantity of HC1 to get 1.35 pH is also determined. Results of
recovery of Na2SiFe from some of the experiments using different amounts of precipitant
are given in Table 3. With 40% excess precipitant in presence of 9.25 ml of
cone. HC1(11.2M), 100% recovery of Na2SiF6 is achieved at 30°C and 1.35pH, without
any trace of cryolite in the product. All experiments were done for 2 hours.
Table 3
(Table Removed)

Example 4
The stoichiometric amount (10.66 ml) of 3M sodium carbonate is slowly added to 100ml
of leach liquor (0.32M H2SiF6 and 0.06M H3A1F6) in a PVC vessel. The pH of the leach
liquor is adjusted to 1.35 by adding requisite quantity of cone. HC1. The PVC beaker is
then kept in a water bath maintained at different temperatures. The precipitates obtained
are filtered, washed and dried in an electric oven at 110°C for more than two hours. The
results of the experiments of precipitation of sodium silicofluoride at different
temperatures are given in Table 4. Results show that with increase in temperature the
recovery of Na2SiF6 increases upto 50°C beyond which it starts decreasing. Using
stoichiometric amount of precipitant, a maximum precipitation of 64.26% of sodium
silicofluoride is obtained.
Table 4
(Table Removed)

Example 5
A 300ml leach liquor with 0.32M H2SiF6 and 0.06M H3A1F6 is taken in the PVC vessel,
to which 32ml of 3M Na2CO3 is added to get complete precipitation of sodium
silicofluoride. From the raffmate, experiments are performed to precipitate cryolite at
different temperatures. To 50ml of raffmate solution 12 ml of 3M sodium carbonate is
added to get pH 4.0. This solution is kept in hot water bath maintained at determined
temperatures for In. The results of precipitation of sodium aluminofluoride at different
temperatures are given in Table 5 show that increase in temperature increases the
recovery of cryolite upto 55°C and beyond this the recovery of cryolite decreases.
Maximum recovery of 94.56 % is obtained at 55°C.
Table 5
(Table Removed)

Example 6
To 100 ml leach liquor containing 0.32M H2SiF6, 0.06M H3A1F6, 1M H3FeF6, 0.009M
Cu, and 0.013M Ni etc, 38ml of 3M Na2CO3 is added slowly at room temperature.
Complete precipitation of sodium silicofluoride is obtained at pH 1.50 which is
confirmed by XRD identification. The filterate after precipitation of sodium silicofluoride
is used for precipitation-recovery of sodium aluminofluoride. On increasing the pH
slowly beyond 1.50 by the same precipitant it is observed that an iron bearing compound
starts interfering with the precipitation of cryolite especially at around a pH of 2.20. But
due to comparatively very low concentration of H3AlFe in the leach liquor, Na3AlFe is
not detected by XRD during the phase identification on the sodium ferricfluoride. In
order to avoid iron contamination problem in cryolite recovery, one of the options is to
extract iron at low pH(~1.0) by solvent extraction using tributylphosphate(10%TBP in
kerosene) prior to the recovery of sodium silicofluoride and cryolite. Results of selective
precipitation of sodium silicofluoride from mixed synthetic solution are given in Table 6.
Table 6
(Table Removed)

The major advantages of the present invention are given here:
1. Cryolite and sodium silicofluoride can be recovered from any fluoride solutions if
these are present in the form of hydrofluosilicic acid and hydrofluoaluminic acid.
Thus, cryolite as well as sodium silicofluoride for metallurgical and other
applications may be produced by this route.
2. Sodium silicofluoride can be recovered by selective precipitation from the leach
liquor a waste solution obtained from refining of the low-grade molybdenite
concentrate.
3. Recovery of sodium silicofluoride and cryolite will make the refining of lowgrade
molybdenite process economical and ecofriendly.
4. Recovery of these fluoride chemicals will make the raffinate free from fluoride to
recover copper and nickel by solvent extraction
5. Recovery of these value added products from the leach liquor of molybdenite
refining may result in turning the whole process attractive with zero waste
generations.


















We Claim
1. A process for recovery of sodium silicofluoride and cryolite from low-grade
molybdenite concentrate which comprises:
a) adding 8-12M of hydrochloric acid to low-grade molybdenite concentrate containing hydrofluosilicic acid, hydrofluoaluminic acid hydrofluoferric acid, copper and nickel,
b) adding 0.5-5 M of sodium salt solution to the above said resultant solution, at a pH level ranging between 0.5-1.5 followed by stirring for a period of 1-2 hrs to precipitate the desired sodium silicofluoride,
c) filtering, washing and drying the above said precipitate at a temperature in the range of 100 - 150°C for a period of 2-4 hrs,
d) increasing the pH level of raffinate in the range of 1 - 4 by adding 0.5-5 M of sodium salt solution to the above said resultant raffinate solution and precipitating the sodium aluminofluoride, at a temperature in the range of 30-70 °C under stirring for a period of 1-2 hrs.
e) filtering, washing and drying the precipitate at a temperature in the range
of 100-150°C for 2-4 hrs.
2. A process as claimed in claim 1, wherein the refining of low-grade molybdenite
concentrate has the following composition
Hydrofluosilicic acid : 0.1-0.32M
Hydrofluoaluminic acid 0.02 - 0.12M
Hydrofluoferric acid 0 - 2M
Copper : 0 - 0.02M
Nickel : 0 - 0.03M
3. A process as claimed in claims 1, wherein the sodium salt used is selected from
commercial grade sodium carbonate and sodium hydroxide.
4. A process for recovery of sodium silicofluoride and cryolite from low-grade molybdenite concentrate substantially as herein described with reference to the examples.

Documents:

797-DEL-2002-Abstract-(01-07-2010).pdf

797-del-2002-abstract.pdf

797-DEL-2002-Claims-(01-07-2010).pdf

797-del-2002-claims.pdf

797-DEL-2002-Correspondence-Others-(01-07-2010).pdf

797-del-2002-correspondence-others.pdf

797-del-2002-correspondence-po.pdf

797-DEL-2002-Description (Complete)-(01-07-2010).pdf

797-del-2002-description (complete).pdf

797-del-2002-form-1.pdf

797-del-2002-form-18.pdf

797-DEL-2002-Form-2-(01-07-2010).pdf

797-del-2002-form-2.pdf

797-DEL-2002-Form-3-(01-07-2010).pdf

797-del-2002-form-3.pdf


Patent Number 243387
Indian Patent Application Number 797/DEL/2002
PG Journal Number 43/2010
Publication Date 22-Oct-2010
Grant Date 11-Oct-2010
Date of Filing 31-Jul-2002
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110 001,INDIA
Inventors:
# Inventor's Name Inventor's Address
1 MANOJ KUMAR NATIONAL METALLURGICAL LABORATORY,JAMSHEDPUR,JHARKHAND,INDIA
2 BANSHI DHAR PANDEY NATIONAL METALLURGICAL LABORATORY,JAMSHEDPUR,JHARKHAND,INDIA
3 TILAK RAJ MANKHAND BANARAS HINDU UNIVERSITY,VARANASI,UTTAR PRADESH,INDIA
PCT International Classification Number C01F 7/50
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA