FIELD OF THE INVENTION
The present invention relates to the recovery of rare earth elements from blast furnace slag using hydrochloric acid to extract lanthanum, cerium, and neodymium. The multi step process involves digesting blast furnace slag containing rare earth elements with a dilute HCL solution while leaving calcium and silica in the blast furnace slag substantially undissolved; separating the solution obtained from the solid residue and recovering the rare earth elements from the solution by ion exchange cum precipitation using amide based resin, and recycling of spent acid in next cycle of leaching as stated in the flowsheet (Fig.1). The rare earth product obtained was essentially a composite of La-Ce-Nd with >93% purity which was confirmed by chemical analysis and SEM (Fig.2). The extraction of rare earth elements from blast furnace slag was benign enough to ensure no alteration to the original matrix of blast furnace slag, making it still competent for cement making.
BACKGROUND OF THE INVENTION
Blast furnace slag is a co-product formed in the process of iron making which consists of silicates, calcium oxide, alumina silicates as well as very traces of rare earth elements. It comprises of about 20% by mass of iron production. Among the various types of blast furnace slag generated, the air cooled blast furnace slag is slowly cooled under atmospheric conditions forming a crystalline structure. This crystalline structure renders its application in construction materials. There are very scanty examples of iron bearing raw materials being used in extraction of rare earths and there hasn’t been any work so far cited in literature to extract these minor rare elements from blast furnace slag. Extraction of rare earth elements from blast furnace slag with acids might destroy the amorphous nature of the slag, thus requiring less stringent and more amenable conditions to cause relinquishment of costlier rare earths from calcium-alumino-silicate matrix

Among the few similar raw materials explored for such a work are the monazite, bastanite, apatite concentrate, where in chemical leaching methods have been applied. Some of the research works are discussed below.
An attempt to extract REEs was tested by Kumari et al (2012) with Korean monazite, yielding 95% rare earth hydroxides by acid leaching method using 6 N HCl at 90oC and 60 g/L pulp density. Another trial by Kumari et al (2014) with Korean monazite induced milling mixture of monazite and NaOH allowing the formation of rare earth hydroxides and it also enables the yield of La and Nd at room temperature after water leaching. Bastanite, a very major mineral of rare earths was treated in another study by Li et al (2012) with roasted and unroasted samples in 1-3 M H2SO4 in the presence of thiourea, the dissolution of rare earth elements was increased considerably from 17.6% to 89%. Another widely used raw material viz., apatite concentrate, was leached with nitric acid by Cheng et al (2013) followed by solvent extraction giving 95%, 90%, 87% and 80% recovery of Nd, Ce, La and Y respectively. Apart the raw materials cited above, there has been extensive research in extraction of REEs from other primary and secondary resources. However, there hasn’t been any work so far cited in literature to extract these minor rare elements from blast furnace slag. The iron making process for producing molten iron results in generation of slag which typically contains substantial amounts of calcium, silica, alumina, titania and other metallic components in trace. Many blast furnace slags have been reported to contain small but valuable quantities of rare earth elements like lanthanum, cerium and neodymium. It would be highly desirable to be able to economically recover the rare earth elements because several of these are commercially valuable. However, the only methods currently available for their extraction from blast furnace slag are cement making or extraction of vanadium/chromium values although in traces, which can’t preferentially dissolve rare earths. However, such procedure is not economically viable for the quantities of rare earth elements available in the slag. Moreover, the presence of large amounts of calcium and

alumino-silicates in the slag makes separation of the rare earth elements particularly difficult because of the difficulty of separating these from the matrix without excessive consumption of reagents.
In view of this, no mention was made of the possibility of separating rare earth elements from the blast furance slag, it is an object of the present invention to provide a simple and inexpensive process for recovering rare earth elements from such a raw material.
The main object of this invention is to provide a process for recovery of rare earth elements from blast furnace slag which comprised of
(a) digesting blast furnace slag containing rare earths together with calcium, alumina and silica values in a dilute mineral acid solution to obtain a final digestion slurry (pH- 1.2-1.5) whereby rare earths are selectively dissolved while leaving calcium and alumino-silicates in the slag substantially undissolved;
(b) separating the solution obtained from the solid residue and recovering the rare earth elements from the solution by ion exchange cum precipitation using amide based resin, and
(c) recycling of spent acid in next cycle of leaching
OBJECTS OF THE INVENTION
Another objective of the present invention is the use of hydrochloric acid in water
after pre-treatment
Yet another objective of the present invention is that the process is carried out in
at least two temperatures.
Yet another objective of the present invention is that lanthanum is selectively
extracted in the first temperature stage, which is further subjected to a second
temperature to dissolve the other rare earths.
Yet another objective of the present invention is that the process uses high pulp
density.

Yet another objective of the present invention is that the process allowsvery low
concentrations of calcium, aluminum and silica values in solution.
Yet another objective of the present invention is that the process recovers the
rare earth elements from the solution by ion exchange.
Yet another objective of the present invention is that the process recovers the
rare earth elements from the solution by precipitation.
Yet another objective of the present invention is that the process recovers the
rare earth elements from the solution as oxalates and/or oxides.
SUMMARY OF THE INVENTION
According to the present invention it has been surprisingly discovered that rare earth elements can very economically be separated from blast furnace slag by the steps of: (a) leaching blast furnace slag with a dilute acid solution to obtain a final digestion slurry to selectively dissolve the rare earth elements, while leaving calcium, alumina and silica in the slag substantially undissolved; (b) separating residual solids from the solution obtained followed by ion exchange separation in amide based resin and (c) recovering the acid values from the solution. It has been surprisingly found according to this invention that rare earth elements in blast furance slag are readily leachable in dilute acids, e.g. mineral acids, while the same rare earth elements cannot be easily leached from the iron ore with the same acid solution. The term "rare earth elements" as used herein includes those elements having atomic numbers from 57 to 71 inclusive, as well as scandium and yttrium. Examples of suitable acids include HCl, HNO3, etc.
An important feature of this invention is the discovery that a dilute acid in the form of chloride ions in water can be used to selectively leach rare earth elements out of blast furnace slag without the concurrent dissolution of all of the Ca-Al and silica compounds. This selective leaching of the rare earth elements also means that less difficulty is experienced in the extraction step. Carrying out the leach at two temperatures further ensures complete adequate recovery of the rare earth elements.

In the procedure of the invention, ground blast furnace slag in the form of slurry is preferably mixed with an aqueous solution of chloride acid. The acid digestion may be carried out in either one or several stages. In a two stage digestion, the temperature is maintained in ambient range whereby much of the calcium, alumina and silica remain unaffected and promote lanthanum dissolution in solution. This slurry is further elevated to high temperature range (60-90oC). The final leachate contains mostly all the rare earths with especially few dissolved impurities of calcium, alumina and silica. The rare earth elements are then extracted from this entire leachate or solution. The rare earth elements can be quite easily recovered from the leachate or solution by known means, such as by the selective precipitation and removal of sparingly soluble salts upon the addition of soluble oxalates, fluorides, carbonates, or by treatment with ammonium or sodium hydroxide to precipitate the insoluble hydroxides, or by ion exchange using solid ion exchange resins in a column, or by solvent extraction using esters of phosphinic acid.
However, it has been reported that it is particularly advantageous to recover the
trace levels of rare earths by ion-exchange. Mainly cation exchangers and elution
by complexing agents are used for separation of rare earth elements using ion
exchange methods. In this process the order of elution of individual rare
earth(III) elements depends on the values of stability constants of formed
complexes. They generally increase from light lanthanides(III) to heavy
lanthanides(III). Ion exchange of rare earth elements in the presence of
chelating ligands on anion exchangers is still a poorly studied field. However, the papers published during the last few years show particular applicability of anion exchangers to this end. According to this method, a solution of the ions to be separated is contacted with an ion exchange resin resulting in equilibrium between resin and solution. Separation of the different ions is made possible by differences in distribution of the various ions between the solid and liquid phases (values of equilibrium constants differ for different ions). These differences are enhanced by selectively complexing the ions in solution and repeating the ion

exchange reaction many times. In practice, a mixture of the elements to be separated are adsorbed from an aqueous solution on a cation-exchange resin and are then washed down the column (eluted) by passing a solution of a complexing agent through the column until separation is achieved. The ion exchange reaction for each ion is repeated many times by this operation and in combination with the effect of the complexing agent results in separation, which are then eluted from the column and collected as successive portions of eluate.
The rare earths loaded are stripped using aqueous solutions of inorganic acids. The dissolved rare earths are then precipitated as insoluble oxalates and carbonates (oxide precursors), from which oxides are recovered by calcination. The stripping and precipitation steps can also be combined. Rare earth metals can be separated from other metals in weakly acidic medium (pH 1-4) by precipitating them as oxalates with oxalic acid. The separation is more sensitive for the light rare earth elements since the property difference between rare earths decreases as the atomic number increases.
In a more specific embodiment of the extracting procedure, ion-exchange technique was employed in a glass column clamped in a vertical position and half filled with water. The dry resin is pre-soaked in water for several hours to remove a large part of the adhering gas bubbles. The slurry of the resin is poured into the column until a height of one foot of settled resin is obtained. The resin in the column is then backwashed with water. The process of backwashing serves to classify the resin according to particle size, break up any lumps that may form through packing, and to eliminate the fines. The resin is conditioned by using 0.5N HCl. The column is then eluted with distilled water and is now ready for another experiment. Loading was carried out at more than or equal to 4.5mL/min, with elution rate of more than or equal to 1.5-2mL/min.
A further advantageous feature of the present invention resides in the fact that a large part of the acid values used in leaching can be recovered and used again in

the process. Thus, the acid values from the leachates and the solution from
which the rare earths have been recovered may be combined and subjected to a
recovery process. The fraction left after acid recycling can be
precipitated/neutralized resulting calcium/alumina based silicate solids can be recycled to the blast furnace slag residue.
In the specification and claims, all references to the pH and temperature refer to measurements made with a glass electrode and PID controller, properly standardized against buffer solutions and calibrated.
BREIF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 shows a flow sheet illustrating in block form a series of steps according to
the teachings of the invention;
FIG. 2 is a picture showing SEM-EDAX based analysis of white rare earth
product synthesized (in inset).
Fig. 3 shows the complete recovery of rare earth metals from blast furnace slag
under a set of conditions as can be seen from Example -1.
DETAIL DESCRIPTION F THE INVENTION
As shown in FIG. 1, ground blast furnace slag 1 is subjected to a pretreatment stage 2. The pretreated blast furnace slag is subjected to a leaching stage 5 where it is contacted with a solution of hydrochloric acid and water from stage 3. It is in the leaching stage 6 where in the solids react with dilute acid at two temperatures yielding a leach slurry 7 from the leaching stage which is conveniently separated from the leached solid residue in stage 8 washing to recover soluble values prior to disposal and the filtrate or leachate liquor can be transferred via line 9 to a ion exchange stage 10-11 (a,b). The ion exchange purification stage 10 may conveniently be a conventional loading-elution setup in

which the leachate liquor 9 comes into contact with a resin 10 containing an amide compound leaving out 13 as raffinate. After contacting the resin and leachate liquor for a period of time depending upon such factors as the volume ratio of resin to aqueous, the resulting rare earth-containing loaded resin can be washed in 11a and the resin can be subjected to elution in 11b by 12 to lead to 14 and 15. The raffinate can be joined to 13 for recycled to 10-11. The experiment can be repeated by 22 to ensure complete loading leaving excess acid in 13-14 which can be recycled for acid recovery by 23-24. The recycled acid can be taken back for leaching by 25, and remaining solid in 26 can be neutralized in 27, to yield calcium rich substrate in 28.
The loaded resin 11b from the rare earth purification stage is back stripped by means of a mineral acid solution12. The eluted solution 15 passes through precipitation stage 16 wherein the compound 17 is added and the precipitated rare earth compounds 19 (FIG.2) are recovered via discharge 18, while the liquid phase is discharged through line 20 to a spent acid treatment, with the spent acid being disposed / recycled at 21. The loading and elution stage may optionally be carried out in multiple steps to obtain separate fractions of the total rare earths present.
The process of this invention is illustrated by examples, which should not be construed to limit the scope of this invention.
EXAMPLE 1: A series of rare earth extractions were conducted on a blast furnace slag sample obtained from an Indian steel works site. The blast furnace slag had the following composition (dry basis):0.15% Fe; 36.25% CaO; 27.13% Al2O3; 32.14% SiO2, 0.08% P, and nearly 0.08% total rare earths. The blast furnace slag was added to 100 ml of hydrochloric acid solutions at various concentrations in varying liquid: solid ratios (L:S) and stirred constantly at temperatures between 30° and 50° C, and after leaching of 30min, the residual mixture was subjected to further high temperatures, after which the remaining insoluble residues were separated, and the pH values of the final slurry and

filtrate were noted. Optionally, the second residue could be further contacted with fresh acid to carry out a third successive leach. The leach liquors from double leaching tests were subjected to analysis and the results are shown in Fig.3 below. The results contained in Fig.3 show the complete recovery of rare earth metals from blast furnace slag under a set of conditions that it is possible, by careful control of the intermediate pH and temperature values to carry out a double leach in which the first leachate solution contains very little of the total rare earth values except La, while the second leachate contains the bulk of the rare earths values and a reduced load of Ca, Al, Si.
EXAMPLE 2: Ion-exchange technique was carried out in vertical glass column with the resin filled until a height of one foot of settled resin is obtained. The resin in the column is then backwashed with water. The process of backwashing serves to classify the resin according to particle size, break up any lumps that may form through packing, and to eliminate the fines. The resin is conditioned by using 0.5N HCl. The final leachate was loaded at a loading rate of 1-2-6mL/min on the resin in single or multiple cycles depending on the concentration of eluted solution generated using HCl at a flow rate of 1-4mL/min. The eluted solution was precipitated resulting a precipitate which was dried in oven, to generate a dull white powder. Table-I shows the results of the analysis of the final REOs, indicating that the REOs with a purity of above 93% were achieved as the final product (FIG.2).


WE CLAIM
1. A process for the recovery of rare earth elements from blast furnace slag
which comprised of:
(a) digesting blast furnace slag containing rare earths together with
calcium, alumina and silica values in a dilute mineral acid solution to
obtain a final digestion slurry (pH: 1.2-1.5) whereby rare earths are
selectively dissolved while leaving calcium and alumino-silicates in the
slag substantially undissolved;
(b) separating the solution obtained from the solid residue and recovering the rare earth elements from the solution by ion exchange cum precipitation using amide based resin, and
(c) recycling of spent acid in next cycle of leaching

2. A process as claimed in claim 1 wherein the rare earths include at least one of the rare earth elements of atomic number from 57 to 71.
3. A process as claimed in claim 1 wherein hydrochloric acid in water was used as lixiviant after pre-treatment.
4. A process as claimed in claim 1 wherein the leaching is carried out in at in at least two temperatures.
5. A process as claimed in claim 4 wherein the process uses high pulp density.
6. A process as claimed in claim 4 wherein lanthanum is selectively extracted in the first temperature stage, which is further subjected to a second temperature to dissolve the other rare earths.

7. A process as claimed in claim 6 wherein is that the process allowsvery low concentrations of calcium, aluminum and silica values in solution.
8. A process as claimed in claim 6 wherein the process recovers the rare earth elements from the solution by ion exchange.
9. A process as claimed in claim 8 wherein the process recovers the rare earth elements from the solution by precipitation.
10. A process as claimed in claim 8 wherein the process recovers the rare
earth elements from the solution as oxalates and/or oxides.