Process For Separating Cerium Concentrate From Ores

Abstract

A process for separating cerium from the other rare earth values in bastnasite ore by roasting the ore followed by leaching with dilute hydrochlorde acid under carefully controlled conditions.

Description

This invention relates to the method for the separation of cerium from concentrates of rare earth carbonate ores. In one particular embodiment, it pertains to a process for the separation of cerium concentrates from bastnasite ore that has been subjected to roasting and certain pretreatment processes.

Bastnasite is an ore that occurs in certain sections of western United States and contains a relatively large amount of rare earth compounds. The rare earths referred to herein will be understood as those elements having atomic numbers 57 to 71 inclusive. The rare earths of most particular interest are cerium, europium, lanthanum, praseodymium, neodymium, terbium, gadolinium, and samarium.

Bastnasite is a rare earth fluocarbonate which contains traces of thorium. The mineral is found in deposits in New Mexico, California, Europe and Africa. The crude ore as mined in the California deposit contains a substantial percentage of rare earth oxides. In addition, the mineral contains barite, calcite, silicates, aluminates and ferromagnesian minerals.

A concentrated bastnasite is commercially available which has the following approximate composition:

Rare Earth Oxides 68 to 70% Fluoride 5 to 8% SiO.sub.2 1.1 to 1.5% CO.sub.2 15 to 20%

This concentrate is the raw material for my novel process. I have found that a cerium concentrate can be prepared in a commercial grade with the concurrent recovery of a similarly valuable didymium (mixed rare earth) fraction by controlled acid leaching of the concentrated bastnasite ore. My novel process differs from the usual industrial processes in that they normally require total dissolution of the ore by caustic and/or acid attack. My novel process gives valuable products with a minimum of reaction steps.

In the first step of my process, the bastnasite ore concentrate is roasted at 1,000.degree. to 1,500.degree.F. This is normally done by the vendor as a final step in his preparation of the concentrate or it may be done as the first step of my novel process. The advantage of roasting the ore is that it drives off additional carbon dioxide and thus concentrates the ore, the rare earth oxide content increasing to about 90 percent. In addition, roasting increases the porosity of the ore and at least partially oxidizes the cerium.

In the second step of my process, the ore is leached by mixing the ore concentrate with dilute hydrochloric acid. The acid concentration must be maintained at less than 5 weight percent. Use of a more concentrated acid results in partial dissolution of the cerium and consequently is to be avoided. Although the acid may be used in concentration as low as 1 weight percent, it is obvious that the rate of reaction is improved by operating at acid concentrations of approximately 3.5 - 5 weight percent.

The leaching is normally carried out at atmospheric pressure. Although satisfactory results would be obtained in operation at higher pressures, no economic advantage is achieved by such operation and operation at atmospheric pressure is preferred. Although no effort is made to increase the temperature, the reaction of the hydrochloric acid with the ore concentrate is exothermic so there is an initial increase in the temperature of reaction. This is not objectionable. The reaction can be carried out at temperatures of from 25.degree. to 50.degree.C. For obvious economic reasons, operation at room temperature is preferred.

The leaching is normally carried out in a weight ratio of ore to dilute acid of 1:1 to about 1:25. The preferred weight ratio is about 1:5 to 1:10. The ore is leached for 6 to 24 hours, preferably 12-18 hours.

Both of these products recovered from the bastnasite ore on treatment with the process of my invention, have direct commercial applications. The cerium concentrate is recovered as the residue from the process and has a purity of about 80 to 90% based on the formula: ##SPC1##

Cerium concentrates of this purity constitute a commercial grade for uses such as a specialty glass ingredient or conversion to cerium oxide for glass polishing compounds. This product may also obviously be used as a raw material for production of other and higher purity cerium compounds.

The filtrate recovered in my novel process is a didymium (mixed rare earth) chloride. This material is also of immediate commercial utility, either as a chloride, or by conversion to other didymium products such as the carbonate, oxide, or fluoride. It also constitutes an improved feed material for extraction of other commercially valuable rare earth concentrates, such as lanthanum and europium.

The invention is further illustrated by the following specific but non-limiting examples.

EXAMPLE I

The feasibility of the leaching of the roasted bastnasite concentrate with a hydrochloric acid was demonstrated in a run in which 30 grams of the roasted bastnasite (65 percent minus 325 mesh) was transferred to a standard reaction vessel and leached with a solution of 25 ml. of 12 normal (37 weight percent) hydrochloric acid in 250 ml. of water. The leaching was carried out by stirring the acid with the bastnasite concentrate for a period of 16 hours. At the end of this time, the material in the reaction vessel was filtered and the filtrate and residues were analyzed. The residue contained approximately 75% CeO.sub.2 /T.O. An analysis of the didymium (mixed rare earth) chloride solution showed the solution contained less than 2% CeO.sub.2 /T.O.

This run demonstrated the utility of my novel process. The hydrochloric acid concentration was about 4 percent and a cerium concentrate containing approximately 75 percent was recovered. The solution recovered was essentially free of cerium.

EXAMPLE II

The possibility of adding the hydrochloric acid to a slurry of a roasted bastnasite was investigated. In this run, 1800 grams of roasted bastnasite concentrate was slurried with 14 liters of water. After the slurry was mixed thoroughly, 11/2 liters of hydrochloric acid was added slowly over a period of about 4-6 hours. After the hydrochloric acid addition was complete, the stirring was continued for about 24 hours and the slurry was filtered. The residue was found to contain approximately 81% CeO.sub.2 /T.O. The didymium chloride solution contained 4.8 percent cerium. It is apparent from these data that satisfactory results can be obtained by slowly adding the hydrochloric acid to a slurry of the bastnasite in water. The purity of cerium in the concentrate was improved without an inordinate loss of cerium in the didymium chloride solution.

EXAMPLE III

In this run, the roasted bastnasite was leached with a weight ratio of bastnasite concentrate to hydrochloric acid of approximately 1. The leaching was carried out using the techniques described in Example I. A total of 300 grams of roasted bastnasite was added to and leached with a solution containing 250 ml. of 37 weight percent HCl and 2500 ml. water (concentration approximately 4 weight percent). The leaching was carried out for a period of 22 hours at room temperature. At the end of this time, the slurry was filtered and the filter cake and the filtrate were analyzed. The filtrate was found to have a cerium purity of 90 percent and the rare earth chloride solution contained only 4.8 percent cerium (CeO.sub.2 /T.O.).

This run demonstrates optimum conditions when a bastnasite ore that has not been subjected to the size reduction is used. The purity of the cerium concentrate was high and the didymium chloride solution did not contain an excessive amount of cerium.

This run also indicated the effect of leaching time on the efficiency of the separation. Successive analyses of soluble and insoluble values, during the course of the leaching indicated the relative rate of solubilization of rare earth vs. cerium values:

Leach % of Total Values Soluble CeO.sub.2 /T.O. Ratio Time (hrs.) CeO.sub.2 RE.sub.2 O.sub.3 Residue Solution ______________________________________ 0 0 0 -- -- 0.25 10 11 44.5 37.0 0.75 14 19 45.5 36.5 1.25 17 23 44.5 36.0 1.75 17 32 46.5 27.0 3.50 13 57 63.0 15.7 4.50 11 65 66.0 12.5 5.50 10 64 65.0 11.5 22 6 88 90.0 4.8 ______________________________________

Under the conditions of this test, with relatively initial high acid concentration, some cerium was dissolved in the first 2 hrs. As rare earth extraction proceeded, and the acidity decreased, most of this cerium reprecipitated into the residue to yield, after dissolution of most of the rare earths, a final rare earth chloride with acceptably low cerium content. Incremental addition of the total acid over the leaching period minimizes this slight initial dissolution of cerium.

The drawing is a graphical presentation of the above data and provides a general indication of the optimum leach time, under these conditions of temperature, initial acid concentration, and ore/acid ratio.

EXAMPLE IV

The effect of the particle size of the bastnasite ore was investigated in a series of runs in which the bastnasite ore concentrate was reduced in size to about 5 microns or less and leached using the techniques described in Example I. In the first of these series of runs, 150 grams of this finely divided roasted bastnasite concentrate was leached with a solution of 1250 ml. of water containing 125 ml. of hydrochloric acid (approximately 4 weight percent concentration). The leaching was carried out for a period of about 20 hours. At the end of this time, the products were separated as described above and the rare earth chloride solution analyzed for cerium. The solution was found to contain 5% CeO.sub.2 /T.O.

EXAMPLE V

An effort was made to determine the best ratio of fine sized bastnasite to acid. In this run, 100 grams of roasted bastnasite concentrate that had been reduced to a size of about 5 microns and leached with a solution of 1250 ml. of water containing 125 ml. of hydrochloric acid. This represents a 50 percent increase, relative to Example IV, in acid/ore ratio. The leaching was carried out for a period of 20 hours. At the end of this time, the slurry was filtered and the residues and filtrate analyzed. The residue was found to have a CeO.sub.2 purity of about 87 percent. The rare earth chloride solution, however, contained 12.5% CeO.sub.2 /T.O.

EXAMPLE VI

The effect of acid concentration was investigated in this run. A total of 150 grams of the bastnasite concentrate that had been reduced to 5 micron size was leached with a solution of 125 ml. of hydrochloric acid in 250 ml. of water (acid concentration of about 15 percent). The leaching was carried out using the techniques described in Example I for a period of about 20 hours. At the end of this time the slurry was filtered and the residue and filtrate analyzed. The cerium concentrate was found to have a CeO.sub.2 purity of 87 percent. However, the didymium chloride solution contained 15% CeO.sub.2 /T.O.

EXAMPLE VII

In this example, the effect of the ratio of ore to acid solution was investigated further. The ore feed was roasted at 1400.degree.-1500.degree.F., and sized to nominally less than 10 microns. A total of 300 grams of this fine-sized bastnasite was added to and leached with a solution of 250 ml. of hydrochloric acid in 2,500 ml. of water (acid concentration approximately 4 percent). The reaction was carried out for a period of about 20 hours. At the end of this time, the slurry was filtered and the filter cake and solubilized materials analyzed. The cerium concentrate was found to have a CeO.sub.2 purity of 88 percent. The rare earth chloride solution contained 7% CeO.sub.2 /T.O.

It is apparent from a review of the data presented in Examples IV to VII that advantages are achieved by reducing the size of the roasted bastnasite concentrate prior to the acid leach. When the material was reduced in size to below 5 microns and the acid solution kept in the proper range, the cerium concentrate analysis showed a CeO.sub.2 purity in excess of 85 percent. Example VI shows the criticality of the acid concentration. In this run, the acid concentration was increased to approximately 15 percent. The rare earth chloride solution contained 15% CeO.sub.2 indicating that an excessive amount of the cerium is solubilized by treatment with acid in a concentration of above 5 percent. It should be noted that the amount of cerium solubilized under more extreme leaching conditions would be termed "excessive" only to the extent that it would decrease the immediate value of the didymium chloride fraction. If the cerium concentrate is the primary product desired, or if the didymium fraction is to be used primarily for feed to other purification operations in which low (i.e., less than 10-15 percent) cerium content is of minor importance, or if the final leach slurry is to be given a terminal treatment for reprecipitation of soluble cerium, it is obvious that minimizing the dissolution of cerium during leaching would be less important.

Obviously many modifications and variations of the invention may be made without departing from the essence of the scope thereof and only such limitations should be a part of the appended claims.

Claims

1. A process for preparing a cerium concentrate and a non-cerium rare earth chloride solution from a bastnasite ore concentrate which comprises the steps of:

a. Roasting the ore concentrate at a temperature of about 1000.degree. to 1500.degree.F for about 1 to 4 hours to drive off a substantial portion of the carbon dioxide,

b. Leaching the roasted ore concentrate with an about 1 to 5 percent hydrochloric acid solution for 6 to 24 hours,

c. Filtering and recovering the cerium concentrate and the non-cerium rare

2. The process according to claim 1 wherein the weight ratio of roasted bastnasite ore to hydrochloric acid used in the leaching is from 1 to 1 to

3. The process according to claim 1 wherein the roasted ore is reduced to a

4. The process according to claim 1 wherein the leaching is carried out at

5. A process for separating cerium values from non-cerium rare earth values from a bastnasite ore concentrate which comprises:

a. Roasting the ore concentrate at a temperature of about 1,000.degree. to 1500.degree.F. for a time sufficient to drive off a substantial portion of the carbon dioxide whereby cerium values are selectively converted to a form substantially insoluble in aqueous acid leach solution and non-cerium rare earth values are converted to a form soluble in said leach solution,

b. Leaching the roasted ore concentrate with an about 1 to 5 weight percent hydrochloric acid solution for a time sufficient to solubilize non-cerium rare earth values, and,

c. Separating the leach solution containing non-cerium rare earth values from the undissolved concentrate containing cerium values.