Monazite sand, extraction

Ytterbium occurs along with other rare earths in a number of rare minerals. It is commercially recovered principally from monazite sand, which contains about 0.03%. Ion-exchange and solvent extraction techniques developed in recent years have greatly simplified the separation of the rare earths from one another. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

There are several processes for commercial thorium production from monazite sand. They are mostly modifications of the acid or caustic digestion process. Such processes involve converting monazite to salts of different anions by combination of various chemical treatments, recovery of the thorium salt by solvent extraction, fractional crystallization, or precipitation methods. Finally, metalhc thorium is prepared by chemical reduction or electrolysis. Two such industrial processes are outlined briefly below. 

In one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metalhc cerium and removed by filtration. Thorium then is recovered from solution. 

Because of its lower cost, mesothorium 1 is frequently substituted for radium in therapy and in the manufacture of luminous watch-dials. The commercial process for extracting it from the by-products of monazite sand was long kept secret, but after Soddy and W. Marckwald independently discovered that it is chemically identical with radium, the process for extracting the latter element from pitchblende was adapted so that it could be used for recovering mesothorium 1 (84,94). 

When Edgar Fahs Smith was investigating monazite sand under the direction of F. A. Genth (1820—1893), the latter always appropriated the zirconium sulfate that was extracted, and would say as he carried it away, Zirconium is not simple there is another element concealed in it, and when I have leisure I shall endeavor to isolate it (18). It was in zirconium ores that large quantities of element 72 were first revealed (19,20,21). 

Derivation By extraction from monazite sand with oxalic or hydrochloric acid and conversion into the oxalate, followed by crystallization. 

The method evolved by Moseley (1887 to 1915) of determining the atomic number enabled chemists to ascertain, as has already been seen, the maximum number of elements that can exist in serial order between any two selected ones. As the atomic numbers of lanthanum and lutecium are 57 and 71, it is clear that it is possible for 13 elements to exist of atomic numbers between these. Now europium was the twelfth to be discovered, but no element corresponding to 61 had been recorded. This should lie between neodymium (60) and samarium (62), and as early as 1902 Bohuslav Brauner had predicted its existence. In 1926 Hopkins, of Illinois, with his collaborators Harris and Yntema, announced the discovery of a new element in the neodymium extracted from monazite sand, the lines of the X-ray spectrum agreeing with those expected for element 61. He called it Illinium. 

The following procedure is used for producing thorium hrom monazite sand. The sand is digested with hot concentrated alkali which converts the oxide to hydroxide. The filtered hydroxide is dissolved in hydrochloric acid and the pH adjusted between 5 and 6, which precipitates the thorium hydroxide but not the main fraction of lanthanide elements. The thorium hydroxide is dissolved in nitric acid and selectively extracted with methyl isobutyl ketone or tributyl phosphate in kerosene. This gives a rather pure organic solution of Th(N03)4. thorium is stripped from the organic phase by washing with alkali solution. 

Solvent extraction by a crown hydroxamic acid in chloroform for the determination of La in monazite sand has been reported by Agrawal and Shrivastav (1997). La can be determined spectrophotometrically in the oiganic phase between 1.2 and 20 ppm, or by ICP/AES with a detection limit of O.lSppb. A two-fold excess of Y, Ce, Pr, or Nd did not interfere with the spectrophotometric determination, and higher concentrations could be tolerated when fluoride or oxalate were present in the aqueous phase. 

A successful process has been developed by the National Chemical Laboratory using cellulose phosphate as a cation-exchange material for the purification of thorium from rare earth elements. Monazite sand is broken with sulphuric acid and extracted with water to give a solution of thorium and rare earth sulphates and phosphates. This is first treated with metallic iron or aluminium to reduce the ferric iron impurity to the ferrous condition. The solution is then fed through a colunm of cellulose phosphate to absorb the thorium. Some of the thorium is present in solution as a cationic phosphate complex, rather than as simple thorium cations, but both forms are retained by the column to a high degree. Rare earth elements, which predominate in the feed solution, are not appreciably absorbed, and the ratio of thorium to rare earths is increased to about 450. ... 

A common procedure begins with a treatment of monazite sand with 50-70% sodium hydroxide at 1,400°C to convert thorium oxide to hydroxide. The filtered hydroxide is then dissolved in hydrochloric acid and the pH adjusted to 5-6 to precipitate thorium but not the main fraction of the rare earth elements. After dissolution of the hydroxide in nitric add, thorium is extracted with methyl isobutyl ketone (MIBK) or TBP in kerosene. Thorium is then stripped from the solvent using an alkali solution. 

The exploitable monazite resources are as a rule alluvial l and occur mainly as monazite sand. Seashore deposits atTravancore in India contain very large quantities of monazite. Other important sources are located on the Atlantic coast in Bahia, Brazil. In Austraha, monazite is obtained as a by-product when seashore sand is treated for extraction of ihnenite. Even yttrium group metals are extracted from monazite, in spite of the low original content. At Mountain Pass in Cahfomia large deposits of bastnaesite have been found. Euxenite is worked in Idaho for extraction of niobium and tantalum. Then REMs are obtained as by-products. 

Dysprosium is the 43rd most abundant element on Earth and ranks ninth in abundance of the rare-earths found in the Earth s crust. It is a metallic element that is usually found as an oxide (disprosia). Like most rare-earths, it is found in the minerals monazite and allanite, which are extracted from river sands of India, Africa, South America, and Australia and the beaches of Florida. It is also found in the mineral bastnasite in California. 

There are a number of minerals in which thorium is found. Thus a number of basic process flow sheets exist for the recovery of thorium from ores (10). The extraction of monazite from sands is accomplished via the digestion of sand using hot base, which converts the oxide to the hydroxide form. The hydroxide is then dissolved in hydrochloric acid and the pH adjusted to between 5 and 6, affording the separation of thorium from the less acidic lanthanides. Thorium hydroxide is dissolved in nitric acid and extracted using methyl isobutyl ketone or tributyl phosphate in kerosene to yield Th(N03)4,... 

Thorium is widely distributed in Nature and there are large deposits of the principal mineral, monazite, a complex phosphate containing uranium, cerium, and other lanthanides. The extraction of thorium from monazite is complicated, the main problems being the destruction of the resistant sand and the separation of thorium from cerium and phosphate. One method involves a digestion with sodium hydroxide the insoluble hydroxides are removed and dissolved in hydrochloric acid. When the pH of the solution is adjusted to 5.8, all the thorium and uranium, together with about 3% of the lanthanides, are precipitated as hydroxides. The thorium is recovered by tributyl phosphate extraction from >6M hydrochloric acid solution or by... 

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