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Chemistry of f block elements
1. Dr. S. H. Burungale
Associate Professor
Head
Department of Chemistry
Yashwantrao Chavan College of Science Karad
2. Monazite is a primarily reddish-brown phosphate mineral that
contains rare-earth elements. Due to variability in composition,
monazite is considered a group of minerals.
monazite-(Ce), (Ce,La,Nd,Th)PO4 (the most common
member),
monazite-(La), (La,Ce,Nd)PO4,
monazite-(Nd), (Nd,La,Ce)PO4,
monazite-(Sm), (Sm,Gd,Ce,Th)PO4,
monazite-(Pr), (Pr,Ce,Nd,Th)PO4.
3. Bastnäsite has cerium, lanthanum and yttrium in its
generalized formula but officially the mineral is divided into
three minerals based on the predominant rare-earth
element.[5] There is bastnäsite-(Ce) with a more accurate
formula of (Ce, La)CO3F. There is also bastnäsite-(La) with a
formula of (La, Ce)CO3F. And finally there is bastnäsite-(Y) with
a formula of (Y, Ce)CO3F. There is little difference in the three
in terms of physical properties and most bastnäsite is
bastnäsite-(Ce). Cerium in most natural bastnäsites usually
dominates the others. Bastnäsite and
the phosphate mineral monazite are the two largest sources
of cerium, an important industrial metal.
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37. Alkaline cracking
A more recent process uses hot sodium hydroxide solution (73%) at
about 140 °C. This process allows the valuable phosphate content of
the ore to be recovered as crystalline trisodium phosphate. The
lanthanide/thorium hydroxide mixture can be treated
with hydrochloric acid to provide a solution of lanthanide chlorides,
and an insoluble sludge of the less-basic thorium hydroxide.
Mineralization and extraction
38.
39. Grinder: Grind monazite ore to ~150 micrometers. Monazite ore
contains 55–60% rare-earth metal oxides along with 24 to 29% P2O5, 5
to 10% ThO2, and 0.2 to 0.4% U3O8.
Digestion: Crushed monazite is mixed with highly concentrated sulfuric
acid (93% acid) at feed temperatures of 150 to 180 °C. The ratio of acid
to ore varies depending on the concentration of the ore (unable to find
ratio range). The digester is stirred vigorously with a robust agitator and
operates at temperatures between 200 and 300 °C. Acid is charged into
the reactor and heated before the ore. The insoluble product coats the
grains of crushed ore. The temperature in the reactor rises due to heat
released from the exothermic reactions. After ~15 minutes, the viscosity
of the solution has increased and the solution is similar to a dough. The
product reacts for 3 to 4 hours. It is then removed from the digester
before the solution hardens. The ratio of sulfuric acid to sand removed
is 1.6 to 2.5.
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40. Dissolution: The contents of the reactor are cooled to 70 °C
and leached with 30 °C water. A ratio of 10 parts water to
mass of ore originally added is used. This leaching process
continues for 12 to 15 hours.
Filtration: All solids from step three are filtered off. Such
solids include: silica, rutile, zircon, ilmenite, and undigested
monazite residues. The resulting solution is called monazite
sulfate.
Dilution: Diluted the monazite sulfate with 6–7 parts water
at 30 °C.
Neutralization: Add NH3(aq) to neutralize to a pH of 1.1 to
form a selective precipitate of thorium-phosphate cake.
Filtration: Collect thorium phosphate precipitate during
filtration of neutralized monazite solution.
41. Dryer: Feed thorium-phosphate cake through a dryer at
~120 °C to create concentrated thorium phosphate.
Neutralization: Add NH3(aq) to remaining monazite solution
to create rare-earth-metal precipitate at a pH of 2.3.
Filtration: Filtrate out the RE precipitate to yield the
concentrated rare-earth metal hydroxides.
Neutralization: Add NH3(aq) to remaining filtrate to a pH of 6.
This creates a uranium concentrated precipitate.
Filtration: Filter remaining solution to yield uranium
concentrate.
The final products yielded for this process are thorium-
phosphate concentrate, RE hydroxides, and uranium
concentrate
42. Acid cracking[edit]
The original process for "cracking" monazite so as to extract the thorium and lanthanide
content was to heat it with concentrated sulfuric acid to temperatures between 120 and
150 °C for several hours. Variations in the ratio of acid to ore, the extent of heating, and
the extent to which water was added afterwards led to several different processes to
separate thorium from the lanthanides. One of the processes caused the thorium to
precipitate out as a phosphate or pyrophosphate in crude form, leaving a solution of
lanthanide sulfates, from which the lanthanides could be easily precipitated as a
double sodium sulfate. The acid methods led to the generation of considerable acid waste,
and loss of the phosphate content of the ore.
