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CHEM411 > Modules 4+5 > Flashcards

Modules 4+5 Flashcards

1
Q

which is the only Ln not to occur naturally

A

Pm

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2
Q

in what state are the Ln elements found

A

In ore

all found together

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3
Q

what are the 4 common ores

A

monazite
xenotime
bastnaesite
rare earth laterites

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4
Q

give details about monazite

A

LnPO4
9 coord
contains thorium
mostly early Ln

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5
Q

give details about bastnaesite

A

LnCO3F
9 coord
mostly early Ln

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6
Q

give details about xenotime

A

LnPO4
8 coord
mostly late Ln

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7
Q

give details about rare earth laterites

A

rich in late Ln

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8
Q

why do monazite and bastnaesite favour early Ln

A

9 coord - favour large ions

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9
Q

why did US ramp up production in early 2010s

A

china had a near monopoly on rare earths

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10
Q

what are 4 main applications of rare earths

A

magnets
alloys
polishing powders
catalysts

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11
Q

which are the 3 elements with the most applications

A

La, Ce, Nd

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12
Q

what are the 2 key factors leading to supply problems

A

growth in consumption

china introducing quotas

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13
Q

what is the process for extracting bastnaesite

A

LnCO3F roast -> LnOF + CO2 Ce(III) ox. to Ce(IV)

Aq. HCl -> LnCl3(aq) + Ce(IV) residue

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14
Q

what is the process for extracting monazite

A

LnPO4 + NaOH —>
Ln(OH)3 +Th(OH)4
HCl/HNO3 —–>
LnCl3/Ln(NO3)3 + Th(IV) residue

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15
Q

what is the next step after extraction of Ln ore

A

separation

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16
Q

what happens in separation by redox

A

oxidation -> Ce(IV)—-hydrolyse –> Ce(OH)4

reduction -Zn->Eu2+—-SO42—–> EuSO4

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17
Q

what are the 2 methods of further separation beyond redox

A

classical methods

fractional crystallation

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18
Q

what happens in fractional crystalisation

A

Ln(NO3)3.2NH4NO3.4H2O

solubility varies with Ln3+ radius

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19
Q

how does ion exchange chromatography work

A

cationic resin has different affinities for different ions

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20
Q

how do affinities of the ion exchange resin change across the series

A

resin has greater affinity for larger ions (early)

- smaller ions eluted first

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21
Q

how are ions processes after ion exchange chromatography

A

Ln(HEDTA) —C2O42—>Ln(C2O4)3 ——> Ln2O3

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22
Q

what is the reaction that takes place in industrial solvent extraction

A

3H2L2 + Ln£+ —-> [Ln(HL2)3]

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23
Q

how does industrial scale solvent extraction work

A

Ln moves to org layer
forms complex with HL
can be separated and repeated
late Ln prefer org layer

24
Q

what is the main way that phosphors can be recycled

A

hydro metalurgy - similiar to ore extraction

25
Q

what are the 3 ways magnets can be recycled

A

reuse them (only for larger magnets)
separate the alloy - H2 decrepitation
hydrometalurgy

26
Q

where are the 3 main sources of uranium

A

kazakhstan
australia
canada

27
Q

where is thorium extracted from

A

occurs as Th3PO4 in monazite

28
Q

where is uranium extracted from

A

occurs mainly as pitchblend U3O8

29
Q

how can ultra pure U be obtained

A

further extraction if U3O8 with (BuO)P=O

30
Q

what isotope of uranium is required for nuclear reactors

A

235U

31
Q

what is the natural compostions of uranium isotopes

A
  1. 3% 238U

0. 7% 235U

32
Q

what uranium compound is used in isotope separation

A

UF6

33
Q

what are the advantages of using UF6 in isotope separation

A

gas - sublimes at 56 C

F only has 1 isotope

34
Q

what are the disadvantages of UF6

A

it is corrosive and forms HF in water

35
Q

how is UF6 made

A

UO2 + 4HF -> UF4 +2H2O —F2—> UF6

36
Q

what is the formula for rate of diffusion

A

rate = 1/root(Mr)

37
Q

what is the formula for separation factor

A

rate of diffusion 235UF6/ rate of diffusion 238UF6

38
Q

what are the 3 methods of isotope separtation

A

Gas diffusion
gas centrifuge
laser isotope separation

39
Q

why is gas diffusion no longer used

A

too expensive

40
Q

explain how a gas centrifuge works

A

spins at 70000 rpm
0.001atm so UF6 stays gaseous
238 is pushed towards the edge of the centrifuge - reused
235 stays in centre and is removed

41
Q

what is the separation factor of gas centrifuge

A

1.2

42
Q

what is a separative work unit

A

unit of measurement of the amount of work needed to separate uranium

43
Q

how does laser isotope separation work

A

UF6 forms dimers thriugh vdw interactions

selective excitation of U235 breaks U235 dimers and moves them to the side of the chamber - u 238 dimers are unaffected.

44
Q

what Kwh per SWU are required for each method

A

Gas Diff 2500
Gas cent 40
Laser 10-15

45
Q

how does uranium work in a reactor

A

split by thermal neutrons

forms lighter fragment and more neutrons

46
Q

how does a pressurised water reactor work

A

35% thermal efficency
need 2.5-3.5% U235
water acts as moderator and coolant
moderator slows reaction to enhance fission

47
Q

how is mixed oxide fuel made

A

239Pu + depleted U

48
Q

how is 239Pu made

A

238U absorbs a neutron followed by B- radiation

49
Q

what are the advantages of using thorium as a nuclear ful

A

abundant
occurs as single isotope
unlikely to produce trans uranium products
ThO2 is easier to handle than UO2

50
Q

what are the disadvantages of using thorium as a nuclear fuel

A

not fissile - needs fissile material added

232U is produced - gamma emitter

51
Q

how much energy does Th contain relative to U

A

200x more

52
Q

why does nuclear fuel need to be reprocessed

A

power output diminishes
fuel rods become U235 depleted
conc of fission products increases - capture neutrons

53
Q

how are nuclear fuels reprocessed

A

U and Pu separated from fission products

U separated from Pu

54
Q

what does spent nuclear fuel compose of

A

97% U/Pu

3% fission products

55
Q

how are fission products dealt with

A 
stored for weeks or months
evaporated to powder
mixed with molten glass
stored in stainless steel cannister
long term storage

CHEM411 (5 decks)

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