CE60013 - Nuclear Chemical Engineering Flashcards
List uranium ppm in v.high grade, high grade, low grade, and v.low grade:
Very high: 200,000 ppm
High: 20,000 ppm
Low: 1,000 ppm
Very low: 100 ppm
What are the main steps of the nuclear fuel cycle?
Ore mining and milling
Refining
Enrichment
Fabrication
Reactor
Transport and storage
Reprocessing
Recycling
Waste treatment
Waste disposal
(~ in order)
Give examples of uranium mineral types:
Oxides (uraninite, pitchblende)
Complex oxides (Davidite)
Silicates (uranophane)
Vanadates (Carnotite)
Phosphates (Autunite)
Describe uranium supply:
High and very-high grade uranium mineral ore resources (uraninite and pitchblende) are exhausted.
Uranium mineral sources now being worked are:
- by-product uranium recovery in gold extraction
- by product recovery from phosphate rock in the fertilizer industry
- Low and very-low grade ore bodies, e.g. Rossing mine, Namibia. Average ores processes in US: 800-1500 ppm U (2015 data).
- Former Soviet Union and United States bilateral agreement on nuclear weapons
What are the types of uranium mine?
Underground
Open pit
In-situ leaching (ISL)
Open-Pit Mines:
Open-pit mining involves removing large quantities of overburden (soil, rock, and other materials) to expose the uranium ore.
Usage: This method is suitable when the uranium deposits are close to the surface and spread over a large area.
Underground Mines:
Underground mining involves tunnelling into the Earth to reach the uranium deposits. This can be done using various methods, such as shafts or adits.
Usage: Used when uranium deposits are deeper or when environmental or geological conditions make open-pit mining less practical.
In Situ Recovery (ISR) or In Situ Leach (ISL):
ISR involves the injection of a leaching solution (usually containing oxygen and sometimes chemicals) directly into the ore deposit, allowing the uranium to be dissolved and pumped to the surface.
Usage: This method is used when the uranium deposits are in a porous or permeable rock formation, and it can be more environmentally friendly compared to traditional mining methods.
Heap Leaching:
Similar to ISR, heap leaching involves piling up crushed ore on a leach pad and applying a leaching solution to dissolve the uranium. The solution is then collected and processed to extract the uranium.
Usage: Suitable for low-grade ore or when economic considerations favor a less intensive mining process.
Placer Mining:
Placer mining involves extracting uranium from loose, unconsolidated sediments such as riverbeds or beach sands.
Usage: Rarely used for uranium, as deposits tend to be more commonly found in hard rock rather than in placer deposits.
By-Product Recovery:
Some uranium is recovered as a by-product of other mining operations, such as copper or phosphate mining, where uranium is present in small quantities.
Usage: This method leverages the extraction of uranium as a secondary product in conjunction with the primary mining activity.
List the main steps involved in uranium ore processing:
Mining ore
Crushing and grinding
Pre-concentration
Leaching
Liquid-solids separation
a. Tailings
b. Ion exchange
c. Solvent extraction
(Then from b and/or c…)
Precipitation and filtration
Drying
Final product
What does traditional beneficiation of mineral ore bodies involve?
- crushing and grinding.
- roasting of mineral to oxidise uranium
- mineral upgrading using gravity and air
- flotation, although they have limited applicability
- thickening of ore pulps, solid/liquid separation
- leaching of mineral slurry, which can be done in-situ.
(Beneficiation: the treatment of raw material (such as iron ore) to improve physical or chemical properties especially in preparation for smelting.)
How is uranium is extracted from the mineral ore body?
Via chemical extraction
Acid or base leaching of uranium:
- H2SO4 or Na2CO3/NaHCO3 mixture are the preferred reagents.
- Uranium forms soluble complexes with both sulphate and carbonate ions in solution
- Competing inorganic sulphates and carbonates are mostly insoluble
In-situ leaching is used when the uranium deposits are located in porous or permeable rock formations.
The leaching solution (acid or base) is injected into the ore body through wells drilled into the deposit.
The solution dissolves the uranium from the ore, and the uranium-bearing solution is then pumped to the surface for further processing.
For uranium solvent (acid) leaching, why is oxidation required before adding the solvent?
Tetravalent U (in UO2) needs to be oxidised to hexavalent U (UO3) to make it soluble (UO3 is soluble).
MnO2 or NaClO3 are often used to oxidise uranium.
What is gangue in nuclear engineering?
The commercially valueless material in which ore is found / the rest of the ore that is not uranium.
How do properties of the gangue affect the solvent used for leaching?
If gangue is silica (insoluble in acid), acid leaching with H2SO4 is employed (cheaper and faster dissolution than alkaline processes).
If gangue is limestone (it consumes acid), Na2CO3/NaHCO3 is preferred as leaching agent.
You want to dissolve the uranium only, and not the rest of the rock.
What are the necessary conditions for in-situ leaching (ISL)?
Ore deposit located in water saturated zone
Aquifer trapped between two impermeable layers
Deposit must have adequate permeability
The deposit must be easily oxidised
Describe in-situ leaching (ISL):
- Well Injection:
Wells are drilled into the uranium-bearing ore zone. These wells may be cased to prevent the leaching solution from spreading to unwanted areas. - Leaching Solution Injection:
A leaching solution, often containing a dilute acid (such as sulfuric acid) or an alkaline solution, is injected into the ore zone through the wells.
The leaching solution interacts with the uranium ore, causing the uranium to dissolve into the solution. - Uranium Dissolution:
As the leaching solution percolates through the ore, it dissolves the uranium from the rock matrix. - Solution Recovery:
The uranium-bearing solution, now referred to as the “pregnant” solution, is pumped back to the surface through recovery wells.
Why is uranium solvent extraction needed?
To separate it from the other, unwanted elements, which should stay in the aqueous phase whilst uranium is preferentially separated into the organic phase.
How does ion exchange work?
Ion exchange separation is a separation technique based on the reversible exchange of ions between a solid resin or polymer matrix and a solution.
The ion exchange resin, typically in the form of beads or a column, is designed with specific groups that attract and bind uranium ions preferentially.
As the solution passes through the resin, uranium ions replace other ions in the resin, leading to the retention of uranium while other ions are released.
Subsequent elution with a different solution, often an acid, reverses the process, releasing the bound uranium ions.
This enables the separation and concentration of uranium
Why is it better for uranium ion exchange columns to be in sulphate form?
- Uranium in the hexavalent state (U(VI)) is more stable in sulfate solutions compared to other anions. Sulfate provides a stable environment for maintaining uranium in the desired oxidation state during the ion exchange process.
- The sulfate environment helps prevent redox reactions that might occur with other anions, ensuring the preservation of uranium in its hexavalent state, which is typically the form targeted for recovery.
- The leaching solutions used to extract uranium from ores or concentrates are often in sulfate form. Using a sulfate form in the ion exchange column ensures compatibility and consistency with the overall uranium recovery process.
- Eluting or recovering uranium from the ion exchange resin is more straightforward when using sulfate solutions.
Why is uranium refining required?
To obtain pure uranium hexafluroide.
UF6 has a bp of 56C (whilst oxides are typically solids). It is easier to convert into a gas and then undergo enrichment.
Why does the uranium process require the production of uranium hexafluoride (UH6) (via reaction for UO2 and HNO3)?
UF6 has a bp of 56C (whilst oxides are typically solids).
The next step of the process is enrichment which requires gaseous feed.
It is easier/less costly and energy intensive to convert it into a gas.
List the main steps involved in uranium refining:
Dissolution (of uranium ore concentrate and nitric acid)
Purification by solvent extraction
Conversion to UO3
Reduction
Hydrofluorination (anhydrous HF and pure UO2 reacted)
a. Metallothermic reduction
b. Fluorination (obtaining pure UF6)
What is uranium enrichment?
The process of increasing the concentration of the isotope uranium-235 (U-235) in a sample of uranium, usually in the form of uranium hexafluoride (UF6).
Fissile isotope U-235 is the one that can sustain a nuclear chain reaction. Natural uranium consists mostly of U-238, which is not as effective for sustaining nuclear reactions.
What techniques could be used for uranium enrichment?
Distillation
Chemical exchange
Diffusion
Centrifuge
Aerodynamics
What does SWU stand for?
Separative Work Units (measured in kg)
SWU is used to describe the capacity of an enrichment plant
How isthe Separative Work Unit (SWU) calculated?
SWU = WV(x.w) + PV(x.p) - F*V(x.f)
Where V(x) is the value function,
V(x) = (1-2x)*ln((1-x) / x)
F = Feed (kg)
P = Product( kg)
W = Waste (kg)
xf = Feed composition
xp = Product composition
xw = Waste composition
What is yellow cake?
Yellowcake, also known as urania, is a concentrated form of uranium ore.
The mined uranium ore is crushed and chemically treated to separate the uranium. The result is ‘yellow cake’, a yellow powder of uranium oxide (U3O8). In yellow cake the uranium concentration is raised to more than 80%.
Why is solvent extraction needed in uranium processing?
Want to recover uranium of high purity and remove unwanted elements.
How is uranium recovered and concentrated?
Liquid-liquid extraction
Ion exchange
- conventional recovery from - clarified liquors
- resin-in-pulp processes
Combined ion exchange / liquid-liquid extraction
Describe a mixer-settler system for solvent extraction:
Solvent (usually solute free, e.g. TBP in kerosene) and a feed (aqueous solution containing uranium) are fed into a mixer.
Impeller produces a well mixed system, increasing contact between the feed and solvent.
What are the 2 main performance indicators of uranium mixer-settler systems for solvent extraction:
Uranium purity in extract phase
High recovery yield
What do the following represent in equilibrium stage theory?
L
H
y
x
i
L - light phase flowrate
H - heavy phase flowrate
y - conc of solute in light phase
x - conc of solute in heavy phase
i - stage number
Light phase is the kerosene / solvent
Heavy phase could be water (immiscible with kerosene)
Concentrations xi and yi (light and heavy phase leaving the stage in opposite directions) would be in equilibrium.
What is the mass balance for the solute in a singe stage, i?
MB:
Lᵢyᵢ₊₁ + Hᵢxᵢ-₁ = Lᵢyᵢ + Hᵢxᵢ
Operating line: [y = mx]
yᵢ - yᵢ₊₁ = (Hᵢ/Lᵢ)*(xᵢ-₁ - xᵢ)
What is the extraction factor, Ei?
Ei is the ratio of the solute leaving in the extract / the sulte leaving in raffinate
Ei = Lᵢyᵢ / Hᵢxᵢ
= (Lᵢ / Hᵢ)*mi
Where mi is the equilibrium slope.
Ei = slope of eq line / slope of operating line
We want a high Ei and more uranium in the light phase. This can be improved by increasing the amount of light phase, L, or having more favourable thermodynamic conditions to promote mi.
How is single stage extraction loss found?
Loss = xn / x0 = 1 / (1+E1)
This is the amount of solute (uranium) remaining in the heavy phase which was not able to be extracted into the light phase.
We want this as close to 0 as possible.
How is uranium recovery calculated?
Recovery = 1 - Loss
= Lyᵢ / Hxᵢ-₁ = E1/(1+E1)
How is overall solute loss for cross-current uranium extraction calculated?
= xn / x0 = 1/(1+E)^n
How is overall solute loss for counter-current uranium extraction calculated?
= xn / x0 = (E-1)/(E^(n+1)-1)
This is known as the Kremser-Souders-Brown Equation, where E is the extraction factor.
Regarding the Kremser-Souders-Brown Equation for counter-current solute loss,
xn / x0 = (E-1)/(E^(n+1)-1)
What do variations in E lead to? (i.e. what happens when E > 1 and E < 1)
- For E > 1, as n increases, Xn / X0 decreases,
i.e. with a large extraction factor, as the number of stages is increased, the amount of solute extracted with each stage decreases. - For E = 1, Xn / X0 = 1 / (1 + n)
With increasing stages, Xn/X0 is decreasing in the above fashion. - For E < 1 and n is large, E^(n+1) tends to 0.
This leads to Xn / X0 = 1 - E
How does acid concentration effect uranium separation?
Separation is improved by increasing acid conc’ in the aqueous phase
When does a pinch condition occur?
When the line, H/L is greater than m.
Pinch scenarios suggest an infinite amount of stages would be needed.
What is the decontamination factor, f(AB)?
f(AB) = ((y.A1 / y.B1) / (x.A0 / x.B0)
We want this to be large.
What are the 3 main scenarios to beware of for pinch conditions?
a) At the feed point
b) at the effluent
c) at the intermediate point
What is the principle of ion exchange?
The ion exchange process involves a chemical reaction in which an ion in the solution is retained by a solid, which in turn releases a different ion into the solution.
Ion exchange is stoichiometric.
Electroneutrality is always preserved.
Ion Exchange Resins, in particular a cross-linked polystyrene / polydivinilbenzene (PS-PDVB) co-polymer, are the most relevant to nuclear applications.
Describe the ion exchange resin cation structure (in lecture):
Na+ cations are linked to SO3- groups by an ionic bond and can be exchanged with other cations.
SO3- anions are fixed to the resin by chemical covalent bonds
Polystyrene and divinylbenzene provide an organic structure insoluble in water
Water fills the pores in the resin structure and allows diffusion of the cations, which is necessary for the exchange process.
Describe the fundamental properties of ion exchange resins:
Ion exchange resins are porous
Ionic species diffuse within the porous structure
The greater the DVB (Divinilbenzene) content of the resins, the greater the degree of crosslinking
Resins with low crosslinking have larger pores and diffusion of ions is faster
If the crosslinking is too low, then moisture content will become excessive and resin will be softer and difficult to use
Cross-linkage of around 8% is commonly used
The styrene/divinylbenzene matrix is hydrophobic and does not absorb water
The resin absorbs water strongly and swells after introduction of ion exchange functional groups
Water absorption is due to hydration of the fixed functional groups and counter ions
Absorption of water results in swelling and hence stretching of the polymer chains, this is balanced by the elastic forces of the polymer
Highly crosslinked resins cannot swell as the polymer chains are restrained
How do sulphonic resins behave?
Sulphonic acid is strongly acidic and can dissociate not only in alkaline but also in acidic solutions over the entire pH range.
The sulphonic form can exchange positive ions such as Na+ and Ca2+:
R-SO3H + NaCl -> R-SO3Na + HCl (1)
2R-SO3H + CaCl2 -> (R-SO3)2Ca + 2 HCl (2)
R-SO3H + NaOH -> R-SO3Na + H2O (3)
Reactions 1 and 2 are reversible, regeneration is not 100% in a batch system hence a column system is needed.
Reaction 3 is not an equilibrium reaction, hence ion exchange may be performed efficiently even in a batch system.
How do carboxyl resins behave?
Weakly acidic carboxyl groups do not dissociate in acid solution and there is no ion exchange ability
No salt splitting ability in neutral solutions e.g. NaCl or Na2SO4
Selectivity similar to strongly acidic cation exchange resins.
- Although not with respect to H+ ions, which are taken up preferentially to any other univalent ion. Hence ease of regeneration with acid.
Can only be used in a limited pH (basic) range
More economical to regenerate
- Ease of loss of captured ions even a flow of water may be sufficient to hydrolyse them and elute ions in the water
How do quaternary ammonium resins behave?
Exchange anions like Cl- and SO4 2-
Resins which have a quaternary ammonium group (triple bond N+) as the exchanging group
- Groups dissociate in the same way as strong alkalis NaOH or KOH and exhibit strong acidity
- The quaternary ammonium exchange group dissociates not only in acidic but even in alkaline solutions
- Resins can absorb mineral acids and split neutral salts (see eqs 1 and 2)
- Ion exchange properties over the entire pH range.
R-NOH + NaCl -> R-NCl + NaOH (1)
R-NOH + HCl -> R-NCl + H2O (2)
How do tertiary ammonium resins behave?
Functional group does not dissociate in alkaline solutions and there is no ion exchange
Ion exchange does take place with mineral acids such as HCl and H2SO4 or with salts such as NH4Cl
Weak acids are generally difficult to capture
Some weakly basic ion exchange resins do exchange with H2CO3 but not with silicic acid
Easy to regenerate, not only with NaOH but also by Na2CO3 or NH3
Captured ions are easily eluted
1) R-N: + HCl -> [R-N:H]+ Cl-
2) [R-N:H]+ Cl- + HNO3 -> [R-N:H]+ [NO3]- + HCl
What is the organic and aqueous phase in U-solvent extraction?
The aqueous phase (Heavy phase) is water-based and can be an acidic, basic, neutral, or a saturated salt solution.
The organic phase (Light phase) is an organic solvent, usually diethyl ether or dichloromethane, which has minimal solubility in water.
For instance, ethanol would be a poor extraction solvent because it forms a solution with water.
How would you modify the system if the concentration of A in the extract needs
to be increased keeping the same initial and final concentrations of A in the aqueous phase?
Increase the H/L ratio (use less organic solvent) while adding more stages.
So, by increasing the H/L ratio and adding more stages:
- More of compound A is absorbed into the organic phase due to increased contact between the phases.
- The concentration of compound A in the organic phase increases, while the concentration of compound A in the aqueous phase decreases.
- This results in an overall increase in the concentration of compound A in the extract phase while maintaining the same initial and final concentrations of A in the aqueous phase.
How to simply calculate extraction factor, Ei?
E = m * (L/H)
Which can then be used to calculate loss via the KSB equation,
Loss = Xn/X0 = (E-1)/(E^(n+1)-1)
Describe how a solvent extraction system fed at an intermediate stage operates and explain why this configuration is advantageous:
Intermediate feeding enables having a scrubbing section, where traces of the solute with less affinity for the extract are removed from the extract.
What is the distribution coefficient?
The distribution coefficient, m, is the ratio of the organic (L) to aqueous (H) phases.
What are the aspects of ion exchange resins and their functions?
Polystyrene substrate - provides an insoluble structure
Divinilbenzene (DVB) crosslink - maintains structure stiffness and rigidity
Ions
As water enters, the polytysrene substrate swells. The DVB cross links are needed to maintain structure and reduce swelling.
What is equilibrium of ion exchange on resin dependent on?
In equilibrium, both the ion exchanger and the solution contain both the competing and the counter ions X and Y
The exchange is normally reversible.
The concentration ratio of the two competing counter-ion species is usually different from that in the solution.
As a rule, the ion exchanger selects one species in preference to the other.
What is the rate determining step in ion exchange kinetics?
In ion exchange the rate determining step is the interdiffusion of counter-ions.
In the bulk solution, any concentration differences are constantly levelled out by agitation or turbulence (both involving convection).
Agitation does not affect the interior of the beads nor a liquid film which adheres to the bead surfaces.
What factors affect the rate of ion exchange?
Rate increases with inverse of the square of the resin bead radius
Rate decreases as resin crosslinking increases
Rate increases as temperature increases, about 4-8 per cent per degree C
Agitation of resin and solution has no effect on rate of exchange
The rate of diffusion within the bead is dependent on the concentration gradient
How do we determine whether ion exchange kinetics are film or particle diffusion controlled?
(XD’δ)/(CDro)*(5+2a[AB])
If above «_space;1, particle diffusion controlled
If»_space; 1, film diffusion controlled
Where:
X = concentration of fixed ionic groups;
C = concentration of solution;
𝐷’ = interdiffusion coefficient in the ion exchanger;
D = interdiffusion coefficient in the film;
ro = bead radius;
δ = film thickness;
a[AB] = separation factor
What are the different ways of radioactive decay?
Alpha - an α particle is the nucleus of a He atom
Beta - a β particle is an electron
Gamma - γ is high-energy electromagnetic radiation
Positron - a positron has positive charge equal to the electron
Electron capture (or K-capture) - when an atomic nucleus absorbs an inner-shell electron (usually from the K shell) and transforms a proton into a neutron while emitting a neutrino and a characteristic X-ray photon
Spontaneous fission - immediately forming fission products
Neutron emission
Proton emission
