Chemistry of Uranium ore processing

Crushed ore is mixed with hot water to a 58% solids slurry. The solids slurry is then processed through 
a series of tanks, where sulfuric acid, sodium chlorate, and steam are used to extract the uranium from 
 the solids slurry. The average leaching efficiency for this process is 98.5%. The uranium-bearing solution
 is then decanted and directed to a solvent extraction (SX) process for further purification. In this 
extraction step, the dissolved uranium is transferred from the feed solution into the organic solvent. 
Next a stripping step recovers the uranium into a sodium chloride aqueous phase after which the
 barren solvent is recycled. The average efficiency of the SX circuit is 99.9%. The high-grade 
“pregnant” strip solution from SX goes to the next stage where magnesia slurry is added to 
precipitate magnesium diuranate. The yellow cake precipitate is then thickened, dried, re-crushed
 and packed into industry standard 220 litre steel drums for shipment to customers.

Uranium meeting nuclear-grade specifications is usually obtained from yellow cake through a
 tributyl phosphate solvent-extraction process. First, the yellow cake is dissolved in nitric acid to
 prepare a feed solution. Uranium is then selectively extracted from this acid feed by tributyl 
phosphate diluted with kerosene or some other suitable hydrocarbon mixture. Finally, uranium is
 stripped from the tributyl phosphate extract into acidified water to yield a highly purified uranyl nitrate,
 UO2(NO3)2.

Summary: The acid leaching process  comprises the following reactions:

Oxidation and dissolution of U(IV): UO2(s) + Cl2(aq) → (UO2)2+(aq) + 2Cl–(aq)

Dissolution of U(VI): UO3(s) + 2H+(aq) → (UO2)2+(aq) + H2O(l)

Neutralization: 6H+(aq) + UO2(CO3)34-(aq) → (UO2)2+(aq) + 3CO2(g) + 3H2O(l)

Precipitation: (UO2)2+(aq) + H2O2(l) → UO4·2H2O(s) + 2H+(aq) + 2H2O(l)

Reduction: (UO2)2+(aq) + C6H8O6(aq) + 2H+(aq) → U4+(aq) + C6H6O6(aq) + 2H2O(l)

Precipitation: U4+(aq) + 4HF(aq) + 2.5H2O(l) → UF4·2.5H2O(s) + 4H+(aq)

Uranium Minerals
Primary uranium minerals
Name	Chemical Formula
uraninite or pitchblende	UO2
coffinite	U(SiO4)1–x(OH)4x
brannerite	UTi2O6
davidite	(REE)(Y,U)(Ti,Fe3+)20O38
thucholite	Uranium-bearing pyrobitumen
Secondary uranium minerals
Name	Chemical Formula
autunite	Ca(UO2)2(PO4)2 x 8-12 H2O
carnotite	K2(UO2)2(VO4)2 x 1–3 H2O
gummite	gum like amorphous mixture of various uranium minerals
saleeite	Mg(UO2)2(PO4)2 x 10 H2O
torbernite	Cu(UO2)2(PO4)2 x 12 H2O
tyuyamunite	Ca(UO2)2(VO4)2 x 5-8 H2O
uranocircite	Ba(UO2)2(PO4)2 x 8-10 H2O
uranophane	Ca(UO2)2(HSiO4)2 x 5 H2O
zeunerite	Cu(UO2)2(AsO4)2 x 8-10 H2O

Uranium Processing :

The crushed and ground ore, or the underground ore in the case of ISL mining, is leached with sulfuric acid:

UO3 + 2H+ ====> UO22+ + H2O

UO22+ + 3SO42- ====> UO2(SO4)34-

The UO2 is oxidised to UO3.

With some ores, carbonate leaching is used to form a soluble uranyl tricarbonate ion: UO2(CO3)34-. This can then be precipitated with an alkali, eg as sodium or magnesium diuranate.

The uranium in solution is recovered in a resin/polymer ion exchange (IX) or liquid ion exchange (solvent extraction – SX) system. The pregnant liquor from acid ISL or heap leaching is treated similarly.

Further treatment for IX involves stripping the uranium from the resin/polymer either with a strong acid or chloride solution or with a nitrate solution in a semi-continuous cycle. The pregnant solution produced by the stripping cycle is then precipitated by the addition of ammonia, hydrogen peroxide, caustic soda or caustic magnesia. Solvent extraction is a continuous loading/stripping cycle involving the use of an organic liquid to carry the extractant which removes the uranium from solution.

Typically, in solvent extraction, tertiary amines* are used in a kerosene diluent, and the phases move countercurrently.

2R3N + H2SO4 ====> (R3NH)2SO4

2 (R3NH)2SO4 + UO2(SO4)34- ====> (R3NH)4UO2(SO4)3 + 2SO42-

* "R" is an alkyl (hydrocarbon) grouping, with single covalent bond.

The loaded solvents may then be treated to remove impurities. First, cations are removed at pH 1.5 using sulfuric acid and then anions are dealt with using gaseous ammonia.

The solvents are then stripped in a countercurrent process using ammonium sulfate solution.

(R3NH)4UO2(SO4)3 + 2(NH4)2SO4 ====> 4R3N + (NH4)4UO2(SO4)3 + 2H2SO4

Precipitation of ammonium diuranate is achieved by adding gaseous ammonia to neutralise the solution (though in earlier operations caustic soda and magnesia were used).

2NH3 + 2UO2(SO4)34- ====> (NH4)2U2O7 + 4SO42-

The diuranate is then dewatered and roasted to yield U3O8 product, which is the form in which uranium is marketed and exported.

Enrichment

The vast majority of all nuclear power reactors require 'enriched' uranium fuel in which the proportion of the uranium-235 isotope has been raised from the natural level of 0.7% to about 3.5% to 5%.  The enrichment process needs to have the uranium in gaseous form, so on the way from the mine it goes through a conversion plant which turns the uranium oxide into uranium hexafluoride.

The enrichment plant concentrates the useful uranium-235, leaving about 85% of the uranium by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level of uranium-235 and then passes to the next stage of the fuel cycle. The other stream is depleted in uranium-235 and is called 'tails' or depleted uranium. It is mostly uranium-238 and has little immediate use. 

Today's enrichment plants use the centrifuge process, with thousands of rapidly-spinning vertical tubes. Research is being conducted into laser enrichment, which appears to be a promising new technology.

A small number of reactors, notably the Canadian CANDU reactors, do not require uranium to be enriched.

Fuel fabrication

About 27 tonnes of fresh fuel is required each year by a 1000 MWe nuclear reactor. In contrast, a coal power station requires more than two and a half million tonnes of coal to produce as much electricity. (1)Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide powder. This powder is then pressed to form small fuel pellets, which are then heated to make a hard ceramic material. The pellets are then inserted into thin tubes to form fuel rods. These fuel rods are then grouped together to form fuel assemblies, which are several meters long. 

The number of fuel rods used to make each fuel assembly depends on the type of reactor. A pressurized water reactor may use between 121-193 fuel assemblies, each consisting of between 179-264 fuel rods. A boiling water reactor has between 91-96 fuel rods per assembly, with between 350-800 fuel assemblies per reactor.

Three stage Nuclear Program [ India ]

In the first stage, Heavy water reactors using enriched uranium derived from India’s limited uranium reserve, would be constructed and begin operating. The use of heavy water reactors meant that India did not need to to develop expensive and power demanding uranium enrichment facilities.

During the second stage, India was to construct Fast Breeder Reactors, which burned plutonium reprocessed from the spent fuel of the heavy water reactors as well as their depleted uranium. India needed to develop breeder technology quickly, because it had limited uranium resources. Breeders allowed India’s uranium supply to be used much more efficiently.

During the third stage thorium was to be bred, and U-233 would fuel Indian power reactors.

 Natural Uranium contains two isotopes, U-235 (0.7%) and U-238 (99.3%). Out of these two isotopes, only U-235 is useful for nuclear reactors, as it is fissile. U-238 is not fissile, similar to Th-232. However, this U-238 gets converted into Plutonium (Pu-239) during its stay inside the Uranium reactors by absorbing one neutron. This Pu-239 can then be extracted and used as fuel in Fast Breeder reactors. Therefore to sustain the Fast Breeder Reactors, enough Plutonium from Uranium based reactors is necessary. The only way it can be done is to have enough operational Uranium based reactors. This is why India is importing Uranium to sustain Uranium based reactors.

As mentioned above, Pu-239 will be used in Fast Breeder Reactors as fuel, but a blanket, or coating of Th-232 will be placed over Pu-239 (the fuel). This Th-232, during its stay inside the Fast Breeder Reactor, will get converted into Uranium-233 by absorbing one neutron. Uranium-233 is fissile but is not naturally occurring.

This Uranium-233  can then be extracted from the spent-fuel, and used as fuel in another type of reactors. Now, if you place a blanket of Th-232 over this Uranium-233 fuel, that blanket will again get converted into Uranium-233 during its stay inside the reactor by absorbing one neutron, and we will have a process where fuel can be re-generated inside the reactor! Though Uranium-233 is the fuel in these reactors, they are also termed as Thorium based reactors.

Thus in order to reach the Thorium based energy generation, building enough Plutonium stock for fast breeder reactors is necessary, which can only be done by having enough Uranium based electricity generation.

                           

                             Aluminium industry

By far the greatest quantity of commercially exploited bauxite lies at or near the Earth’s surface. Consequently, it is mined in open pits requiring only a minimal removal of overburden. Bauxite beds are blasted loose and dug up with power shovel or dragline, and the ore is transported by truck, rail, or conveyor belt to a processing plant, where it is crushed for easier handling. Refining plants are located near mine sites, if possible, since transportation is a major item in bauxite costs.

The production of aluminum from bauxite is a two-step process: refining bauxite to obtain alumina and smelting alumina to produce aluminum. Bauxite contains a number of impurities, including iron oxide, silica, and titania. If these impurities are not removed during refining, they will alloy with and contaminate the metal during the smelting process. The ore, therefore, must be treated to eliminate these impurities. Purified alumina usually contains 0.5 to 1 percent water, 0.3 to 0.5 percent soda, and less than 0.1 percent other oxides. The Bayer process, with various modifications, is the most widely used method for the production of alumina, and all aluminum is produced from alumina using the Hall-Héroult electrolytic process.

Purified alumina is dissolved in molten cryolite and electrolyzed with direct current. Under the influence of the current, the oxygen of the alumina is deposited on the carbon anode and is released as carbon dioxide, while free molten aluminum—which is heavier than the electrolyte—is deposited on the carbon lining at the bottom of the cell.

The Bayer process involves four steps: digestion, clarification, precipitation, and calcination.

In the first step, bauxite is ground, slurried with a solution of caustic soda (sodium hydroxide), and pumped into large pressure tanks called digesters, where the ore is subjected to steam heat and pressure. The sodium hydroxide reacts with the aluminous minerals of bauxite to form a saturated solution of sodium aluminate; insoluble impurities, called red mud, remain in suspension and are separated in the clarification step.

Following digestion, the mixture is passed through a series of pressure-reducing tanks (called blow-off tanks), where the solution is flashed to atmospheric pressure. (The steam generated in flashing is used to heat the caustic solution returning to digestion.) The next step in the process is to separate the insoluble red mud from the sodium aluminate solution. Coarse material (e.g., beach sand) is removed in crude cyclones called sand traps. Finer residue is settled in raking thickeners with the addition of synthetic flocculants, and solids in the thickener overflow are removed by cloth filters. These residues are then washed, combined, and discarded. The clarified solution is further cooled in heat exchangers, enhancing the degree of supersaturation of the dissolved alumina, and pumped into tall, silolike precipitators.

Sizable amounts of aluminum hydroxide crystals are added to the solution in the precipitators as seeding to hasten crystal separation. The seed crystals attract other crystals and form agglomerates; these are classified into larger product-sized material and finer material that is recycled as seed. The product-sized agglomerates of aluminum hydroxide crystals are filtered, washed to remove entrained caustic or solution, and calcined in rotary kilns or stationary fluidized-bed flash calciners at temperatures in excess of 960° C (1,750° F). Free water and water that is chemically combined are driven off, leaving commercially pure alumina—or aluminum oxide—a dry, fine, white powder similar to sugar in appearance and consistency. It is half aluminum and half oxygen by weight, bonded so firmly that neither chemicals nor heat alone can separate them.

Refining four tons of bauxite yields about two tons of alumina. A typical alumina plant, using the Bayer process, can produce 4,000 tons of alumina per day. The cost of alumina can vary widely, depending on the plant size and efficiency, on labour costs and overhead, and on the cost of bauxite

In a modern smelter, alumina is dissolved in reduction pots—deep, rectangular steel shells lined with carbon—that are filled with a molten electrolyte consisting mostly of a compound of sodium, aluminum, and fluorine called cryolite.

By means of carbon anodes, direct current is passed through the electrolyte to a carbon cathode lining at the bottom of the cell. A crust forms on the surface of the molten bath. Alumina is added on top of this crust, where it is preheated by the heat from the cell (about 950° C [1,750° F]) and its adsorbed moisture driven off. Periodically the crust is broken, and the alumina is fed into the bath. In newer cells, the alumina is fed directly into the molten bath by means of automated feeders.

The results of electrolysis are the deposition of molten aluminum on the bottom of the cell and the evolution of carbon dioxide on the carbon anode. About 450 grams (1 pound) of carbon are consumed for every kilogram (2.2 pounds) of aluminum produced. About 2 kilograms of alumina are consumed for each kilogram of aluminum produced.

The smelting process is continuous. Additional alumina is added to the bath periodically to replace that consumed by reduction. Heat generated by the electric current maintains the bath in a molten condition so that fresh alumina dissolves. Periodically, molten aluminum is siphoned off.

Because some fluoride from the cryolite electrolyte is lost in the process, aluminum fluoride is added, as needed, to restore the chemical composition of the bath. A bath with an excess of aluminum fluoride provides maximum efficiency.

The real problem among nations

      We are south Asian people. We are thinking in a wrong direction. We are quarreling among ourselves; Therefore, we remain divided and we suffer. Many a time we do it deliberately. There are many common issues to be solved collectively. If we work as united minded people, we can easily solve many problems. But when we divide, our problems start accumulating.

The step-mother attitude between India and Pakistan is one such example. If we start working and using our energy for common good work, it will be ever green heaven. India occupied a portion of Kashmir and Pakistan occupied another portion of Kashmir. The state is kept divided. Many wars were fought over this issue. If we think with a cool mind, we can realize it. what we gain out of it ?. We produce terror groups to create problems. Instead we can facilitate local people to work for betterment. Religion need not become an obstacle for development activity. We should learn how to respect the other man’s point of view. Technology abolished the boundaries between nations. Now-a-days many things can happen among peoples without the knowledge of governing class.

       If all the three parties sincerely make efforts to eliminate mutual hatred it would be still better situation. We do not need terror camps. We need world class teaching centers of science and arts and humanity. I think we can see towards west for learning skill developments. The Hinduism, the Islam, the Christianity and many more practices are only fine efforts to bring the people to be together for cooperation and not for creating isolation among people of a region. I think live and let live is best policy. War is not a solution against natural problems.

Why should one hate another person? We have same appearance, similar language and we originated from common ancestors.

       After Urie incident, India made surgical attack in POK and there were heated arguments from both ends. Throughout the week I went on monitoring the allegations and debates between the two contries. Sometimes I was under confusion in identifying who is Indian and who is Pakistani. We are all south Asians. We are all brothers of same mother the south Asia. We are running militant camps to kill our own people. And we feel proud of our skill in killing people. Sometimes we kill our own children for none of their fault. We want to make them Jihadis. Where are, we going really? The world is only one and we cannot hide in a single small world, our planet the earth. Internet connectivity will pull us out of our hiding holes. See the power of Facebook, the social media connecting us and exposing our activities both good and bad.

There are many natural problems like food, water, sanitation, health,peace of mind and so many. We need to fight against these odds together. We need to see our children play happily. But are we diverting from the basic way of “live and let live” path?

Humans are very small organisms of this ever-changing planet. It eliminated many races. Out time is limited on this small earth. You fly in a high altitude; can you distinguish the human beings. No, it is a single place for all. Suffering, inequality, illiteracy, superstition and many such things are our real enemies. Let us fight on these issues collectively for mutual common benefit of our people.

                                                                               Saturday, October 8, 2016

                                          Silicate Structures 

As we discussed in a previous lecture, the relative abundance of elements in the Earth's crust determines what minerals will form and what minerals will be common.  Because Oxygen and Silicon are the most abundant elements, the silicate minerals are the most common.  Thus, we will spend some time here discussing the structure, chemistry, and occurrence of silicate minerals.  Our systematic discussion of the common rock forming minerals will follow in the lectures throughout the remainder of the course.

Element Wt% Atomic% Volume% O 46.60 62.55 ~94 Si 27.72 21.22   ~6 Al   8.13   6.47 Fe   5.00   1.92 Ca   3.63   1.94 Na   2.83   2.34 K   2.59   1.42 Mg   2.09   1.84 Total 98.59 100.00 100

In order to discuss the silicates and their structures it is first necessary to remember that the way atoms are packed together or coordinated by larger anions, like oxygen depends on the radius ratio of the cation to the anion, Rx/Rz.

Rx/Rz C.N. Type 1.0 12 Hexagonal or Cubic Closest Packing 1.0 - 0.732 8 Cubic 0.732 - 0.414 6 Octahedral 0.414 - 0.225 4 Tetrahedral 0.225 - 0.155 3 Triangular <0.155 2 Linear

Since oxygen is the most abundant element in the crust, oxygen will be the major anion that coordinates the other other cations. Thus, for the major ions that occur in the crust, we can make the following table showing the coordination and coordination polyhedra that are expected for each of the common cations.

Ion	C.N.  (with Oxygen)	Coord. Polyhedron	Ionic Radius,  Å
K+	8 - 12	cubic to closest	1.51 (8) - 1.64 (12)
Na+	8 - 6	cubic to octahedral	1.18 (8) - 1.02 (6)
Ca+2	8 - 6		1.12 (8) - 1.00 (6)
Mn+2	6	Octahedral	0.83
Fe+2	6		0.78
Mg+2	6		0.72
Fe+3	6		0.65
Ti+4	6		0.61
Al+3	6		0.54
Al+3	4	Tetrahedral	0.39
Si+4	4		0.26
C+4	3	Triangular	0.08

The radius ratio of Si+4 to O-2 requires that Si+4 be coordinated by 4 O-2 ions in tetrahedral coordination.  

In order to neutralize the +4 charge on the Si cation, one negative charge from each of the Oxygen ions will reach the Si cation. Thus, each Oxygen will be left with a net charge of -1, resulting in a SiO4-4 tetrahedral group that can be bonded to other cations.  It is this SiO4-4 tetrahedron that forms the basis of the silicate minerals.

Since Si+4 is a highly charged cation, Pauling's rules state that it should be separated a far as possible from other Si+4 ions.  Thus, when these SiO4-4 tetrahedrons are linked together, only corner oxygens will be shared with other SiO4-4 groups.  Several possibilities exist and give rise to the different silicate groups