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Extraction of Niobium
Typology: Study Guides, Projects, Research
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PROJECT REPORT
ON
“GENERATION OF DATA FOR COUNTER CURRENT EXTRACTION BY
CLASSICAL SHAKE FLASK METHOD” & “EFFLUENT TREATMENT”
BY
PILLALAMARRI AVINASH
ROLL NO : 100810802005
3 rd^ YEAR, B TECH (CHEMICAL ENGINEERING)
AT
SPECIAL MATERIALS PLANT
NUCLEAR FUEL COMPLEX
DEPARTMENT OF ATOMIC ENERGY
HYDERABAD
UNDER THE GUIDANCE OF
Mr.K.V.MIRJI
Sr. MANAGER (SMP)
DEPARTMENT OF CHEMICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
WARANGAL
This is to certify that Mr.Pillalamarri Avinash has done his project work under my guidance during the period from 19-05-2013 to 20-06-2013 on the topic entitled “ GENERATION OF DATA FOR COUNTER CURRENT SOLVENT EXTRACTION OF TANTALUM AND NIOBIUM BY CLASSICAL FLASK METHOD” & “EFFLUENT TREAMENT” with reference to NUCLEAR FUEL COMPLEX.
▲ Uranium oxide plant
▲ Ceramic fuel fabrication plant
▲ Enriched fuel fabrication plant
In the last few decades, consumption of electricity has been increasing in India at the rate of 10% per annum. 25% of the present needs are met by hydro electricity, 72% by thermal power and 3% by nuclear energy. India occupies 2% of world’s landmass and currently generates 3% of global electricity. However, India has a share of 16% in world’s population. To achieve a moderately high level of economic growth, the domestic electricity generation capacity needs to be increased manifold.
Nuclear power is one source, that can generate electricity at costs competitive with coal-fired power stations in certain location. A tone of uranium fed in to nuclear power station produces as much heat as about 25,000 tones of coal taken over the life times of the stations.
One important advantage of nuclear power is that it avoids a wide variety of environmental problems arising from burning fossil fuels like coal, oil and gas. The problems that have received the most publicity have been ‘global warming’, which is changing the earth’s climate, acid rain, which is destroying forests and killing fish; air pollution, which is killing tens of thousands of people every year; the destructive effects of massive mining of coal and oil spills which do great harm to ecological systems.
A nuclear power plant is much similar to a coal fired thermal plant except the way heat is produced to raise steam. At the heart of a nuclear power plant is the fission reaction in the nuclear fuel, uranium, which takes place in the reactor core. In a nuclear fission, a neutron hits the nucleus of an atom of the uranium fuel and spills it, in which two or three neutrons are released and used to cause fission in other uranium atoms. Fission of a single atom of uranium yields energy equal to 200MeV (million electron volts) in comparison to only 4eV in the oxidation of one carbon atom. Therefore, on equal weight basis the total energy from the nuclear fission of one tonne of uranium is about as much as that produced from 2.5 million tonnes of coal combustion. Natural uranium consists of two forms (isotopes) of uranium i.e., U-238, (99.3%) and U- (0.7%). It is the less abundant U-235 that leads to fission reactions. U-235 is the only natural isotope that can be made to undergo fission by thermal (slowed) neutrons. However concentration of U-235, as compared to U-238 can be increased by the process called enrichment. An enrichment of about 3 to 4 percent provides considerable flexibility in the design and operation of nuclear reactors although slowing down of neutrons still remains a necessity.
The surplus neutrons produced in the chain reaction are allowed to interact with other atoms to produce even more neutrons. In such a case, the reaction will continue over a time, until fuel is depleted. However, not all the atoms are available to fission other U-235 nuclei as some of them ’escape’ or are absorbed by the surrounding materials. Three scenarios may be envisaged, considering such a neutron economy. First, if more than one neutron is available for reaction, the rate of fission increases with time and the reaction is ‘super critical’. Second, exactly one neutron is available for fission reaction such that reaction rate is constant and the reaction is ‘critical’. Third, less than one neutron is available for reaction and number of fission decreases with time or the reaction is ‘sub critical’. In a nuclear reactor, an increase in the number of neutron is allowed initially to reach the required reactor power and then maintain at that level. The reaction rate is lowered to reduce power level or to shutdown the reactor by decreasing the number of available neutrons e.g. by inserting a neutron absorbent like boron or cadmium.
The chief potential health hazard in a nuclear fuel cycle is the radiation exposure for uranium mining reactor operations, fuel reprocessing and accidents in nuclear facilities.
The Nuclear industry has, from its beginning, given great attention to public health and safety. It carries operations under different Acts, Regulations and codes of practice etc., based on international accepted safety standards. Radiations from radioactivity releases to the environment from normal operation of a nuclear plant is small compared to the natural background radiation from outer space and the material of earth’s crust with which has lived since the creation of the world. It is even less than the additional radiation that we would get from a single chest x-ray.
All reactor units and fuel fabrications plants have elaborate safety systems build into them and or therefore ‘fail safe’ to ultimate degree possible.
Risk :
Every human activity associated with some risk. Risk is defined at the probability of occurrence of an undesirable effect as a result of an action or lack of it. We are subjected to a small risk all the time whatever we do-even if we stay at home. On an individual basis each person has leant accept an element of risk involved in travelling, smoking, eating, drinking etc. the following activities involve a risk of one death in a million.
Exposure to OAO mSV of ionizing radiation, which is half a day’s occupational exposure at the annual dose equivalent level or living three years in the vicinity of a nuclear power station.
The nuclear fuel complex (NFC), a unit of Department of Atomic energy, Government of India, has been an integral part of Indian Nuclear Power program since its inception in 1970’s
NFC produces nuclear components such as fuel tubes for PHWR and BWR, fuel tubes for breeders, Zircaloy tubes and structures, and non-nuclear components such as seamless stainless steel tubes, special materials having high technological applications in atomic energy, defense, space and electronic industries.
This report is based on the study of solvent extraction process carried out for separation of metals, mainly concerned with the separation of columbite-tantalite ore to obtain Niobium as a metal of interest. It involves processes like Ore dissolution, scrubbing, stripping, precipitation, repulping, drying, calcinations, etc.
The production of these materials involves a variety of highly sophisticated equipment, advanced techniques, clean working environment and specialized technical skills. The different techniques used in this plant include multi stage liquid- liquid extraction, pyro-metallurgical reduction, electron beam melting, vacuum distillation, packed column distillation, sub- boiling distillation, zone refining, electro-refining, etc.
SMP is engaged in regular production of high purity materials such as antimony, bismuth, cadmium, indium, selenium, tellurium, zinc, gallium, phosphorous oxy chloride, gold, GPC, etc., in addition, plant products tonnage level tantalum pentoxide and Reactor grade Niobium such as rod, sheet, crucibles, thermo wells, reactors, etc. The high purity materials are used in semi conductor technology for the synthesis of compound semi conductors, and as dopants, diffusants, solders, etc.
Tantalum is used in a variety of temperatures and corrosive atmospheres. Tool grade tantalum pentoxide finds its applications in tool industry. Reactor grade niobium is used in nuclear industry for alloying of zirconium to produce ZrNb alloys. Zirconia is used for making artificial diamonds. Titanium by virtue of its excellent corrosion resistance, lightness, good alloying property, finds extensive application in aero space industry and chemical industries.
The source material for producing Zirconium metal is the mineral Zircon. It occurs in association with other minerals in the beach sand deposits of Kerala, Tamilnadu and Orissa. Indian Rare Earths Limited – a unit of DAE, processes these deposits to separate the individual minerals and supplies the Zircon sand.
Zircon is zirconium silicate containing 67% zirconium in association with about 2% hafnium. The removal of hafnium is an important step in the nuclear metallurgy of zirconium as hafnium is an absorber of neutrons. The permissible limit for hafnium is only 100 parts per million in zirconium.
▲ The first step of removal of silica involves the treatment of zircon with caustic soda at 600degC. ▲ The resulting frit is washed with water to remove water soluble sodium silicate. ▲ (^) The zirconium hafnium values are brought into solution with concentrated nitric acid. ▲ The separation of zirconium from hafnium and other impurities is achieved by solvent extraction using Tributyl phosphate in kerosene as the solvent, where the zirconium is preferentially extracted into the organic phase leaving hafnium and other impurities remain in aqueous phase. ▲ The zirconium-laden organic is then stripped to recover pure zirconium in aqueous solution. ▲ Zirconium hydroxide is precipitated from the pure solution using ammonia and the hydroxide is filtered, washed, dried and calcined in a rotary kiln to obtain pure zirconium oxide. The oxide is hammer milled to fine powder.
To ensure adequate mechanical strength in zirconium components in the reactor core and for dependable corrosive resistance at elevated temperatures and pressurized water environment, zirconium has to be alloyed with other elements. Zircaloy-4 containing 1.5% Tin and minor amounts of iron and chromium is used for the fuel cladding, calandria tubes and other fuel components. Zirconium -2.5% Niobium alloy is preferred, for pressure tubes by virtue of its higher mechanical strength and better behavior in the reactor environment.
▲ As molten zirconium is too reactive to be contained in conventional refractories, a special melting technique is adopted for producing the alloy ingots. ▲ Crushed sponge is briquetted with the required alloying additions and a consumable electrode is fabricated by welding together the briquettes. ▲ The electrode is melted in a vacuum arc furnace, employing a deep water-cooled copper hearth. ▲ The electrode which melts in the arc heat, gets chilled a solid ingot is formed in the cylindrical copper mould. ▲ Two or more first melted ingots, after surface cleaning and trimming, are welded together to form a large electrode for remelting in a bigger arc melting furnace to produce a more homogenous ingot.
The zirconium alloy ingots, which are hot extruded in the form of mother blanks are conditioned and subjected to cold rolling/ pilgering in multi pass with intermediate vacuum annealing to meet the specifications of finished products in terms of dimensions, texture, microstructure and mechanical properties.
Vacuum arc remelting :
Generated by the electric arc melts the tip of the electrode and a new ingot is progressively formed in the water-cooled mold. A high vacuum is being maintained throughout the remelting process.
VAR Advantages :
The primary benefits of remelting a consumable electrode under vacuum are:
Electron Beam Welding :
Electron Bean Welding (EBW) is a fusion joining process that produces a weld by impinging a beam of high energy electrons to heat the weld joint. Electrons are elementary atomic particles characterized by a negative charge and an extremely small mass. Raising electrons to a high energy state by accelerating them to roughly 30 to 70 percent of the speed of light provides the energy to heat the weld.
An EBW gun functions similarly to a TV picture tube. The major difference is that a TV picture tube continuously scans the surface of a luminescent screen using a low intensity electron bean to produce a picture. An EBW gun uses a high intensity electron beam to target a weld joint. The weld joint converts the electron beam to the heat input required to make a fusion weld.
The electron beam is always generated in a high vacuum. The use of specially designed orifices separating a series of chambers at various levels of vacuum permits welding in medium and non-vacuum conditions. Although, high vacuum welding will provide maximum purity and high depth to width ratio welds.
EBW Benefits
