|Title of Invention||
"AN IMPROVED PROCESS FOR THE PRODUCTION OF NIOBIUM CARBIDE"
|Abstract||An improved process for the production of niobium carbide This invention relates to an improved process for the production of Niobium Carbide (NbC) and an intermediate hard composite product consisting of well dispersed NbC phase in a metal matrix. The process comprises fusing a mixture of mobium oxide containing source, carbon source and binder at a temperature above 15000C under inert atmosphere for a period in the range of 15 to 30 minutes,, grinding the above said fused mass to make powder, removing free carbon by known method , leaching the above carbon removed powder with acid to obtain mobium carbide by conventional methods.|
|Full Text||This invention relates to an improved process for the production of Niobium Carbide.
Particularly this invention relates to an improved process for the production of Niobium Carbide (NbC) and an intermediate hard composite product consisting of well dispersed NbC phase in a metal matrix.
o Niobium carbide exhibits high melting point (3613 C)
and hardness, a high resistance to the action of molten metals and metal vapours, high electrical conductivity and good emissiv-ity. These properties determine the field of application of NbC, namely for the manufacture of heating elements for high temperature furnaces and of evaporating plants for aluminium, for protective coatings, for coating thermoionic emission elements, and for use in heat-resistant alloys. NbC powder is used in increasing quantities in sintered cemented carbides in order to replace more expensive TaC. It is also used to prepare Nb metals.
NbC is most often prepared by caburization of Nb2 O 5
with carbon black and less frequently by the reaction of two
elements Nb and C. In the first method finely divided mixtures of
Nb2 O5 and carbon black are pressed into cylindrical blocks and converted to NbC in high frequency furnaces in the presence of
hydrogen under vacuum at 1600 - 18000 C. Similar procedure is
followed in the second method to prepare NbC by the reaction of Nb and C. Reference may be made to Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, A Wiley-Interscience Publication, New York, 1982, Vol.4, p,496.
The major drawbacks of the above processes are that the high frequency furnaces are expensive. The processes become more expensive as these are carried out in the presence of hydrogen under vacuum. The processes also require pure raw materials.
Due to the increase in the demand of NbC, it is necessary to develop an improved process which over comes the draw-backs of the hitherto known processes employed for its production.
The main objective of the present invention is to provide an improved process for the production of niobium carbide.
Another objective of the present invention is to provide an improved process for the preparation of niobium carbide from impure niobium oxide containing starting material such as columbite. ,Yet another objective of the present invention is to prepare an intennediate product comprised of niobium carbide dispersed or surrounded by a matrix of a metal mixture formed from some of the metal oxides in the material mixture. The invention also aims at producing the said product in thermal plasma at lower cost and with high throughput.
Accordingly the present invention provides an improved process for the production of niobium carbide which comprises: fusing a mixture of niobium oxide containing source as herein described, carbon source and binder as herein described at a temperature above
15000C under inert atmosphere for a period in the range of 15 to 30 minutes, grinding the above said fused mass to make powder, removing free carbon by known method as herein described, leaching the above carbon removed powder with acid to obtain niobium carbide by conventional methods such as herein described.
The niobium oxide used may be in the form of impure material such as columbite. Carbon used may be in various form such as graphite powder, activated carbon etc.. The reaction mixture (columbite and carbon) is mixed thoroughly and introduced into plasma either in loose form or in the form of pellets.
The said thermal plasma may be formed for example, by striking an arc * between a graphite crucible which forms the bottom electrode and top graphite electrode having an axial hole to introduce the plasmagen gas such as argon. An extended or expanded plasma arc is formed when the two electrodes are separated without extinguishing the plasma.
The reaction mixture is heated in the thermal plasma in excess of 15000 C. Because of high energy content and high enthalpy of the thermal plasma, the heating is very rapid. Moreover, the ionic species generated in thermal plasma coupled with the high heat transfer in thermal plasma can be instrumental in the formation of niobium carbide in much faster rate compared to the conventional route. Alternatively, any suitable furnace, a resistance type, induction arc etc. can be used, but the use of thermal plasma may be more advantageous.
The carburized mass is obtained in the form of fused, semi fused and well sintered mass. From the metallographic observation it has been found that a multi phase material is formed.
This intermediate material is considerably hard and can have a number of uses such as grinding material, for surface hardening etc. .
The carburized mass is further ground to a finer fraction such as 20 um or below to liberate the niobium carbide phase from the matrix. The well developed niobium carbide phase is found to be surrounded by a metal matrix phase filling the inter granular spaces. The extent of fineness, is decided by the grain size of the niobium carbide in the carbide product.
The carburized product might contain certain amount of free carbon either from graphite crucible used for plasma heating or from the charge. The said carbon is removed by employing conventional froth flotation, centrifuge or any other known techniques employed for the purpose.
The carbon removed powder is then leached with an aqueous solution of a mild acid such as sulphuric acid, hydrochloric acid to obtain the final product viz. niobium carbide.
The following examples are given to illustrate how the process of the present invention is carried out in actual practice and should not be construed to limit the scope of the invention.
In all the examples the impure niobium oxide containing material used is columbite, with the chemical composition given in Table 1.
Table 1 Chemical analysis of columbite
Example 1 : Hundred (100) parts of ground columbite, 25 parts of carbon as graphite powder and 1.5 parts of sodium chloride is thoroughly mixed with addition of a binder (polyvinyl alcohol) and compacted to pellets. The pellets are kept in a graphite crucible and heated in an extended arc thermal plasma with current = 350 A : voltage = 45 V for about 30 minutes at a tempera-
o ture above 1500 C. Argon is used as the plasma gen gas which
also provides a protective atmosphere for the niobium carbide. An intermediate product in the form of fused mass is obtained. In this intermediate product, well developed niobium carbide phase is found to be surrounded by a metal matrix phase.
The said intermediate product was further ground to make fine powder below 20 um and subjected to heavy media separation with or without centrifuge to remove free carbon, if any.
The powder almost devoid of free carbon is leached at 700 C, with
an aqueous acid solution containing 5% H2 SO4 and 3% HF to remove
the metal impurities, primarily iron. The final product is basically NbC with small amount (below 4%) of TaC. The other impurities are below 1%. The presence of TaC in the final product is not detrimental for its use in cutting tool applications.
Example 2 :
250 parts of ground columbite, 70 parts of carbon as activated carbon and 3.0 parts of sodium chloride is thoroughly-mixed and compacted to pellets. Plasma heating (current = 400 A,
voltage = 50 V) of the pellets kept in graphite crucible is
carried out for 45 minutes. An intermediate product with well
developed niobium carbide in a metal matrix phase is obtained.
This said product is further ground to below 30 um and subjected
to heavy media separation to remove free carbon. This is followed
o by acid leaching at 70 C with aqueous acid solution containing 2%
HF and 5% H2 SO4 to obtain NbC powder. The main advantages of the invention are :
1. High quality product of NbC is obtained by carbothermic
reduction of low cost impure niobium oxide containing materials
such as columbite.
2. High rate of production at low cost is obtained by utilizing high enthalpy argon thermal plasma, which also maintains a protective atmosphere for oxidation of niobium carbide.
3. An intermediate hard product comprising of well distributed NbC phase in a metal matrix is obtained which can have a number of uses such as grinding material, for surface hardening and other wear resistance applications etc.
1. An improved process for the production of niobium carbide which comprises: fusing a mixture of niobium oxide containing source as herein described, carbon source and binder as herein described at a temperature above 15000C under inert atmosphere for a period in the range of 15 to 30 minutes, grinding the above said fused mass to make powder, removing free carbon by known method as herein described, leaching the above carbon removed powder with acid to obtain niobium carbide by conventional methods such as herein described.
2. An improved process,as claimed in claim 1 wherein niobium oxide containing source such as columbine is used.
3. An improved process as claimed in claim 1-2 wherein carbon source used is such as graphite powder, activated carbon.
4. An improved process as claimed in claim 1-3 wherein binder used is such as polyvinyl alcohol, polyvinyl acetate.
5. An improved process as claimed in claims 1-4 wherein removal of free carbon is effected by method such as froth flotation, centrifuge.
6. An improved process as claimed in claims 1-5 wherein acid used in such as H2 SO4, HCL, HF.
7. An improved process for production of niobium carbide substantially as herein described with reference to the examples.
|Indian Patent Application Number||508/DEL/1998|
|PG Journal Number||49/2004|
|Date of Filing||26-Feb-1998|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI -110001,INDIA|
|PCT International Classification Number||C01A 33/00|
|PCT International Application Number||N/A|
|PCT International Filing date|