MANUFACTURE OF BORON CARBIDE

Figure 1. Boron carbide process in operation.

Boron carbide is produced in a heat resistance furnace using boric oxide and petroleum coke as the raw materials. In this process, a large current is passed through the graphite rod located at the center of the cylindrical furnace, which is surrounded by the coke and boron oxide mixture. Heat is generated at the surface of the electrode, due to which boron oxide reacts with the coke to produce boron carbide (Figure 1). The process is inefficient in terms of the production of boron carbide as only 15% charge gets converted into boron carbide. No published attempt has been made to optimise the process using mathematical modelling. Also, experimentally not much work has been done. Therefore, in this first ever study both mathematical and physical modelling has been carried out.
 
A laboratory scale hot model of the process has been fabricated and installed with necessary accessories, such as powder supply unit and electrode cooling unit. The furnace is made of stainless steel body and high temperature ceramic wool insulation. In order to validate the mathematical model, temperature has been measured at various locations in the furnace. Similarly, the product has been collected from the various locations, at the end of each experiment, to analyze them for various species. Temperature inside the furnace ranges from 2600 oC (near the core) to 900 oC (near the charge surface). Temperatures have been measured using pyrometer, C, B and K type thermocouples. A special device has been made to measure the core temperature more precisely. Also, experiments have been conducted to correlate emissivity of the graphite with temperature (Figure 2) in order to get correct measurement of the core temperatures using pyrometer.


Figure 2. Plot of temperature vs. relative emissivity for graphite electrode.
In mathematical modelling, simultaneous heat and mass transfer model has been developed for the resistance-heating furnace, considering boron carbide formation as a typical carbothermal reduction. Coupled transient, partial differential equations have been worked out. These equations have been solved numerically, using the implicit finite volume method to obtain the profiles of temperature and volume fraction reacted (B4C formation) in the furnace. Model’s predictions are validated against experiments and initially it is found that the match is poor. This led us to think to find out the uncertainties associated with either the experiments or modeling. It is found that porosity can affect the simulation results significantly. Therefore, experiments were performed to measure the porosity of the mixture/product at various temperatures (Figure 3). Similarly significant inaccuracies are found in temperature measurements, thermal conductivity of coke, etc. After incorporation of the above corrections a good match is found between the computed and experimental results (Figure 4). This gives a fine example that how a mathematical model can be used to improve the physical model. A Graphical User Interface for the process has also been developed. This process has been optimised experimentally now. A 2-D mathematical model with GUI is also available for the process.

                   Figure 3. Plot of temperature vs. porosity                                 Figure 4. Comparison of simulated (1D & 2D)
                   variation for boric acid and graphite mixture.                            results with experimental core temp.
 
This technology and sofware are ready to transfer to interested industries (Currently under negotiation with Indian Armour Systems Pvt. Ltd.). Interested industries/person, even to setup a plant or transfer of technology can contact us.

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