how low voltage smelting transformer can be modified to a higher voltage to be used in ferronickel production
You need to provide more information about the original use of the transformer. Nickel smelting is a very specialized application that calls for a 'submerged arc'. Basically, the arc is fairly short and will essentially be submerged in a layer of slag on top of the molten material. It is a semi-continuous process in that periodically, some of molten nickel is poured off. The stream of molten nickel is blasted with water that causes the nickel to solidify into small pellets. Steel melting furnaces, by contrast, involve a much longer arc. The slag layer in the furnace is much thinner, and much of the arc is above the slag. It is a batch process in that once the 'campaign' has been completed, the entire melt is 'tapped' - poured into a tundish either to be cast into ingots or to feed a continuous casting process to make billets, slabs or even plates. There are other smelting processes for other materials - for example, silicon is produced via a smelting process. I would guess that if the voltage requirements of the nickel smelting process are different from the requirements of the original utilization, then it would be necessary that the transformer be redesigned and rewound - something that I would not expect to be very practical.
