TIN SMELTING OPERATION USING A REVERBRATORY FURNACE

This work attempts to describe the processes involved in tin smelting operation and refining of ingot obtained afterwards with particular emphasis on The National Metallurgical Development Center, Jos, Plateau State, Nigeria. Cassiterite which is an important ore of tin after being mined is crushed, ground, and then concentrated to remove gangue. The processing method(s) (usually gravity, magnetic, or both) applied depends on the properties of the constituent elements present in the ore. The concentrate obtained after processing is charged into an already pre-heated furnace at a temperature of 1,400oc, which is taped after a period of about 6hours and then refined using heat treatment to obtain tin of high grade. However, alluvial deposits of cassiterite may not require further concentration after mining due to the sorting effect provided by the stream before smelting operation is carried out.

MATERIALS AND METHOD
This part of the project deals with the materials used for tin smelting operation and refining, including methods used in the operation using the reverberatory furnace of the National Metallurgical Development Centre(NMDC)Jos , Plateau state, Nigeria.

MATERIALS USED
• Cassiterite: The cassiterite used here was gotten from Dogo Na Hauwa village in Jos South Local Govt. Area of plateau State occurring as an alluvial deposit containing 67%Sn. The cassiterite was brought to NMDC weeks before the smelting operation in bags, each containing 50kg of cassiterite, up to 24 bags (total of 1.2tonnes). It was stored in the concentrate store of NMDC before time of use. The cassiterite is the primary material (ore of tin) in the smelting operation, because this is where tin (the product) is extracted from.
• Limestone: The limestone used was gotten from Gombe State with 57.6%CaCO3. The lumps of limestone were transported using a trolley to the primary crusher (Jaw crusher) of NMDC Mineral Beneficiation Pilot Plant (MBPP). It was charged into the crusher then conveyed to the roll crusher (secondary crusher) for further communition. It was then screened with a screen of 2.00mm aperture. The Limestone helps to liberate cassiterite from its gangue, raises the melting point of the slag, and also reduces the melting point of cassiterite.
• Anthracite: The Anthracite was sourced from Makeri Smelting Company in Plateau state containing 86.5%C. Anthracite here serves as a reducing agent to remove oxygen from cassiterite and also to supply heat.
• Diesel: Diesel used was gotten from NNPC Jos, Farin gada branch. It provides heat to the furnace for the extraction process.
• Mold: This is a hollow container with a particular shape into which molten material product from the furnace is poured, so that when the substance hardens it takes the shape of the container. The one used for this operation is made of Metal and having a rectangular shape.
• Rambling rod and tapping rod: The tapping rod is used to remove the melt after smelting through the tapping hole, while the rambling rod is used to spread the material mixture in the furnace and also to push the melt towards the tapping hole after smelting, through the charging point.
• Ladle: This is a big long spoon with a long handle and a deep cup-shaped part used in removing ash and slag after tapping. One had perforated bottom while the other had no perforation.
• Shovel: The shovel is used in mixing and in charging the mixture of cassiterite, anthracite, and limestone into the already fired furnace.
• Weighing balance: It is used to measure the weight of materials before mixing and also to measure the weight of product per mold obtained.
• Pyrometer: This is a type of thermometer used to measure high temperature. It is not inserted directly in the furnace, but installed close to the furnace to measure the temperature of the furnace.

METHODS
This involves the procedure applied in the tin smelting process and also refining process of cassiterite in the NMDC reverberatory furnace. The procedures used are explained below:
1. PREPARATION OF THE FURNACE FOR SMELTING
Before smelting proper, the furnace was checked to be in a good working condition and was cleaned to remove materials present from previous operation. Small holes in the furnace that can allow heat loss to the environment were closed using fire clay. The thermometer (pyrometer) was checked to ensure that it was functioning properly to be able to record the temperature of the furnace during operation. Firewood was placed in the furnace, and then hot air was supplied to the furnace through the tuyeres, by pumping air, using a pump, and diesel each coming from different pipes to meet at a junction and then projected to heat the furnace. The furnace was allowed to heat for 8hours and the pyrometer recording was observed to be 1,4000c.

1. CHARGING/CASTING OPERATION OF MATERIAL MIXTURE
   1.2 tons of cassiterite, 48Kg limestone, and 96Kg anthracite were weighed separately and thoroughly mixed together using a shovel.
   After mixing, the mixture was charged into already preheated furnace at a high temperature of 1400˚C through the charging point using the shovel. After which rambling or stirring with rambling rod was done to spread the mixture in the furnace.

The firing continued for 6hrs, till concentrate melt enough to give a pool of molten tin. The molten tin was tapped using a tapping rod through the tapping hole of the furnace into the tapping pot. Impurities floating at the top of the pot were removed using a ladle with perforated bottom and the remaining molten material was cast into different molds to solidify.

1. REFINING OF SLAB USING HEAT TREATMENT
   The slabs obtained after casting were refined using heat treatment. The slabs were placed into an iron cauldron or ‘kettle’ with a small fire burning beneath to make the tin molten. Firewood and charcoal was used as source of heat here with gas being pumped from a gas cylinder to produce more heat. The heat was maintained at a temperature slightly above the melting point of tin but below the melting points of the impurities (melting point of tin is 231.90c). Air was then pumped into the molten tin through a pipe, to float the impurities. These impurities were collected in a surface layer of dross, which was skimmed. The remaining clean tin was ladled into molds to cool and solidify ready for sale.

Refining becomes necessary if the percentage purity of tin after smelting in the reverberatory furnace is not up to ASTM standard. After smelting the percentage purity of tin obtained was a little above 96.0%. The refining process further purified the obtained tin to a percentage purity of 99.85% which is up to ASTM standard.

TIN PRODUCT OBTAINED