Tin does not occur as the native element but must be extracted from various ores. The main commercial tinfields of the world are West of England, Brittany, and Erzgeburge; Burma, Siam, Malay States, and Indonesia; Bolivia; Nigeria; Australasia; and Southern Africa. Cassiterite (SnO2) otherwise known as tinstone, is the only tin mineral that is in sufficient abundance in the earth’s crust to have any commercial value. Beside cassiterite small quantities of tin may also be recovered from complex sulfides such as stannite, cylindrite, franckeite, canfieldite and teallite. When chemically pure, cassiterite contains 78,6 per cent tin but, when contaminated with impurities, the tin content varies between 70 and 75 per cent. The following pictures show appearance of Cassiterites in two of its most common form: a) Very brilliant black crystals of cassiterite on white mica and b) Gemmy, doubly terminated cassiterite on quartz-mica matrix.
There are different uses of tin di-oxide after its extraction from ores. In conjunction with vanadium oxide, it is used as a catalyst for the oxidation of aromatic compounds in the synthesis of carboxylic acids and acid anhydrides. Tin (IV) oxide has long been used as an opacifier and as a white colorant in ceramic glazes in the form of earthenware, sanitaryware and wall tiles. It can also be used as a polishing powder sometimes in mixtures also with lead oxide, for polishing glass, jewelery, marble and silver.
There is limited data on tin determination in natural samples. This partly resulted from the resistance of cassiterite to acid attack, commonly used for chemical decomposition of minerals. Furthermore, the solution containing Sn requires careful handling because Sn is readily oxidized and precipitates in concentrated nitric acid as hydrous stannic oxide or metastannic acid.
3 Sn + H2O + 4HNO3 = 3 H2SnO3 + 4 NO
But the precipitated exhibits powerful absorption properties for certain ions, for example, iron, lead, copper, nickel, zinc and antimony. The amount of impurities may be determined by adding to the impure weighed oxide fifteen times its weight of pure ammonium iodide and heating for 15-20 minutes at 425-475OC or until no further fumes are evolved. The tin is volatilized as stannic iodide:
SnO2 + 4 NH4I = SnI4 + 4NH3 + 2H2O
The loss in weight gives the weight of pure stannic oxide present in the precipitated. Gravimetric estimation of tin is time consuming; moreover, co-precipitation of other metal ions requires the removal of these ions to achieve actual tin content.
On the other hand, volumetric estimation of tin by iodimetric titration has also some disadvantages because throughout the entire process CO2 atmosphere to be maintained strictly to avoid areal oxidation of Sn+2 to Sn+4 leading to volatilization of tin at 1140C. Moreover, standard iodine solution is volatile to some extent. The Iodine Solution as well as distilled water issued during the analysis should be entirely free from dissolved oxygen. Thus, alkali fusion techniques and the reduction of Sn by graphite were used for decomposing the minerals. These methods are conducted under an open environment and require a high temperature of about 1200°C, which may cause loss of volatile tin.
In the present work, complexometric method using ethylene di-sodium salt of EDTA and lead nitrate has been used to determine the tin content in the ore. Upon dissolving the ore in acid, a mixture of Sn (IV) and Pb (II) ions (if any) may be complexed by adding an excess of standard EDTA solution. The excess EDTA being determined by titration with a standard solution of lead mixture: the total lead plus tin is thus determined. Sodium fluoride (NaF) is then added to this which displaces the EDTA form Sn(IV) – EDTA complex. The liberated EDTA is determined by titration with a standard solution of lead nitrate at pH 6 while conducting titration.
Sn4+ + H2Y-2 = SnY + 2H+
Pb2+ + H2Y-2 = PbY-2 + 2H+
a) Determination of Tin (Sn) from Cassiterite Ore
A weighted amount of sample is dissolved in 10 ml concentrated HCl and 2 ml concentrated HNO3 upon gentle warming on water bath. The solution is digested for 5 minutes to expel nitrous fumes and chlorine, allowed to cool slightly. 25 ml standard 0.2(M) EDTA solution is added to it and boiled for 1 minute. The whole solution is then diluted to with water, cooled and volume made up to 250 ml in a volumetric flask. Without delay 25 ml solution is pipetted into three different conical flasks. To each flask 15 ml 30% hexamine solution, 100 cc water and a few drops of 0.2% xylenol orange is added. The solution in each flask is then titrated with standard 0.01(M) lead nitrate solution until the colour changes from yellow to red. Now 2 gm sodium fluoride (NaF) is added to each flask due to which the solution acquires a yellow colour owing to the liberation of EDTA from its tin complex. The solution is again titrated with standard lead nitrate until a permanent red colour is obtained. Effect of zinc was also studied by the addition of known amount of Zn++ solution in the sample solution and no effect due to presence of zinc was observed.
b) Determination of other metal oxides in Cassiterite Ore
As the other metal oxides are present in lesser amount, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) has been used in the study to determine the oxides present in the ore.
The following table shows the full composition analysis of Cassiterite Ore sample received at the laboratory. As expected, average tin di-oxide content as evident from the five replica analysis is 89.69 % which falls in the range of 88-95% of SnO2 which is normally present in high grade Cassiterite. The repeatability of tin content determined is also quite good leading to a standard deviation as low as 0.15.
Complexometric determination of tin in Cassiterite Ore is a fast, accurate and inexpensive technique with less stringent requirement of maintaining the environmental condition. The repeatability of the method is good resulting low standard deviation. Other elements interference is also negligible in this process. This technique , consequently, can be used for the determination of tin.