An ore contains arsenic and antimony as impurities. How are they removed during the process of roasting? (a) by conversion into \(\mathrm{As}_{2} \mathrm{O}_{3}\) and \(\mathrm{Sb}_{2} \mathrm{O}_{3}\) (b) by conversion into \(\mathrm{As}_{2} \mathrm{O}_{5}\) and \(\mathrm{Sb}_{2} \mathrm{O}_{5}\) (c) by conversion into both trioxides and pentoxides (d) by conversion into sulphides

Purifying ore is an essential process in metallurgy, where valuable metals are separated from unwanted materials or impurities. One method of purification is roasting, a thermal treatment that prepares the ore for subsequent processing stages. In roasting, the ore is heated in an oxygen-rich environment, which leads to chemical reactions that help remove impurities, such as sulfur, arsenic, and antimony, by converting them into gases or more easily manageable compounds. This process not only purifies the ore but also enhances its reactivity, making it ready for reduction or other metallurgical operations.

Understanding ore purification is critical to grasp the importance of subsequent impurity removal steps, and how adjustments to temperature and atmosphere can selectively target different impurities within the ore.

The removal of impurities is a pivotal step in the refinement of ores and can be achieved through various methods, including roasting, which is particularly effective when dealing with sulfide ores. The goal is to utilize heat and oxygen to transform the impurities into forms that can be easily separated from the desired metal. In roasting, compounds like arsenic and antimony are oxidized and then volatilize or form slag, which can be physically removed, leaving behind a higher purity metal concentrate.

Importance of Controlled Conditions

It is essential to control the roasting conditions such as temperature, oxygen supply, and reaction time to ensure efficient impurity removal without losing the valuable metal content.

Arsenic (As) and antimony (Sb) are two common impurities found in metal ores that can be problematic in the smelting process. During roasting, these elements react with oxygen to form oxides: arsenic forms arsenic trioxide (\(\mathrm{As}_{2}\mathrm{O}_{3}\)), and antimony forms antimony trioxide (\(\mathrm{Sb}_{2}\mathrm{O}_{3}\)). At higher temperatures, these elements can also form pentoxides (\(\mathrm{As}_{2}\mathrm{O}_{5}\)) and (\(\mathrm{Sb}_{2}\mathrm{O}_{5}\)). These oxide byproducts can be captured using filtration systems as they volatilize (turn into gas) due to their low boiling points, thereby removing them from the ore.

Volatile Nature of Oxides

The volatile nature of arsenic and antimony oxides is beneficial as it simplifies their removal from the reaction chamber. Effective abatement systems ensure these toxic oxides do not escape into the environment.

The chemistry of roasting is based on oxidation-reduction reactions that occur at high temperatures. When a sulfide ore is roasted, sulfide ions are oxidized to form sulfur dioxide (\(\mathrm{SO}_{2}\)), while metal ions are reduced to form the corresponding metal oxides. This transformation is crucial for metal extraction, as these metal oxides are more amenable to reduction into pure metals.

Oxidation States and Reactions

Understanding the chemistry of roasting involves appreciating the role of oxidation states and the thermodynamics of different reactions. In the case of arsenic and antimony, the multiplicity of oxidation states allows for the formation of both trioxides and pentoxides depending on the conditions within the roasting furnace, highlighting the tunable nature of the roasting process.