Mr. Stephen Kwegyir, MSc

Abstract:

Tin is one of the earliest metals used in human history. The amount of tin produced and consumed worldwide in the last ten years has been estimated to be between 300,000 - 400,000 tons annually [1]. Not only is tin an essential constituent of tin bronze, it is also a critical component of alloys for making solders, which are essential for the major drivers of green energy transition; electric and autonomous vehicles, solar PV, semiconductors, etc. [2]. Tin from cassiterite, SnO₂ (main source of tin), has over the years been processed via the pyrometallurgical route. Sulfurization and roasting are primary steps in the process, which are carried out to thermally enrich SnO₂ content in case of low-grade concentrates. Afterwards SnO₂ is treated in reactors, where carbon-based reducing agents are used to reduce tin to the metallic form at high temperatures [3], after which the resulting tin produced is further refined to obtain a marketable grade [4]. The carbothermic reduction of cassiterite, has however, seen several drawbacks such as the generation of environmentally harmful waste gases (e.g., CO₂), high energy and equipment costs, as well as low selectivity with regard to impurities contained in the ore which are difficult to be separated at elevated temperatures [5]. 

A hydrometallurgical extraction route is proposed as a potential alternative processing method for tin extraction from cassiterite to achieve a higher degree of sustainability. This is because it ensures the reuse of chemicals in the process loop and allows for a higher metal recovery at a significantly lower energy consumption and greenhouse emissions [6]. Three different acids, (methanesulfonic acid, sulfuric acid, and oxalic acid) were investigated for their potential to leach tin from cassiterite, and they all proved futile, which supports already existing literature regarding the high chemical stability of cassiterite. A pre-treatment step was deemed necessary to render tin water soluble for subsequent hydrometallurgical processes. 

A reduction of cassiterite in a hydrogen-controlled environment to produce SnO slag, from which tin can easily be leached in acid or alkaline media was investigated.  The formation of SnO slag can be accompanied with the production of a tin metal phase depending on the H₂/concentrate ratio used. During experimentation, a high purity tin nugget (99.5 wt.%) was produced at a reduction temperature of 1300 ⁰C at 30 g H₂/ kg concentrate. The slag formed was soluble in sulfuric acid solution, from which tin extraction is being examined. Other pre-treatment options such as soda roasting and alkaline fusion are being investigated with regard to technological, economic and environmental feasibility.