Study into the electrochemical dissolution of tin present in tin-iron alloys for tinplate recycling

Abstract

In the current industrial age the amount of secondary material resources is expected to increase at a fast rate. Within the steel supply of these secondary resources, the share of End-Of-Life(EOL) scrap will become bigger over time compared to new industrial scrap.
The waste stream of EOL steel scrap is heterogeneous and contaminated, complicating existing recycling routes used for industrial scrap. A way to combat the contamination is to apply heat treatment to the EOL scrap. This study investigates the implications of heat treatment on EOL tinplate, a tin coated steel product used for packaging.
Tin is considered a tramp element in a steel melt, thus the coating must be removed before recycling. Heat treating tinplate causes the tin coating to form an intermetallic alloy with the steel substrate, as a result tin atoms become more difficult to separate from the steel scrap product.  The reduction/oxidation behaviour of the two most prevalent tin-iron intermetallic alloys, FeSn2 and FeSn, has been mapped through Cyclic Voltammetry(CV) in both an alkaline and acidic electrolyte. Pure tin and the steel substrate of tinplate scrap were similarly investigated to compare all elements in the system. The goal is to achieve if selective separation and recovery of tin. Further analysis of the anodic dissolution through Chronoamperometry(CA) analysis allow insight in how the current response over time. Based on the CA results, a detinning process is tested over trials of new industrial scrap and EOL municipal scrap. The process has a high detinning rate for new scrap, while for EOL scrap the detinning is hindered by a presence of paint or lacquer. Trials on heat treated EOL scrap showed a small reduction of the negative impact of lacquers and paint. At the same time it was observed that most of the free tin transformed into FeSn2.
The data from Cyclic Voltammetry, Chronoamperometry and detinning trials were used to propose a possible detinning line design for heat treated EOL tinplate scrap.