Sulfur stream identification and selective removal of heavy metals from ironmaking blast furnace dust

Abstract

The main waste products of ironmaking at Tata Steel IJmuiden are CO2¬(g) and a solid waste stream consisting of mainly carbon and iron-bearing compounds. This solid waste is generally called Zn rich filter cake which contains too much Zn to be reintroduced into the iron making process. Besides Zn, Pb and Cd are also present. These filter cakes originate from the flue dusts that are expelled from the top of the blast furnace. Currently the solid waste is separated based on size distribution which allows the recycling of most of the materials and concentrates the heavy metals into the Zn rich filter cake. Previous research has shown that these cakes are resistant to regular leaching methods for selectively dissolving the contained heavy metals. This is due to their sulfide form. The methods that do work (such as acid leaching) also dissolve additional material such as iron, which is the main component of the waste stream. Therefore Tata Steel Europe is investigating whether alternative options are available to selectively extract these heavy metals and reintroduce the remaining material as secondary ore back to ironmaking.

In this research the option of adding oxidants with and without additional microwave treatment is investigated. Based on what is applicable in reality, literature and thermodynamic evaluations several oxidants were selected to oxidize the metal-sulfides to metal-oxides which can be readily leached in ammonia based media. Furthermore the material was characterized and an investigation was done on where the sulfur originates and whether this stream can be reduced or avoided. The quality of the results was verified by comparing the heavy metals dissolved into the leaching solution by ICP analysis and by determining the heavy metal content in the solid material before and after leaching with XRF. It was found that 98% of the sulfur originates from the blast furnace and only 2% from the addition of sodium sulfide at the water cleaning department.

Leaching experiments were done with a 40 °C leaching solution, a 5:1 liquid to solid ratio. Leaching solutions were all ammonia based due to its selectivity to heavy metals. Leaching the sample material without microwaving or addition of oxidants the highest Zn, Pb and Cd recovery was 54%, 0.5% and 24% respectively. Potassium permanganate as the oxidant resulted in the highest extraction of Zn at 67% in a 2M ammoniumsulfate, 9M ammonia leaching solution. Ferric chloride in a 2M ammoniumsulfate, 9M ammonia leach solution reached extractions of Zn and Cd at 64% and 45% respectively for the IJmuiden filter cake. For Pb the best extraction was 5% in a 2M ammoniumcarbonate, 9M ammonia leach solution with the same oxidants. Microwaving the sample during leaching resulted in a higher extraction for both Pb and Cd, whilst matching the non-microwave oxidant leaching extraction for Zn. The highest extractions were 61%, 42%, 83% for Zn, Pb and Cd respectively. Aging has also been proven to have an detrimental effect on the extraction of heavy metals, however the aging effect is negated when using microwave treatment or oxidants. Extractions for lab reactor leaching of fresh samples without added oxidants or microwaving with an ammonia sulfate solution, resulted in extractions of 62%, 0%, 40% for Zn, Pb and Cd respectively.

It has been verified that most of the sulfur comes from the blast furnace. Further it has been reaffirmed that the metal sulfides this sulfur forms are difficult to leach out. One study states that metal sulfides can and will form in the blast furnace. The current study supports that, even though no direct study of the blast furnace itself was done. Oxidants and microwave assisted leaching seem to work well for Pb and Cd, but are less effective on extracting a higher percentage of Zn. It is recommended to use this method in combination with another process, a different ironmaking process such as HIsarna would be preferable due to the reduction of ZnS to ZnO which is readily leached with an ammonia leaching solution.