Pyrometallurgical removal of zinc from basic oxygen steelmaking dust – A review of best available technology

Abstract

Approximately 20 kg of dust and sludges are produced per ton of liquid steel produced via the blast furnaces (BF)/basic oxygen furnace (BOF) production route. Many of these dusts are recycled through the sinter plant or blast furnace route without issue, but high zinc content dusts are routinely landfilled. Hydrometallurgical techniques, such as alkaline leaching, that are often utilized to remove zinc from electric arc furnace dusts are inappropriate for recovery of material from BOS dust due to the lower zinc concentration present and extra post-processing steps to utilize the separated iron product. Pyrometallurgical treatment through a rotary hearth furnace (RHF), in processes such as FASTMET®, can currently be considered as the most commercially attractive option for the processing and recovery of iron and zinc units when employed as part of an integrated steelworks. The crude zinc oxide produced is suitable for sale to zinc smelters, and the direct reduced iron produced provides process benefits through use, such as reduced blast furnace coking rates and increased productivity. The advantages and disadvantages of variations and alternatives are reviewed with regard to future developments.

Introduction

An integrated steelworks, consisting of blast furnaces and a basic oxygen steelmaking facility, is an example of effective materials efficiency in 21st century manufacturing. The most common process for the production of steel is the basic oxygen steelmaking (BOS) process, accounting for well over a billion tons of steel produced globally (World Steel Association, 2017a,b). A significant volume of BOS dust is created as a by-product of the production of liquid steel through the BOS process, which is generally recovered through the off-gas cleaning system. BOS dust is also known as basic oxygen furnace (BOF) dust, BOF fume, or BOS slurry, and varies depending on the origin of the material. For simplicity, this paper defines BOS dust as the fine ferrous by-product produced during the blowing period in an oxygen converter/furnace at a steelmaking facility, which is wet-scrubbed from the off-gas system of the steelmaking plant and will refer to the material as such when citing references using differing nomenclature.

Typically, BOS dust is highly metalized and has iron content (>60 % Fe_Tot) comparable with good quality ores, and therefore could be a valuable secondary source of iron units for an integrated steel plant. While the recirculation of fine iron-bearing materials back into the blast furnace is commonplace, when these materials are contaminated with zinc, the typical process routes become unsuitable. Because the blast furnace process is remarkably sensitive to volatile metals most critically zinc these, zinc-containing materials have been ordinarily disposed of through landfill. The reason for the sensitivity to zinc is that once charged into a blast furnace, any zinc component is reduced to elemental Zn. Due to the low boiling point of the metal (907 °C) compared to the furnace temperature range (1600–1650 °C), the vapor rises back through the furnace stack and re-condenses (Singh, 2012), leading to condensation of scaffolds (accretions) of zinc on the walls of the furnace. These deposits can affect both solid and gas flow through the furnace negatively impacting productivity and risking damage to the furnace lining through burden slips. Zinc is also known to attack refractories in the upper stack of the furnace and therefore potentially impact on campaign life (Narita et al., 1981). As such, the concentration of zinc loaded to a blast furnace is tightly controlled, with levels around 100–120 g/THM generally permissible (THM = tons of hot metal). In context, for a plant producing 10 Mt of liquid steel per annum, this would allow for a maximum 1 kt of zinc to be charged to the furnace per annum if these limits are to be followed. The dilution of zinc bearing wastes and reintegrating them into existing steelmaking processes are therefore not suitable for processing zinc-bearing wastes on a sustainable scale. The volume of zinc re-entering the steelmaking process through galvanized scrap steel recycling is simply too much.

Zinc and iron are routinely married together through the hot-dip galvanization process, with the zinc providing galvanic protection for the steel. It has been postulated that without substantial improvements to the recovery rate of zinc from its industrial uses (principally hot-dip galvanizing) global zinc reserves will be outstripped by demand as early as 2050 (Daigo et al., 2014). It is therefore clear, that without a go-between process to economically remove zinc from the steel material cycle, the process of hot dip galvanizing will be dependent on depleting global reserves of zinc. A number of hydrometallurgical techniques, such as alkaline leaching, are often utilized to remove zinc from electric arc furnace dusts (Lin et al., 2017; Shawabkeh, 2014; Dutra et al., 2006); however, these are inappropriate for recovery of material from BOS dust due to the lower zinc concentration present, in the latter, and extra post-processing steps to utilize the separated iron product. Nevertheless, ammoniacal leaching has been reported to have results for wastes from BOS furnaces where the zinc content is 2.82 % (Gargul and Boryczko, 2015).

There are several emerging novel techniques for the pyrometallurgical separation of zinc from steelmaking dusts, with one of the most promising being rapid microwave carbothermal reduction (Sun et al., 2008) which has advantages such as rapid reaction times (99.99 % Zn removal in 15 min at 1100 W) and feasible capture of a metallic zinc product. Hybridized pyrometallurgical/hydrometallurgical techniques have also been studied such as chlorination roasting followed by a leaching step (Jaafar, 2014). These have advantages in the lower processing temperatures required (750 °C) and hence lower energy input when compared to other pyrometallurgical techniques. However, the disadvantages of a poorly valorized ferrous product and comparatively low Zn removal (97 %) mean upscale is unlikely. These technologies are still in their infancy, having not achieved pilot scale operation as of the time of writing, and as such will not form the main basis of this review – instead focusing more on proven scalable technology and how they might be applied to process BOS dust.

The need to adopt commercial routes to the re-use of zinc waste from BOS dust is part of a broader move to lower the environmental impact of the steel industry (Allwood et al., 2010; Kim and Worrell, 2002) and ensure its economic sustainability (Fisher and Barron, 2019; Lobato et al., 2015). Thus, there are significant incentives to find ways to as much waste as possible. Herein, we review approaches to the re-use of BOS dust, with particular focus on those that provide the best available technology to supplement an integrated steelworks in processing its zinc-bearing by-products, and to make recommendations to the industry based off viability, technical feasibility and environmental considerations. The goal is to ensure that BOS dust is considered a material with potential as resource rather than a waste.

The objective of this paper is to provide a short to medium term outlook for the feasibility of pyrometallurgical separation techniques for BOS dusts. The recycling of low zinc bearing materials is routinely performed at most steel plants through dilution in the blast furnace burden. Very high Zn bearing materials such as Electric Arc Furnace dusts are very well studied and reviewed (Lin et al., 2017; Walburga Keglevich de Buzin et al., 2017) due to the hazardous nature of the material. Materials that are moderately contaminated with Zn such as BOS dust are comparatively understudied, despite the fact they are produced on a far greater scale.