Hydrolysis Driven Fluoride Losses in Aluminium Smelting: Mechanisms of Fluoride Generation From Condensed Fume Particulates in Gas Treatment Centres

Abstract

Gas Treatment Centres (GTC) are an essential part of the modern primary aluminium smelter, responsible for capturing and returning toxic fluoride emissions into the electrolysis process. The most common technology used is injection type dry scrubbers, with smelter grade alumina (SGA) as the sorbent material. Modern GTCs can capture > 99.5 % of total fluorides, significantly reducing the impact of this industrial process on the surrounding communities and environment.

HF emission released from the stack beyond GTCs with injection type dry scrubbers is known to fluctuate with ambient temperature, observed during the warmer months (summer syndrome), and is even affected by diurnal variations. This release of HF is unexplained by existing process and operational relationships, where research and development have been predominantly focused on HF scrubbing. Evidence for HF generation through hydrolysis from particulate fluorides in the GTC system has been produced in previous laboratory and pilot plant studies. HF emissions were observed to increase exponentially with a significant increase when temperatures exceed 110 °C. However, many questions remain, including the exact particulates and phase(s) involved, how they are generated, and their reactivity as a function of temperature and humidity.

To answer the above questions, it is first necessary to identify which fluoride particulate materials reside on the filter bag fabric (the last location of the GTC system before treated off-gas is released). Microscopy analysis of GTC particulates and used filter bags samples have shown the filter cake to comprise of a condensed fume matrix (sub-micron particulates) with dispersed fines (< 45 μm). Two laboratory scale rigs were designed and built to understand and evaluate these materials: a particulate fluoride generation rig to supply fresh fluoride particulates and an HF generation rig for testing different samples under GTC operating temperature and humidity. This study identified several significant HF generating phases within laboratory-produced condensed fume. Including a polymorph of FeF2 (PDF# 00-03-0143), iron fluoride hydrates (FeF2 · 4H2O, Fe2F5 · 7H2O and β-FeF3 · 3H2O), and two polymorphs of aluminium fluoride hydrate (α and βAlF3 · 3H2O). Of these, β-AlF3 · 3H2O is likely the most abundant phase to exist in industrial smelters, reported to be present in potroom particulates and hard grey scale (HGS). Further, the TGA derivative weight loss peak for this phase ranges from 115 – 150 °C, depending on its concentration level in the material. These temperatures coincide with observations of exponential increase in HF generation from 110 °C onwards in previous studies. Temperature is the primary controlling parameter of HF generation, dictating thermal dehydration and decomposition of HF generating phases. Humidity (source of hydrogen) is a necessary component required in small to moderate levels (3 – 20 g H2O/kg N2 wet) to complete the hydrolysis reaction. Hydrogen for HF generation mainly originate from humidity in the gas stream but is also released in small amounts through dehydration of hydrate phasesin the condensed fume. Moisture in the system also plays a role in forming these reactive (HF generating) metal fluoride and hydrate phases. Formation occurs through the reaction between HF gas or liquid, with an iron or aluminium source (mainly the deposition surface), favoured by high humidity and low temperatures (< 100 °C).

This study identified several significant HF generating phases in condensed fluoride fume particulates, and the role of metal fluoride hydrate phases was highlighted. HF gas generated through these processes at GTC filter bags is a source of currently uncontrolled HF emission from smelters.