Fluoride Emission Control During the Non-Salt Processing of Aluminium Smelter Dross

Transcription

1 Fluoride Emission Control During the Non-Salt Processing of Aluminium Smelter Dross Graham Taylor AECOM Christopher McClung Weston Aluminium

2 Located at Kurri Kurri, NSW, Australia (adjacent to primary aluminium smelter). Local sensitive land uses include grape growing and beef cattle grazing. Recovers aluminium from aluminium smelter dross and recycles scrap aluminium. Produces approximately 24,000tpa of aluminium from 40,000tpa of dross and scrap aluminium. Uses a non-salt process to recover aluminium. Control of fluoride emissions is essential. Weston Aluminium

3 What Is Dross? Dross is an unavoidable by-product of aluminium production. Variable aluminium content depending on the process producing it (30-70% aluminium, 20-25% aluminium oxide and 3-5% silicon oxides). Aluminium smelter dross can contain up to 5% of cryolite (Na 3 AlF 6 ) the source of fluoride emissions. Aluminium Nitrides and carbides as well as metal oxides derived from the molten alloys. Gas evolution can include H 2, NH 3, and CH 4.

4 Recovery of Metal from Dross Traditionally recovered using the Rotary Salt Furnace process. More than 1 tonne of salt cake is produced for every tonne of dross treated. Landfill disposal creates environmental issues (air / water / groundwater / heating). Recovery and re-processing of salt residues required expensive and limited disposal options. Development of salt free aluminium dross treatment processes eliminate the salt dross disposal problem and produces by-products which can be reused.

5 Weston s Dross Process Non salt recovery process. Processes dross from most Australian smelters and from New Zealand. Pre-processing required to remove nonmetallic portion prior to remelting (30% of fines removed) - Large lumps removed (rumbled separately to remove bath); - Remainder passes through grizzly, mill then screen deck prior to remelting. Pre processed high aluminium content dross fed to 2 Rotary Furnaces: Gas fired; Operation temperature 720 o C to 780 o C; 0.3 to 3 Rev/min; Mixed by 5m long claw fitted to forklift.

6 Weston s Dross Process (cont.) Recovered metal to sows or to reverbatory furnace for holding prior to casting. Residual solids processed through metal recovery unit / rotary cooler / ball mill to recover additional high metal product to be fed back into the rotary furnaces. Up to 10,000tpa of ALDEX is produced for on-selling and/or disposal.

7 Rotary Furnace Fluoride Control 1994 emissions directed to a cyclone and baghouse. Baghouse maxiumum temperature 130 o C. 1400m 3 /min. HF emissions mg/Nm 3. Laser monitoring of rotary furnace emissions to meet approval/licence limit of 1.4mg/Nm 3 (1hr.ave) increased fluoride content in dross/smelter aluminium waste results in a modification to inject lime to dry scrub fluoride emissions variability of fluoride removal results in a conversion of the dry scrub fluoride removal to a wet/dry scrubbing system and removal of quad cyclone.

8 Wet/Dry Fluoride Scrubbing System Fluoride emission from the rotary furnace is temperature dependent (720 o C to 780 o C). Fine hydrated lime is stored in a 60t storage silo. Lime slurry (density approx. 1.16kg/L) produced. Atomised slurry injected using compressed into reaction chamber where the high exhaust gas temperature dries the slurry producing a fine lime to react with fluorides (and sulphur oxides). High surface area promotes fluoride fixation as CaF 2. Fine reacted lime (plus CaF 2 and particulates) is collected in a baghouse. The collected product is recycled into ALDEX. Fluoride content of reacted lime approx %. Continuous monitoring of baghouse discharge to ensure it complies with 1.4mg/Nm 3 (1hr.ave) fluoride limit. Original laser monitor was a LASIR replaced in 2006 by a NEOGAS monitor. Neogas Laser monitor configured to Citect PLC, which also records other process control parameters in real-time.

9 Operational Issues Blockages of dry lime silo outlet. In-line blockages of lime slurry (filtration is essential). Quality of lime (reactivity index). Dispersion of lime slurry in reaction chamber. Baghouse temperature (maintain above dewpoint). Maintenance and calibration of the laser monitoring unit. Maintenance of baghouse. Monitoring of operational parameters essential for ongoing compliance (Fluoride emissions every 10 seconds, Baghouse DP, Spray status, Baghouse temp, inlet temp, slurry tank status, dry lime silo tank status, slurry density, alarm status (visual/audible) etc). Process control (feed quality, temperature (hence fluoride emissions).

10 Y-Axis Label Wet/dry Fluoride Scrubbing System Performance

11 Conclusion Weston Aluminium has progressively refined its non-salt based aluminium dross reprocessing facility to minimise its impact on the environment. The process facilitates the efficient recovery of aluminium from the dross and the production of potentially valuable residual ash that can be recycled for use in the steel and cement industries. Fluorides liberated during the dross processing are being effectively captured in Weston s custom built wet/dry scrubbing system. Weston s wet/dry system can maintain fluoride emissions continuously below 1.4mg/Nm 3.

12 Thank You