Presentation on Dross treatment

Srihari Chodagiri

Brief on Sohar Aluminium and about Aluminium dross generation in

Casthouse

Ways of handling the hazardous dross

Project background

Project implementation and results

Continuous improvements

Conclusion

Today we shall discuss:

Joint venture between Oman government and Rio Tinto Alcan

Started primary aluminium production in 2009

State of the art AP-35 technology used for Smelting

Production of 360,000 tonnes of pure aluminium annually

World longest pot line and highest designed capacity for casting lines in it’s time

Sohar Aluminium LLC

Two standard ingot casting lines(30 T/hr) using 4 holding/tilting furnaces of 80T each – Main source of dross

Metal siphoned into furnaces but dross formation inevitable

Lowest melt loss in RTA smelters through use of best practices

One sow casting line (25 T/hr) – Does not form dross (Dross on metal crucible sinks into bottom mixing with bath which is removed during crucibles cleaning later)

Casthouse

Produced as aluminium by-product due to re-oxidation of aluminium when liquid metal comes in contact with air

Essentially consists of Al2O3, Bath, carbides, oxides, nitrides, fluorides, dirt and other impurities

Classified as class 4.3 hazardous material by U.S Department of Transportation(DOT), UN 3170

Mainly formed in the furnaces due to reaction of molten aluminium metal with air inside the furnaces

Skimmed into bins before starting the casting – around 100 tonnes of dross gets generated every month

Casthouse - Aluminium dross:

Dross consists of rich mixture of metal, oxidised aluminium and bath that can be entirely made use of.

Thermiting is burning at very high temperatures (due to chemical reaction) – temperatures can reach to the order of 1500C

Thermiting needs to be stopped as soon as possible to preserve the metallic content in dross

Casthouse - Aluminium dross(contd)

Different methods of cooling are used like dross pressing, inert gas cooling, dumping on open ground and cooling down, etc.

Metal particles can be collected and can be charged back into furnace. But fumes from wet dross are very harmful to the people

Segregated material can then be given to dross processor who could further extract metal and mix the remaining material into refractory

Ways of handling dross:

Early 2009, Sohar Aluminium started facing problem on how to deal with the dross pile up which crossed 500 tonnes of this hazardous material accumulated on landfill

Containers stuffed for onward transportation have been struck at Port terminals for clearance

With no foreseeable dross processor available in country and with increased realization that transportation across international boundaries is becoming almost impossible to deal with, management decided to set up a cross functional team to come up with solution to this burgeoning problem.

Project background

Based on team members’ experience, it was planned to initiate processing of the Casthouse dross internally in smelter making use of the available equipment in bath plant.

Bath treatment plant in Carbon is designed to recycle the cover bath on spent anodes material, bath/dross mixture from crucible cleaning and tapped bath from pots

Alumina is mixed into this recycled material which is processed at the rate of 150 tonnes every shift

Comparative analysis on bath/dross mixture removed from crucible cleaning and the dross from furnaces was done at Laboratory revealed no significant difference in the elements present.

Project implementation and results

Cover Bath

Anode Block

Anode Block

Liquid Aluminium

Tapped Bath (Electrolyte)

Cathode Cell

Anode Block

Casthouse dross to be tumbled in Bath circuit

Casthouse dross

Project implementation and results(contd)

Initial trials done with dross lumps of pressed dross after breaking them with crust breaker and results were found encouraging.

But several operational problems surfaced with the use of this pressed drosswhere it was found getting increasingly difficult to process at rotary tumbler and that it needed smaller chunks of dross.

Also, it is observed in Casthouse that thermiting keeps taking place on a large number of dross bins reducing the value of dross material while dross pressing operation for one bin is taking place.

As well many a times, thermiting does not stop even after completion of dross pressing.

Project implementation and results(contd)

Project team decided to explore new ways of dross treatment to be able to stop thermiting as quickly possible as well as to maintain dross lump size to be small

Based on experience of team members, suffocating the dross by cutting down the oxygen/air would be an effective way.◦ 6 new bins with lids matching were made for trials

Project implementation and results(contd)

Upon further trials, it has been identified that treatment using inert gas is effective to stop thermiting as well keep the dross lump size to be small

Several trials were carried out to calculate the inert gas requirements, time and temperature studies for cooling, etc

At the same time, analysis of this dross material is carried out in lab in comparison to bath/dross material from crucible cleaning and no abnormalities were found. Given that Sohar Aluminium produces only pure Aluminium with no alloying elements added, no detrimental impurities are observed, with onus on effective thermiting control

Project implementation and results(contd)

Material Composition

(%)

Metal crucible cleaning

(%)

Skimming

station (%)

Siphon cleaning(%)

Bath crucible cleaning)

(%)

Dross, Ar+N2

Cooled (%)

Dross, N2

Cooled (%)

Treated in Dross press

Al2O3 6.42 15.8 7.91 1.9 75.4 62.2 69.5Bath 83.23 50.91 62.53 97.8 15.4 12.2 17.7

Metal Portion 9.18 32.38 29.15 0.0 8.8 24.9 11.5Carbon 0.6 0.42 0 0.05 0.22 0.72 1.28

        Elemental Composition (%)

Si 0.024 0.137 0.188 0.02 0.02 0.04 0.03Fe 0.025 1.375 0.176 0.06 0.07 0.03 0.03

This treated dross is continuously supplied to bath plant where it is fed into rotary tumbler mixed with other recycle material which then gets separated at drum magnetic separator where the metallic balls are collected into a bin

Project implementation and results(contd)

These balls obtained at bath plant are taken for segregation manually from which the metal balls are directly brought back to Casthouse for metal recovery.

This way of handling the dross has been found to be effectively working with no detrimental effects observed in this process of bath recycling circuit.

Only instances reported were when improperly cooled down dross resulted in ignition of recycle material on floor attributed to thermiting present in dross and this is addressed with the improvements made for dross treatment in Casthouse

Project implementation and results(contd)

The recovered metallic balls were directly brought back and charged to furnaces for immediate metal recovery.

The balance dross material which is quite low as compared to other additives like tapped bath, alumina is mixed in the bath circuit for recycling

Project implementation and results(contd)

Several improvements were put in place in Casthouse to effectively stop the dross thermiting

◦ Developing chambers to put bins inside for cooling instead of lids

◦ Installing 1000 litre liquid argon tank for continuous inert gas supply

Continuous improvements

Redesigning of the skimming bins to match furnace height. This has resulted in many advantages Reduced the number of skimmed bins Eliminated the need to empty the bin inside

Casthouse in bigger skip Quick cooling due to optimized enclosure inside the

cooling chamber

Automating the dross treatment initiation, operators only needed to press start.

Continuous improvements

Emptying of dross inside Casthouse has completely been eliminated with these improvements put in place and the cooled down dross bins were directly taken to bath plant for immediate processing

This has resulted in the improving cleanliness in Casthouse to a great extent.

Continuous improvements

The project was able to completely address the issue of dross accumulation. In fact, as outcome of the project, company was able to realize the benefits of this internal processing to completely make use of the dross material

Several new projects have been taken to carry out recycling of waste in many other areas that lead to several cost improvements

I would like to sign off saying this new methodology adopted quite clearly proves, as could be seen from the results of the last 4 year period, that dross could be entirely made use of provided◦ there is good control on thermiting, ◦ there is siphoning of metal for less dross generation ◦ there is no alloying of elements◦ it gets thoroughly mixed in the bath circuit.

Conclusion

Thank you