ALUMINIUM DROSS PROCESSING-A CASE STUDY

By:

Waheed Ahmed

General Manager

Cast Aluminium Industries

WHAT IS ALUMINIUM DROSS

Aluminium dross is the mixture of metallic aluminium and non metal part mostly Aluminium oxide.

Dross is formed on the surface of molten Aluminum or its alloys by oxidation.

Dross is a valuable by product of any Aluminium Smelter or Aluminium Foundry.

Dross is recycled to recover the valuable Aluminium Metal world wide.

Typical dross generation from a primary operation can be from 0.8% to 1.3% of Aluminium output.

From the secondary smelters it can reach up to 10%.

TYPES OF ALUMINIUM DROSS

Broadly there are two types of drosses White Dross (Primary Dross) Black Dross (Secondary Dross)

The drosses are heterogeneous in nature because of so many steps involved at generator’s site. The recoverable metal content varies from 10% to 70%.

EFFECT OF SMELTER’S PRACTICE ON DROSS QUALITY

Following key steps are involved Drossing Temperature. Fluxing Practice before drossing. Skimming Practice. Cooling of dross as soon as possible.

WAYS TO COOL THE DROSS. Air Cooling. Rotary Dross Cooler. AROS Argon Cooler. Dross Press (Most Widely used Practice)

ALUMINIUM DROSS PROCESSING TYPES

On site Processing

Off Site Processing

ONSITE PROCESSING

In this process the dross is skimmed from the molten metal and is transferred directly to the Hot Dross Processing Machine or furnace. The equipment uses the hot dross thermitting reaction of Aluminium Metal and the Al metal is separated from the oxides by means of specific gravity and the agitation.

The metal is either cast into the ingots or the liquid metal is fed directly to the smelters furnaces.

Following are the challenges associated with this process, Relatively low metal recovery as some metal is used as

fuel. Unstable process encourages Al Metal thermiting. Specialized Equipment requires more maintaince.

OFF SITE PROCESSING

The off site processing of Al dross is the most commonly done practice in the world. In this practice the processor removes the dross from the smelters premises and takes it to the processing facility. The dross from the smelter is taken on the following 2 basis.

The processor buys the dross The processor takes the dross on tolling basis

i.e. the metal goes back to the smelter.

There are a number of challenges associated with the off site processing of the Aluminium dross specially when on tolling basis like, Transportation of dross from Smelter in safe and

environment friendly manner. The storage of dross at the processing facility in

an environment friendly manner. The processing of dross with the BAT to

maximize the metal recovery. The safe and sound disposal / usage of salt

slag / NMP with no liability towards the smelter. The effective & efficient processes of

communication.

CHALLENGES IN OFF SITE PROCESSING

STEPS IN OFF SITE PROCESSING

Segregation Process

Pressed Skull Breaking Process

Tapping Process

Enrichment of aluminium

dross.

Smelting Process in Tilting Rotary Furnace.(TRF)

Chemical Analysis

Met

al &

Dro

ss M

ass

Bala

nce

to C

lient.

Aluminium Dross Physical Quality Check

Client

NMP<1mm

Loose Dross

DROSS STORAGE & ENRICHMENT

The dross is off loaded in covered shed / designated bunker.

The quality inspection team keep record of each delivery by visually inspecting the quality of dross.

Any observation of foreign items and un melted alloying metals is immediately reported back to the generator.

The breaking of dross pressed lumps is done by the JCB Jack Hammer.

The loose material is passed through a enrichment process where by NMP is taken out the process.

PROCESSING OF DROSS

There are many methods and technologies available for the processing of dross. The key criteria of selection should take into account the local environmental laws, smelter’s vision and industrial infrastructure of the country.

TYPES OF OFFSITE PROCESSING

The major types of off site processing are as follows. The Salt Process. Most stable process,

Maximizes the waste thus requires more energy and down stream handling of salt slag requires bigger plant.

The Low Salt process. Stable process, does not contribute much in increasing the waste but still requires down stream recycling of salt slag

The No Salt process. Unstable process, Can be made stable and requires less work down stream to recycle salt free waste.

SALT PROCESS

The Salt Process uses between 20-30% of fluxes that is the combination of sodium and potassium chlorides with a small portion of cryolite.

The challenges associated are, The washing of salts. The inertization of oxides. The reuse of oxides in other allied industries.

LOW SALT PROCESS

The Low Salt process uses between 8-14 % of fluxes having the same composition as of the salt process.

The challenges associated are the same as that of the salt process. The washing of salts. The inertization of oxides. The reuse of oxides in other allied industries.

SALT LESS PROCESS

The Salt Less process uses no additional fluxes at all.

The challenges associated are, To achieve the maximum recoveries equivalent

to the more established salt processes. The control of thermitting reaction during the

melting of the dross is of utmost importance and requires experience & special technology / tools.

Manageable environmental challenges. No washing is required and relatively easy use

of oxides in other industrial sectors.

SALT SLAG / NMP HANDLING , STORAGE & DISPOSAL

Salt slag handling, storage and disposal should be in accordance with the local authorities rules & regulations. It should be kept in mind that the smelter should not get any complaint for the salt slag or NMP. A no liability must be provided to the smelter.

REPORTING MECHANISM

Mass balance of the complete incoming material with traceability.

Chemical Analysis : The analysis of each heat to be marked on the sows as per smelter’s requirement along with the heat number for tracebility.

All the steps involved in the dross processing must be reported to the smelter on weekly, fortnightly and monthly basis.

The visits between the smelters & the processor keeps the relationship close and the technological collaboration not only improves the benefits of both the organizations to the maximum but also helps to develop a trust between the organizations.

DISPATCH OF METAL TO SMELTER

Metal is weighed and the specific heat numbers are marked along with the weighs on the metal and is sent to the smelter in an safe and environment friendly manner.

Before the dispatch of metal to the smelter the metal is checked for the quality assurance so that there is no complain in the final product from the smelter and is safe for the use of smelter as well.

CONCLUSION

Dross is not a waste but a valuable by product of the smelter.

Dross Handling at smelter is of utmost importance.

Reporting mechanism and complete traceability of the dross by the processor is key a factor for smelter’s peace of mind.

Vision of the smelter / authorities is the vital factor in achieving the “Zero Waste Status”.

Down Stream handling of the waste is very important in selecting the dross processing process.

Thank You