Dross Processing in India Growing Opportunities

Abstract : As the aluminium industry in India is rapidly expanding,

both primary as well as secondary industries are gearing up

to producing or processing larger volumes of aluminium

metal. Increasing domestic and imported scrap will further

add to this existing volume to remelt units. High cost of melt

loss, environmental concerns and maximizing metal

recoveries shall be key driving factors for Indian aluminium

primaries as well as secondaries to implement better

methods, work practices and technology. This paper looks

at the current & future challenges relating to dross

processing & handling in Indian Primary & Secondary

Industry.

Introduction :

Metal loss as dross during the melting of aluminium is

and should be a major concern to all aluminium producers

and remelters. Cost reduction, the preservation of metal

units and environmental awareness all make limiting dross

formation during the melting process a major goal for cast

houses and remelt centres. Dross handling and processing

to recover metal is equally important. Few decades ago

what was considered normal recovery of 30% of dross

weight will not be accepted today. With current technology

over 65% will be expected as the minimum return of metal

Technology Gap

The fast expansions in the Indian aluminium industry

due to strong economy, major infrastructure projects in

pipeline, huge developments in the automotive sector and

increased building constructions will continuously increase

the domestic demand for Cast, rolled & extrusion products.

The rapid pace of this expansion is creating a technology

gap, where industry presently used to traditional methods

need to change its mind set and invest in the state of the art

technology to stay competitive. The present practices of

melting, melt handing and dross processing in India have

not yet adopted current technology. This technology gap

(Fig. 2) needs to be filled to meet the triple bottom line for a

sustainable industry.

from a given dross weight. All this has been possible as a

result of continuous developments in the technology to meet

the environmental concerns, energy conservation,

profitability and community impact for the sustainability as

shown in Fig. 1 for long term sustainability of the industry the

triple bottom line concept will be essential.

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Dr. M. Nilmani & Mr. H. T. Makhijani NCS (Australia) HTMA (India)

Indian Aluminium Industry

Primary SectorAluminium SmeltersCast HousesCapacity to implement Environmental Processes

Secondary Sector (Formal Sector) Govt. incentives l Waste Audits are carried out l Gradual awareness about environmental issues.

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Tertiary Sector Obsolete technology employed l Recycling / Waste Processing prime business l No information Documented. l Clusters Scattered all over country. l Heavily Dependent on Manual labour.

(Unorgainsed Sector / Informal Sector)

Next 5 Years

>1000 kta

>1000 kta

200 kta

>2000 kta

>2000 kta

400 kta

Industry Practice - Key Issues

This section will address issues relating to the current

practices that need attention to become efficient. Large

gains are to be made with improved changes made to the

system.

Melting Practice :l

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Furnace Operation

Pouring Temperature

Scrap Charging

De-coating & submerged melting

Burner Control

Door Seals Leakages Positive Pressure

Both the organized and unorganized secondary sectors

are increasing their presence in aluminium recycling.

According to the published figures and discussions with the

importers about 80,000 tons per annum scrap is imported

and processed in India. The volume is on rise. In addition

over 320,000 tons of scrap is collected and recycled within

the country. All types of scraps are processed UBC, Scrap

from machine shop, Cables, conductors, foundry and many

other places like wise.

Scrap Melting :

Obviously, the thinner the gauge, the greater is the

surface area (per unit weight of charge) exposed to the

Current Metal Production & Processing

At present the Indian Aluminium Industry, both in Primary

as well as in Secondary, has production / processing

capacity of close to Two Million Tons. Primary Smelters -

Nalco, Hindalco and Balco produced around one million

tons on average in last two years.

Secondary Industry including the following processors

produced over 700,000 tons of aluminium for different end

uses.

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l Aluminium Conductor Units

l Pressure Die-Casting Units

l Foil Plants

l De-Oxidising Agents Producers

l Aluminium Vessels / Pressure Cookers Producers

l Recycling Unit (At Taloja - Capacity : 30 000 TPA)

l Secondary Alloy Producers, etc.

Extrusion Units

In the next five years with the current greenfield and

brownfield expansions, it is anticipated that the production

levels will be more than double as indicated in Table 1.

Reducing import tariff will further increase availability of

metal / scrap for the secondary industry.

Metal Loss and Dross Generation

In India conservative estimates of the dross produced is

120,000 tons, based on dross produced from various

sectors i.e. 1-2% from primary smelters, contaminated light

section scrap 6-10% and machine turnings (unbaled,

oxidized) in excess of 15%. Minimizing this quantity will

bring a big gain to the plants. The quantity of metal in dross

will vary from operation to operation in the range 15% to

70%. Efficient recovery of this metal will bring big gains to

the operation. As per estimates the quantity of dross

available for processing will be more than double by the end

of year 2010 as shown in table 1.

In India both in Primary as well as in Secondary

Industries dross is often sold to outside agencies either

against payment or alternatively on “Toll” basis. Based on

discussions following information was collected to give

indication to the current state of secondary industry.

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furnace environment. Hence the quantity if surface oxide

film formed is greater for thinner gauges (Fig. 3). Oxidizing

furnace atmosphere, direct flame impingement and

excessive temperature will increase oxidation. Therefore

fine scrap should be charged submerged either under the

metal heel in a reverberatory furnace or through a vortex

created by a suitable rotor.

Chemical composition of charge is very important in

affecting oxidation kinetics. When the oxide layer is dense,

continuous & protective, oxidation falls rapidly after the

oxide skin is formed. Conversely when the skin formed is

porous, the oxidation continuous. Alloying elements Mg, Ca,

Na, and Li are known as oxide formers and cause aluminium

melt to dross at a rapid & continuing pace (Fig. 4). The

principal alloying element which will act in this way is Mg.

Therefore “high Mg” alloys eg 5182 and 5152 will form more

dross than low Mg alloys such as 1100 and 3004.

Contaminants

Scrap may carry a coating of paint or lacquer (e.g. UBC),

or contain cutting oils or rolling lubricants. These non-

metalic organics have an appreciable effect on melt loss. If

the scrap is charged to a 'dry hearth' (i.e. no pre-existing

melt), the volatiles will burn readily, prior to the melt forming.

Hence there is little effect on dross generation. However, if

the painted scrap is directly submerged, the volatiles

generated will take part in metal oxidation. Therefore

decoating is recommended for the coated scrap.

The oxidation rate of A-! varies with temperature and

holding time of the melt. The initial oxide that forms is

protective and keeps the oxidation rate to low level. After a

certain induction period (which depends on temperature,

alloy composition and atmospheric conditions), the

protective nature of the oxide skin is lost and a phenomenon

known as 'breakaway oxidation' occurs. Fig. 5 shows when

this breakaway condition occurs for a various holding

temperatures.

Both in Primary as well as Secondary plants it has been

observed that the metal temperature in Holding Furnaces is

often higher than necessary - often due to large drop in

temperature in Launders. This leads to conditions where

thermiting starts and metal is oxidized at the faster

rate.Temperature of the bath goes out of control leading to

huge dross formation. Use of preheated launders with

insulating covers is recommended to reduce metal pouring

temperature.

Whereas Primary Industry by and large is well provided

with temperature measurement instruments, the

Secondary Industry often lacks basic instrumentation to

measure temperature correctly. This results in higher metal

temperatures.

Many places furnace doors are not well sealed, leading

to heat losses. To compensate the burners are run high

resulting air ingress & insufficient control of burners make

the furnace a dross generator. Having a small positive

pressure inside the furnace will prevent the ingress of air.

Heavy firing and/or direct flame impingement can cause

thermiting of the dross layer. If the burner is of high velocity,

direct impingement can cause surface turbulence which will

create further dross. Emphasis must be placed on the melt

transfer without turbulence and using degassing, fluxing,

temperature homogenizing devices that do not create

surface turbulence.

Skimming practice is often not looked after carefully.

Huge amount of metal is caught up with the dross. Boom

design and operator skill becomes important to control this

loss from the furnace bath.

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Metal Recovery from the Hot Dross

If the dross removed from the furnace if left in front of the

furnace it will fume and pollute. This process is further

accelerated by thermiting leading to direct loss of metal by

oxidation. In the primary operations hot dross from the

furnace is collected into drain pan mounted below the skim

pan. Up to 20% of metal is recovered in the drain pan in case

of smelter dross.

There are various methods employed for higher metal

recovery

Cool dross as fast as possible to stop fuming &

thermiting

Cool under reducing atmosphere to stop oxidation

Use of Dross Press

Agitate the hot dross vigorously to get metal out

(MRM Japan)

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Air cooling is commonly practiced in India by spreading

hot dross on shop floor or leaving it in thick steel pans where

it cools slowly while thermiting and fuming continues. If

thermiting is not controlled total metal in the dross can turn

to waste (oxide), as shown in Fig. 6

Drum coolers have been employed by the primary

industry world over. However, its popularity is going down as

new processors are emerging. Drum cooler is a double

jacketed slanted rotating barrel slowly spinning on its

horizontal axis. Hot dross enters from one end and cools

down by the time it discharges out. Cooled dross is

classified and concentrated in various size fractions. Large

fraction rich in aluminium is recycled back in-house while

finer fractions is used in other applications such steel

deoxidizer.

In recent years use of dross press has become popular.

Dross Presses (Fig. 7) have been used widely in foreign

countries to cool hot dross and obtain maximum in-house

recoveries. Since the cooling cycles are very short - often no

more than a few minutes - further thermiting of the dross is

prevented. Hence metal recoveries are comparatively high.

The Process is environmentally friendly since quick cooling

action eliminates fumes generation and the left over dross -

as hard pressed skull is easy to handle / transport than dross

with fines.

Recovery of Metal From Dross

Current practices in India are more towards coal or gas

fired crucible furnaces of various sizes ranging from 50kg to

250kg. These operations are inefficient and questionable

environmentally. Rotary salt furnaces are also used. Both

use a salt mixture of NaCI & KCI to submerge dross and

release aluminium from oxides. The major problem with this

technology is the safe disposal of salt slag. To minimize the

use of salt, new processes have been developed.

Tilting rotary furnace is one of them. These furnaces use

one fourth of the salt used by normal salt furnaces and

employ oxy-fuel burners to speed up the heating cycle and

control the temperature to maximize metal release without

metal loss. Besides higher recoveries as high as 88 to 93%,

a reduction of more than 50% in the flue gas volumes brings

less load and keeps low temperature of the bag house.

Tilting Rotary Furnaces are available in various sizes to

process 5000 tons to 20,000 tons of Dross per year.

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Zero Waste

Though the unorgainsed industry sector may use many

primitive technologies, it has

not stopped them to be

creative to find maximum

recovery of aluminium from

the dross to the extent of

extracting all

f r o m t h e

dross to take

it to the zero

w a s t e a s

s h o w n i n

Fig. 8.

Conclusion

With major expansions in the aluminium industry now,

there are many opportunities to improve melting and melt

handling practices in order to minimize dross formation. A

few areas in which increased melting yields can be

achieved include scrap preparation, submerged charging,

furnace design, burner operation and charging practice.

Currently India is processing substantial quantities of

scrap that results in excessive dross formation. There is a

scope for employing new technology to minimize melt loss,

improve metal recovery from dross by using tilting rotary

furnace technology and meet environmental needs.

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