Recycling of Photovoltaic (PV) Batteries

Batteries are used whenever electrical energy is needed, but there is neither a align="left">direct connection to the public electricity</div>

grid nor a generator-based stand-alone

supply. Batteries store electrical energy as

chemical energy. During discharge, the

chemical energy is re-converted into

electrical energy. Depending on the

battery system, this process is either

irreversible or reversible. There are two

types of batteries: 'primary batteries' and

'secondary batteries'.

Lead-acid batteries are called .secondary

batteries. or accumulators since they are

rechargeable. They again can be divided

into starter and industrial batteries. Starter

and industrial batteries are used to provide

large quantities of energy (e.g. to start a

car, operate electric vehicles, as energy

storage medium for solar applications, as

short-term emergency power source, etc.).

Units generally weigh from a few

kilograms to one ton.

In the lead-acid battery sector, starter

batteries have by far the largest share. In

1995, approx. 96 million units were

produced worldwide (source: Battery

Council International). An annual

production growth rate of < 2% is

expected. Especially in developing

countries, where the number of cars is

growing over-proportionately, high growth

rates in the use of lead-acid batteries are

to be expected. Studies carried out in

Botswana indicate that the number of

batteries used in the automobile sector will

grow by 40-50% over the period from 1995

to 2005 (source: GTZ waste management

project). If we consider China alone, the

most populous country of the world, which

currently starts to introduce private car

transport, it is obvious that high growth

rates in the consumption of starter

batteries must be expected in the future,

especially in developing countries.

Returning used lead batteries to the

recycling loop has a long tradition. Thanks

to the compactness of a battery, its high

lead proportion (>95%) and relatively high

metal prices, it has been worth while for

consumers to return their own or collected

car batteries to the scrap trade or

secondary smelters. The return rate of

spent batteries was thus already high in

times when catchwords such as resource

conservation and environmental

protection, recycling, closed-loop materials

management etc. did not yet play a role.

Even today, the success of lead battery

recycling in developing countries

continues to be determined largely by the

potential earnings of scrap collectors and

traders. In industrialised countries,

statutory requirements to take back spent

batteries have compensated for the loss of

economic incentives in spent battery

return.

In most European countries, battery

retailers are under obligation to take back

spent batteries. Lead batteries also come

from repair workshops, the reprocessing of

scrap car bodies and at municipal

collection centres. In Germany, for

example, this well functioning and effective

collection system has led to a return rate

of more than 95% for starter batteries and

almost 100% for industrial batteries. In

developing countries, too, return rates of

up to 80% can be achieved where buyingup

structures for spent batteries are in

place. In Zimbabwe (source: Central

African Batteries) for example, the entire

demand for local battery production is

covered by recycling of used batteries.

1. Battery scrap – raw material for

recycling

The major source of raw material for lead

recycling are starter batteries from motor

vehicles. Modern car batteries consist of a

PP (polypropylen)-casing, plates (grids

and paste), connectors/poles and bridges,

and PP-separators as insulators between

the plates (Fig 1). Paste consists of Pb,

PbO

reactions which take place during charge

and discharge of a lead acid battery are:

charging:

2PbSO

2and PbSO4. The electro-chemical4+ 2H2O→PbO2+ Pb + H2SO4

discharging:

PbO

2+ Pb + H2SO4→2PbSO4+ 2H2O

Older types of batteries have a hard

rubber casing and PVC (polyvinylchloride)-

separators instead of casings and

separators from PP. In some developing

countries (e.g. Zimbabwe) these types of

batteries are still produced and in use.

Therefore, recyclers have to be aware that

batteries in hard rubber casing and PVCseparator

will come in with battery scrap.

Depending on the type of battery, its size

and design, the material composition of a

starter battery varies. Table 1 compares

an old type of battery in hard rubber

casing with a modern type of battery. The

lead-bearing components of a battery are:

Tab. 2: Approximate composition of Pbbearing

components of a starter

battery

The grids of old types of batteries have a

higher Sb (antimony)-content (~4%) than

the modern maintenance-free batteries

(~2%), which instead add Ca(calcium)

<0,5% to their grid alloy.

2. Recycling of lead-acid batteries

2.1 General considerations

As already mentioned, lead-acid battery

recycling has a long tradition, especially in

industrialised countries. The battery and

scrap trade takes back spent batteries free

of charge or even pays the metal value.

Because the metallic fraction of a battery

consists largely of lead, metallurgical

reprocessing of battery scrap was never a

serious problem. Recently it has been

rather the stricter environmental

requirements that have caused problems

for secondary lead smelters and made

lead recycling less economically viable.

Lead recovery from spent accumulators

can take two basic routes. Either the

components of an accumulators like lead,

plastics, acids, etc. are at first separated

and then processed individually, or the

acid is extracted first and the batteries are

processed as a whole. In the first case,

recycling materials are recovered from all

components of a battery. In the second

case, only lead is recovered (partially also

residual battery acid), whereby organic

components are consigned to energy

recycling. In view of the high pollution

control standards implemented in

secondary lead smelters of industrialised

countries, modern lead recycling does not

pose a significant health hazard to the

local population or the environment.

In developing countries spent lead

batteries are recycled both in industrial

facilities and by informal small enterprises.

Industrial recycling smelters use both the

grid metal and the lead-containing paste to

produce secondary lead. The informal

sector, in contrast, often only uses the

metallic parts of old batteries (grids,

terminals, bridges) to produce articles

such as solders or weights for fishing nets.

The other parts of the battery are simply

dumped in the environment.

Even industrial recycling facilities in

developing countries employ many manual

techniques due to cheap labour. Batteries

are often broken up, emptied, separated

and charged to the furnaces by hand. The

lead extracted is refined and cast into

ingots manually. This creates a potential

hazard for the workers, the surrounding

population and the environment (soil,

ground, water resources, etc.) in general.

Some secondary smelters also buy up

pre-sorted battery fractions, e.g. grids and

lead paste without casings and separators,

in addition to complete batteries. The lead

smelters thus save several processing

stages and do not have to deal with casing

and separator wastes. They are therefore

willing to pay a higher price for the

material supplied. This practice is very

harmful in environmental terms. Through

the dispersed pre-sorting activities, lead-

Grid metal, poles, bridges

Pb 96-98%

Sb 2-4%

(Ca) <0,5%

Paste

PbSO

PbO (PbO

PB 21%

100%

4

gate

Phone: +49 (0)6196 / 79-3093, Fax: +49 (0)6196 / 79-7352, Email: gate-id@gtz.de, Internet: http://www.gtz.de/gate/

Fundamentals of the Recycling of Lead-Acid Batteries

containing residues and wastes arise in

many places and it becomes impossible to

control their proper disposal.

2.1 Metallurgical aspects of lead

recycling from battery scrap

As described before, the lead bearing raw

materials extracted from lead-acid battery

scrap are:

Pb(Sb)

and bridges

PbO (PbO

PbSO

paste

While the first component needs only

melting, the two other components have to

be converted by chemical/metallurgical

processes to obtain lead metal, which

takes place in the furnace.

The first type of chemical reaction

converts PbO (PbO

reduction process:

2 PbO + C

PbO

The second type converts PbSO

PbS, again through a reduction process:

PbSO4 + 2 C

Finally PbS is converted into Pb through

the following reactions:

PbS + Fe

or

PbS + 2 PbO

PbS + PbO

The above mentioned chemical reaction

are sum reaction. That means that there

are intermediate steps in between. The

reactions take place in the melting furnace

at high temperature (900-1200 °C) and

need additives, which are . carbon (in the

form of coal) and iron (in the form of iron

swarf). Impurities are collected in the slag

which requires for example soda ash as

liquidifying and slag forming constituent.

The product of the smelting operation is

crude lead, which needs subsequent

refining, and soda slag as residue. Since

soda slag is water soluble and therefore

hazardous when brought to landfills,

modern lead recycling plants use silica

slag (fayalite slag), which is water

insoluble. It requires, however, a much

higher furnace operation temperature of

approximately 1400°C.

The refining of crude lead takes place in a

refining kettle at temperatures between

400 and 550°C. If only battery scrap is

used for lead production, two subsequent

refining steps are required:

1. Removal of Cu which might have

entered the melts through copper

wires.

2. Removal of antimony originated from

the former grid metal to produce pure

lead

While the removal of Cu is done in adding

elementary sulphur, Sb can be removed

by selective oxidation or by adding sodium

nitrate (NaNO

stirred and a dross formed. The impurities

are now removed from the melt by

skimming of the dross formed. It is obvious

that the success of the refining has to be

controlled by chemical analysis. The

refined metal is cast into ingots for

shipment, sale or further manufacturing.

2.3 Technical steps in battery recycling

In developing countries lead-acid battery

scrap is normally processed in rotary drum

furnaces using liquid fuel as energy

source. Lead bearing feed materials are

either whole battery packs (grids and

paste) where the separators have been

removed or two separate fractions

a) grid metal only and b) paste and other

fines

The flow sheet in Fig. 2 shows a semimechanised

process option for small to

medium scale battery recycling in

developing countries. In this option grids

and paste are separated and individually

processed. Since the grids (2/5 of the total

material) are already in the metal stage,

metallurgical process of converting

they do not need to go through the whole

PbO/PbSO

need to be molten at some 500°C (low

temperature melt), refined and cast into

ingots. Thus, energy and time are saved.

2.3.1 Dismantling of battery cases

and feed preparation

Used batteries are emptied by hand and

the acid is collected in plastic barrels. If

the full barrels are kept motionless for

some time, solid impurities will settle at the

bottom of the barrels. This process of

sedimentation may be assisted by adding

some flocculent. The purified acid is then

decanted and packed for sale. Possible

customers for the recycled acid is the

mining and metallurgical industry which

uses acid in various leaching operations.

The remaining battery sludge is

neutralised with lime. After passing

through a filter press the filter cake may be

charged together with the fine fraction into

the melting and reduction furnace.

In a next step the tops of the acid-free

batteries are cut off by a guillotine shear

and the grid packs are removed from the

battery case. They are fed to a perforated

grinding drum, which rotates in a water

basin. By moving the feed in the drum an

autogenous grinding process starts which

separates the grids from the separators

and, more important, the paste from the

grids. At the same time the perforation of

the drum acts as a sieve. The fines are

separated and carried away by the water.

A bit of lime added to the water neutralises

the acidic solutions and prevents the drum

from massive corrosion.

Instead of this labour intensive method

whole batteries may be crushed in a

hammer mill (Fig. 3) and fed to a grinding/

washing drum for separation.

In both cases, the slurry is pumped

continuously or batchwise to

sedimentation tanks, where the solids

settle at the bottom. The clarified liquid is

returned back to the grinding operation,

while the sludge at the bottom of the tanks

passes a filter press or is left to sun-dry.

The filter or sun dried cake is the main

feed for the melting and reduction

operation which will produce almost pure

lead.

The second fraction . the coarse material

(basically grids and separators) . leaves

the grinding drum at its lower end.

Separators and grids are separated from

each other by hand sorting using a slow

moving transmission belt (Fig. 4). The

metal fraction is the main feed for the low

temperature melt producing a PbSb-alloy.

Empty battery cases and covers with the

attached poles, bridges and remaining grid

parts are charged to a wet hammer mill,

where metal parts and remaining paste

are separated from the plastic. The output

of the mill passes a perforated drum,

where solids and slurry are separated.

While the solids (metal and plastic parts)

are hand sorted, the slurry with the fines is

added to the slurry obtained from the

grinding drum. The solid metal parts

supplement the feed of the low

temperature melt.

The plastic residues of the dismantling

operation either have to be dumped (in the

case of PVC-separators) or can be used

as fuel (PP, cellulose, hard rubber) in

cement factories. In this case it is

important that no lead remains in the

plastic product.

2.3.2 Melting and reduction operation

of paste and battery fines

The filter or sun baked cake of paste is

charged to a short rotary drum furnace

(Fig. 5) where the charge is melted

together with slag forming constituents

(soda ash = Na

additives (Fe-swarf, coal). The ratio of the

feed materials Pb-fines : Fe-swarf : Soda

ash : coal is approximately 10:2:1:0,5. The

energy needed for the process is obtained

from the burning of the coal within the

furnace and by an additional burner

running on heavy fuel oil, paraffin,

diesel, waste engine oil, etc. To save

energy and to achieve a higher furnace

temperature the combustion air should be

preheated.

Depending on the temperature and the

amount of feed material in the furnace, the

reaction time will be 2-3 h. Due to the

difference in specific weight the molten

lead produced settles at the bottom part of

the furnace. When enough lead has

accumulated, it is tapped into a mobile

ladle and transported in liquid stage to the

refining kettle.

With less PbO/PbS in the slag and more

Pb-metal produced the viscosity of the

slag increases. This hampers the

separation of the small Pb-droplet from the

slag. To overcome this problem either

more soda ash has to be added or the

temperature in the furnace must be

increased. Both solutions have negative

effects. While the first measure increases

the amount of slag which finally needs to

be discarded, the second measure leads

to higher energy consumption and

evaporation of lead into the off-gas.

It is more advisable to tap the lead before

the optimum of recovery is achieved and

to leave the remaining lead-rich slag in the

furnace for a second or third cycle with

new feed material.

After a number of production cycles the

amount of slag in the furnace will be too

large to continue the operation. By adding

a bit more coal and fresh soda ash a slag

wiith a low Pb content (<9% Pb) can be

achieved which is then tapped from the

furnace together with the finally produced

lead. While the lead metal is forwarded for

refining the slag has to be dumped.

Off-gas and flue dust from the operation is

sucked of and treated in the off-gas

cleaning system.

2.3.3 Melting of grids, terminals and

bridges

The coarse fraction of the crushed battery

scrap is fed to a crucible furnace, melting

kettle or rotary drum furnace. By adding a

bit of soda ash the charge is melted and

stirred for some while. During this

operation insoluble impurities will settle on

top of the melt and join the soda ash slag,

which is skimmed off at the end of the

melting operation. Gases and flue dust

from the process are soaked away and

passed over to the gas cleaning system.

The melt is cast into ingots or transferred

in liquid stage to the refining kettle.

2.3.4 Refining of crude lead

First, the lead tapped from the furnace has

to be cleaned from residual oxides and

slag. For that purpose a bit of pitch and

saw dust is added. After stirring for a while

the impurities settle at the surface and are

skimmed off (Fig. 6).

Crude lead originating from battery scrap

is normally alloyed with copper and

antimony (with traces of Ca, Sn, As, Zn).

In order to remove the unwanted elements

two further refining operations have to be

carried out.

By adding sulphur to the lead melt and

after stirring for some time, a Pb/Cu

(and if present with minor parts of

Zn, Sb, As) is formed and skimmed off.

This de-copperisation step should be

carried out at least two times to secure the

refining result.

The de-copperised lead still contains a

large amount of antimony (and maybe

some Sn, As). All of these elements can

be removed by oxidation. For that purpose

air or oxygen-enriched air is blown into the

melt and stirred. The different oxides

formed settle at the surface and can be

skimmed off. The oxidising process is

completed when mainly lead oxide is

formed.

Instead of oxidising the impurities by

injecting air, sodium nitrate (NaNO

be added. Here again a dross containing

the impurities (and lead) is formed, which

is skimmed off afterwards. All refining by

byproduced

or residues should be processed

to recover lead and other valuables

components.

The refining processes and the purity of

the refined lead are monitored by chemical

analysis.

The off-gases of each of the processes

are collected and fed into the central gas

cleaning system of the plant.

2.3.5 Gas cleaning system

Due to the lack of environmental

legislation and monitoring, and due to lack

of funds industrial operations in developing

countries often have very poor emission

control and off-gas cleaning systems.

Because of the hazardous potential of the

majority of the elements and compounds

which are involved in lead smelting and

refining (Pb, Sb, As, SO

gas cleaning standard must be achieved

and should be compulsory.

Therefore, all fumes, gases and dusts

which are generated during the different

production steps should be collected and

treated in a central gas cleaning system. A

standard off-gas treatment system

normally consists at least of a hot dust

chamber and/or hot cyclone, a venturi

washer and a wet scrubber (Fig. 7).

From the furnace the hot gases pass

through a hot dust chamber and/or a hot

cyclone where most of the coarse dust

particles are separated from the gas

stream. From there the off-gas feed into a

wet gas cleaning system which consists of

a venturi washer and a wet scrubber.

The task of the venturi washer is the

collection of the fine dust particles. Water

and off-gas is mixed under high turbulence

and gas/water spray velocity, forming a

fine slurry, which is pumped to a

sedimentation tank.

The more or less dust free off-gas

afterwards enters a wet scrubber. Here,

the main task is the removal of the SO2-

gas of the off-gas. By adding lime to the

scrubber liquid, the SO2 in the off-gas will

react with the lime water forming gypsum.

The gypsum is insoluble in water and

precipitates. Again, the fine slurry of the

second scrubber is pumped into a

sedimentation tank. The clean gas leaves

the whole process via the main chimney.

In the sedimentation tank small amounts

of lime and flocculent neutralise the slurry

and assist the sedimentation of the fines.

The sludge passes a filter press producing

a filter cake. The extracted water is recirculated

to the scrubbers while the filter

cake is fed back into the melting and

reduction furnace.

3. Environmental considerations

As already mentioned, the potential health

and environmental risk involved when

processing battery scrap is very high.

Depending on the level of mechanisation

and environmental standards, the

following environmental hazards can arise:

•

acid spilled when batteries are emptied

•

battery scrap is stored without

protection

•

(e.g. lead-containing dust, soot, SO

chlorides, dioxins, etc.) when battery

scrap is smelted due to:

-

including its organic parts (casing,

PVC separators in older battery

types)

-

vapours during the smelting and

refining process

-

treatment

•

the corresponding landfill design that

would prevent leaching and dust

formation

•

refining process

•

such as battery casings and PVC

separators.

Workers, too, are exposed to raised levels

of harmful substances in such facilities.

This generates considerable health risks if

appropriate precautionary measures are

not taken (respiratory equipment, washing

facilities, separate eating and resting

rooms, regular examinations, etc.).

Existing Documents:

• Presentation: recycling systems for PV-batteries, Andi Michel, Addis Ababa, October 2008
• Gate, GTZ: Vest, Heino: Fundamentals of the Recycling of Lead-Acid Batteries, 2002 (279 kB)
• Pv-batteries, Silvia Schubert

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