Benefits of Modified Lithium Baths for Smelters

More and more smelters are using modified Lithium Bath to reduce temperature, increase conductivity, and increase throughput while reducing emissions. The setback is the need for Lithium removal. It opens a new equipment market, and hopes for older smelters.

By Helge O. Forberg

In the 1950’s, smelters in the old Soviet Union were probably the first ones to use modified lithium bath on a wide scale. It is well known since then that modified lithium bath, with a lithium fluoride content of 2-4 % LiF and an aluminum fluoride content of 4-8 % AlF3, allows a higher electrical bath conductivity, therefore a lower bath temperature, more metal throughput, lower fluoride emissions and lower cash cost. This is why today, approximately 20% of all smelters are using lithium modified bath. Their number will keep growing, even more that the deregulation of energy markets will create more incentive to cut costs while keeping investments low.

Many world leaders use lithium bath today

Reynolds Metals (now Alcoa) was the first aluminum company in the West to have all their smelters converted to lithium modified bath by 1972. VAW (now Hydro Aluminium) was another early user. Today, lithium bath users include Alcoa, Alcan, Pechiney (now Alcan), Century Aluminum, PPH Billiton, RUSAL, Hydro Aluminium and VENALUM. Their smelters are located in Canada, USA, Russia, Germany, France, Greece, Venezuela and Brazil. Most of these are older smelters, but a few are newer ones.

If bath temperature goes down by 5°C, Faraday Efficiency goes up 1%; and most users observe a drop of 12 to 18°C… and fluoride emissions reduced by at least 40%.

Compared to the standard bath used in many smelters, lithium modified bath has a lower melting point and a higher electrical conductivity. As a result, pots on lithium bath can operate on 12-18 degrees C lower bath temperature than pots operating on the standard bath. This will increase the current efficiency: For each 5 degree C reduction the current efficiency increases by 1 %. In addition, the lithium bath has a lower vapor pressure, resulting in a 40-50 % lower pot room fluoride emission. Due to the higher electrical conductivity of the lithium bath the pots can either operate with a higher anode-cathode distance at the same line current - or operate at a higher line current, resulting in increased production and reduced costs.

The net result is that the lithium bath improves a smelter’s competitiveness and extends its economic life. Major benefits can be summarized as:

• Operating bath temperature reduced by 12 to 18 degree C
• Current efficiency increased by 1.5 to 3%
• Specific power consumption decreased by 2.3 to 4 %
• Net carbon consumption decreased by 1 to 2 %
• Fluoride emission decreased by 40 to 50%
• Reduced aluminum fluoride consumption
• Improved pot stability and less “noise” or amplitude in the momentary variations in pot resistance
• Increased production through a combination of improved current efficiency and increased line current

But traces of lithium end up in the pot metal…

Traces of the lithium in the bath will end up in the pot metal. Thus, for a bath containing 3% LiF one can expect 24 parts per million (ppm) lithium in the pot metal. During and after tapping the lithium and sodium in the molten aluminum will burn off. By the time the crucible has reached the cast house, the lithium concentration has most likely been reduced to 8-10 ppm.

A number of value-added aluminum products require less than 2 ppm lithium. The market specification for aluminum going into higher priced products is 1 ppm lithium maximum. Unless the market specification for lithium can be met when the aluminum is in the furnace and ready for casting, the aluminum has to be diverted into lower grade products that will carry lower or no premium. The metal premium and the ability to produce value added products are playing an increasingly important role in the overall smelter economics. This is therefore the reason for many smelters being reluctant to convert to lithium modified bath.

Up till now a number of lithium removal methods have been used. Two well known metal treatment systems for lithium removal are TAC (proposed by STAS in Canada) and HYCAST (developed by Hydro Aluminium). With the TAC system, lithium can be reduced to about 4 ppm. The HYCAST system is somewhat more efficient and can within a reasonable time reduce the lithium to 2 - 3 ppm. However, HYCAST has been taken off the market as Hydro decided to use it exclusively in its own plants. The other systems are less efficient.

Soon, a new system for Lithium removal in molten aluminum

A new system has been developed for the removal of lithium in molten aluminum. The method has been patented by Helge Forberg & Nolan Richards and the equipment will soone be in the catalogue of a reputable Smelter Equipment Supplier. The removal of lithium is accomplished fast and economically by chemical reactions through a combination of mechanical and chemical agitations. The removal is carried out in a three-ton transfer crucible at a metal treatment station built in the cast house. After the completion of the treatment the metal is transferred into the cast house furnace and another crucible would be ready for treatment. The turn-around time at the treatment center is approximately 15 minutes. The objective of the treatment is to meet the aluminum market specification so the aluminum produced in a smelter using lithium modified bath can be sold at a premium and used in valued products.

Improved smelter economics

With the availability of a fast and economic method for removing lithium from molten aluminum, smelters on lithium bath can now obtain a premium for their metal and have the capability to produce value added products. These smelters do not have to divert a portion of their production into lower grade products any more.

As a result, the economics of smelters on lithium modified bath has improved significantly. The molten aluminum cost has always been lower in smelters using lithium bath. However, this cost advantage was often lost due to the handicap of not being able to meet the aluminum market specification of 1 ppm lithium maximum. This barrier has now been removed. Due to the overall smelter economics involved and the competitive advantage of lithium bath, it is expected that more smelters will be converting.

Helge O. Forberg