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StrikoWestofen GmbH

Inhouse-Recycling of Aluminium chips

 

A rewarding cycle

In-house recycling of aluminium chips


More and more foundries are meeting their customers' requirement to supply completely machined aluminium castings. This is leading to an increased accumulation of chip material, but with the right recycling system there is the very attractive possibility of recycling this material cost-effectively in-house.

The question of whether there is a real demand on the part of a foundry to recycle chip material in-house cannot generally be answered with a simple yes or a no. The preconditions for achieving cost-effective recycling and a short return on investment include reliable plant and equipment components, a melting furnace with a high metal yield, and trained operating personnel, besides an adequate amount of chips. Creating these preconditions requires thorough plant and equipment planning so as to realise excellent compatibility among the individual components.

 

The furnace is equipped with a special "pocket" and a charging system for the feeding in of the aluminum chips.

In-house recycling offers several benefits. Among other things, it provides the possibility to save the costs of purchasing ingot material, mostly as ingot stacks. This saving is all the more significant where special and, hence, expensive alloys are used. With chip recycling, therefore, the alloy remains "in-house". Another point is the recovery of the cooling lubricants from the machining processes. Also, interim storage of the chip material becomes unnecessary or can be minimised and transport distances become shorter because the chip material does not need handling so frequently. Selling of the chips similarly becomes unnecessary. Once the production based on chip recycling has been adopted, and become routine, there is even the possibility to source such material from external suppliers inexpensively and thus benefit still further.

But the disadvantages of in-house recycling should not remain unmentioned, either. These include, first and foremost, high investment costs. Chip recycling involves the handling and collecting of the material, its pre-treatment and melting and, as a rule, cleaning of the waste gases. The pre-treatment of the chip material includes crushing, drying, removal of organic coatings, separation of ferrous metals, and screening. Additional costs of operating and maintaining the plant and equipment together with those of employing trained personnel are also incurred. Moreover, additional control of the aluminium melt quality is necessary as well as, under certain circumstances, re-alloying of the metal bath. It is similarly advisable to have the possibility of interim storage for the chip material in the event of plant and equipment malfunctions as, generally, no replacement recycling facilities are available.

Low energy consumption

Generally, foundries prefer furnaces that only melt chip material in quantities from 500 to 1000 kg/h (maximum). However, there are also combined chip melters as an alternative. These are shaft furnaces designed to melt ingot and bulky return material and which are also fitted with an internal "pocket" for the feeding of chips. The advantage of these combination chip melters is their low specific energy consumption since it is possible to make use of the waste gas heat from the furnace chamber to preheat the feedstock in the shaft. It is also possible to produce the entire alloy required in a relatively confined space using one melting unit. The furnace's optimised holding chamber dimensions ensures the melting of the ingot, return and chip material, on the one hand, and the required pouring temperature on the other. Prior to transfer to the pouring furnace, an impeller treatment cleans the melt in the transfer ladle and removes oxide or carbide inclusions from the metal.

Short payback period

Foundries often evaluate a melting unit mainly from the aspect of metal yield as the financial bonus of an enhanced metal yield drastically shortens the payback period of the plant and equipment. Other aspects include low energy consumption and low space requirements.
To be able to achieve a high metal yield, the chip material must reside for only a very short time, if at all, on the molten metal bath surface and be drawn as quickly as possible into the metal bath. The longer the time spent by the chips in a hot, oxygen-bearing atmosphere, the greater is the oxidation of the material's surface. The separation of oxide and metal becomes more and more difficult with increasing thickness of the chip's oxide skin and, where it is no longer possible to recover the metal enclosed in the chip, the metallic yield drops significantly. Having a suitable melting technique is therefore an important condition for cost-efficient chip recycling.
Other factors influencing the metallic yield include the alloy type as well as the form and quality of the chips. Both a high amount of fines in the chip charge and a low chip thickness will reduce the yield. The more critical these chip characteristics are, the more important it is to have a technically perfected system for melting the chips. For this reason, a pre-treatment facility that supplies the driest chip material possible should be used upstream of the melting furnace. Moist material not only reduces the metal yield, but also leads to heavy fume generation and soot formation. This, in turn, makes cleaning of the waste gases necessary.

Melting under exclusion of air

Mainz-Kastel based StrikoWestofen GmbH uses a metal pump fitted with the Lotuss® system from Metaullics Systems Co., Solon/USA, for maximum metal yield. The system fulfils the chip melting technology requirements very well by drawing the fed in chip material straight under the metal bath surface, where it melts under exclusion of air, comparable to immersion melting.
For this application the foundry systems manufacturer has upgraded the STRIKOMELTER® with a refractory lined chip pocket, fitted on the front side, to accommodate the mechanical metal pump and the Lotuss® system (Figure 1). The block for the latter is a permanent cast-in part of the pocket. The pump rests on steel beams and can be easily raised out of the metal bath for maintenance or cleaning purposes. The Lotuss® system and pump form one process unit. The pump draws in the metal from the furnace's holding chamber and feeds it into the Lotuss® system. Its shape which has a tangential inlet and an outlet centrally located in the bottom generates a so-called vortex and, as a result, a downward flow of metal which draws the chips rapidly beneath the metal bath surface, melting them under exclusion of air. The circulation of the metal leads to a thermal as well as chemical homogenisation of the metal bath. The continuous movement of the metal bath is coupled with a reduction of the bath-heating energy requirement to nominal and with high tapping temperature consistency. By varying the delivery rate of the metal pump it is possible to adjust the system to the chip quality and required melting rate and to optimise effectiveness and cost-efficiency.

Furnace with chip pre-treatment

In August 2007 a wheel foundry started up an MCM (Chip Melter with Metaullics System) type furnace with a 7 t bath capacity and melting rate of 800 kg/h (Figure 2). The furnace is of tiltable design and operated with two burners that have a total output of 1000 kW. The natural draft of the stack draws off the waste gases. A furnace pressure control system keeps the pressure in the furnace constant. The chip pre-treatment stage consists essentially of a chip crusher, ferrous material separator, centrifuge, metering screw and screw feeder. The furnace waste gas, which has a temperature of around 500° C, serves to preheat and simultaneously dry the chip material. It is conducted via insulated piping to the screw feeder where it transfers heat to the chips. The cooled gas passes from there back into the waste-gas stack of the furnace. The temperature of the chip material when fed into the charging system is about 200 °C. The chips are virtually devoid of any oil thanks to the hot waste gases.
Detailed measurement of the furnace performance took place as part of the start-up. The chip material comprised mainly turnings of AlSi7 alloy. An analysis of the chip shape and quality and of the residual moisture found the material to be "well-suited for melting". At the end of the melting cycle only a small amount of dross was found on the metal bath surface in the holding chamber (Figure 3). It was possible, without any problems, to draw the dross toward the clean-out door, where residual metal was removed by adding a small amount of salt flux and relatively dry dross drawn over the sill, out of the furnace. The skimmed-off amount of dross corresponded to 1.5% of the originally weighed-in quantity of chips. There were, in addition to this, a few kilograms of dross from the pump pocket of the melting chamber. The metallic yield attained was over 98 % altogether.


When is in-house recycling worthwhile?

As a general rule, at current aluminium and chip material prices, in-house recycling can be cost-effective from a melting rate of 300 kg/h upward, or around 5 t chip and material per day. Mathematically, such a facility is amortised after only a short period. A rough calculation already demonstrates this if, in a first estimate, the price of ingot material is put at 2000 €/t and the revenues from chips at 1000 €/t, and an annual melting rate of 1500 t (5000 hrs pa at 300 kg/h) is achieved. The precondition for a short return on investment, however, is that the plant and equipment are correctly designed and dimensioned and that all the plant and equipment components operate reliably and trouble-free. Also, the metallic yield from the melting process has to be right, because each percent of metal lost greatly reduces the benefit of recycling the chip and material.

STRIKOMELTER ® MCM-T 7000/800 with Lotuss® system can be tilted for tapping and has a bath capacity of 7 t.

At the furnace fitted with the Lotuss® system it was possible to demonstrate a metallic yield of 98 %.

 

 

 

 


 

 

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