Latest developments in recycling aluminium dross and scrap using tilting rotary furnaces
The latest developments in aluminium recycling using tilting rotary furnaces (TRF ) taking as reference a new 30 000 TPY secondaries plant in France.
The plant is equipped with two
12 tonne TRF for melting general commercial scrap and a 7.5 tonne TRF
primarily used for melting aluminium dross together with two 30 tonne
refining/holding furnaces, swarf dyer, casting equipment and automated
ingot stacker. The paper also compares the new plant with the owner’s
original plant that is equipped with traditional fixed axis rotary
The SNR company was founded about 40 years ago on a site 50 km to the west of Paris in St Arnoult. The plant is equipped with:
The annual production capacity
at St Arnoult is at present approximately 15,000 tonnes and each shift
operates with 11 people. The plant specialises in gravity and pressure die
If we compare the concept of the tilting rotary furnace (TRF) with the traditional fixed axis rotary furnace (FARF) the advantages are evident. The chamber of the FARF is nothing more than a rotating tube or corridor, with in its simplest form, a burner at one end and a flue at the other. The furnace operates under negative pressure and draws in cold ‘parasite air’ from outside the chamber thus making it virtually impossible to control the atmosphere of the furnace which is nearly always oxidizing (air rich). This condition makes it necessary to use large quantities of salt (NaCl/KC) to protect the charge and obtain acceptable metal yields. The charge, as it approaches liquidus, remains more or less static. As with all high temperature furnaces heat transfer is achieved primarily by radiation and re-radiation (via the refractory lining) but not as efficiently as in a TRF.
The TRF furnace on the other hand is closed chamber having a single entrance and exit point. The door carries the burner and the flue. The furnace operates under positive pressure and when the burner is firing there is a total absence of parasite air ingress, therefore the furnace can operate in a slightly ‘reducing’ (gas rich) atmosphere which eliminates the necessity to use a protective salt cover. Additionally, as the furnace operates at an angle, the charge is ‘mixed’ with a similar action to that of a concrete mixer. This improves homogeneity and heat transfer resulting in very high thermal efficiency.
Dross Engineering furnaces
SNR compared the tilting
rotary furnaces offered by several suppliers before opting for those
designed and built by Dross Engineering.
The TRF units built by Dross Engineering have found applications across the range of non-ferrous industries and are used to melt zinc and aluminium and reduce lead, tin and bismuth oxides thus showing the remarkable versatility of this furnace concept.
Tilting Rotary Furnace (Converter Furnace) range
1000 dm3 corresponds to approx. 2,70 ton of aluminium melt.
*Vari-gas: For explanation click
here. Full version is available as standard on all furnaces from Dross
The SNR Prémery Plant
The Prémery plant has an annual capacity of 30,000 tonnes and employs 7 foundrymen per shift.
The plant is equipped with a
bag house filter designed by the French subsidiary of the German company:
INTENSIV. The company has a long history designing dry process particulate
arrestment for the cement industry, foundries and mines etc… The
particularity of the INTENSIV bag-house system is their dual stage reverse
air cleaning which ensures optimum performance and low compressed air
consumption. In the case of the SNR Prémery plant the media used and the
reverse air system enable particulate emissions of less than 0.5mg/Nm3 to
A major consideration in any installation of this type is the choice of combustion system. For tilting rotary furnaces there are basically two options:
Most fabricators of this type of furnace fit oxy-fuel burners giving as arguments in favour of such a choice:
However, before making a final choice the end-user should take into consideration other points that are not so favourable:
Those who have a long experience in the industry can testify to the fact that ‘there is nothing new under the sun’! 20 or more years ago the trend (in France and Europe) for Secondary Smelters was to equip their furnaces (all their furnaces; rotary and dry hearth) with oxy-gas burners. Effectively improved production rates were achieved (the furnaces were hotter and so melted at higher rates) and improved consumption rates were recorded. But at what cost? Equipment and refractory problems and safety issues had for consequence that all installations reverted to air-fuel burners. Why? Combustion control of oxy-fuel burner systems is extremely specialised and, at the time, the systems available lacked the computing power and programme depth for such burner systems, within the specific application of secondary aluminium smelting in the furnaces of the period, to be really successful.
Oxy-fuel presents considerable
advantages provided the combustion control system is sufficiently well
adapted and also operator friendly.
The Vari-gas combustion control system, originally developed for air-fuel burners by Dross Engineering and the software engineers of their industrial partner, PELSS enables the end-user to:
The Vari-Gas system is primarily a furnace control tool that, in the hands of an experienced furnaceman , enables exceptional thermal and metal yields to be obtained.
As we have already seen, the basic concept of tilting rotary furnaces lends itself to high thermal yields. Heat transfer rates to the charge by radiation (and re-radiation) and conduction are optimised. When these types of furnace are fitted with oxy-fuel burners, reduced N2 concentrations result in stronger gas radiation and there is a considerable increase in the heat available to melt the charge (approximately 50% more in relation to air gas burners at temperatures of 1100˚C with 95% pure oxygen and natural gas).
But it is worth noting that higher ‘localised’ flame temperature is not the only way to achieve higher heat transfer rates and thus improved productivity, increased ‘ambient’ temperature has a greater effect. Oxy-gas burners available today give good flame homogeneity and greater uniformity between flame and furnace temperatures. With Air Liquide’s Albatch burner a highly luminous flame is produced that is up to 3 times wider than conventional oxy-gas burners. Fuel and oxygen are introduced into the furnace through separate injectors with the oxygen being injected above the fuel to improve stability, prevent ‘lofting’ and reduce flame peak temperature. Thermal decomposition of the fuel is improved leading to the formation of highly radiant carbon particles.
If we were to compare the thermal yield and fuel consumption with a fixed axis rotary furnace fitted with a traditional air-fuel burner, there is in fact ‘no comparison’. The concept of the fixed axis rotary is well known for its mediocre thermal yield and, in addition, the salt bath requires heating before you even begin to melt the charge of metal.
Theoretically, the energy required to melt one tonne of aluminium is 310kW at 100% efficiency, i.e. 31Nm3 of natural gas (with a LCV of 10). In a fixed axis rotary with an air-gas burner, thermal efficiency is very poor and energy consumption per tonne melted are in the region of 100Nm3 natural gas (minimum). On the other hand in a TRF with optimised thermal efficiency energy returns of 50-60Nm3 with an air-gas installation and 35-38Nm3 (plus 70 – 75Nm3 oxygen) can be expected. When using the Vari-gas system, if the charge contains organics such as grease, lacquer etc… it is possible to exploit this ‘latent’ energy rather than sending the unconsumed hydrocarbons to the bag-house filter; In such cases we have recorded energy returns of less than 20Nm3 (in some cases as low as 12Nm3) per tonne melted.
It is difficult to talk about metal yields for the many different categories of aluminium scrap as there are far too many variables. However for melting a charge of clean new cast we have recorded yields in excess of 97%. The operator of the two site in question, Prémery and St Arnoult, confirms that over the range of scrap recycled through the two plants they have recorded improved yields of several percentage points at the TRF site (Prémery) as opposed to the site operating fixed axis rotaries (St Arnoult) and there is the considerable additionally advantage of ‘salt-free’ operation at Prémery.
The tilting rotary furnace is proving itself to be a high performance production tool enabling both secondary metals smelters and foundries to benefit from considerably improved production (up to 45%) and fuel savings (over 50%). SNR’s production facility at Prémery came on stream in Summer 2005 and now operates 3 shifts daily. The company’s objectives in significantly increasing their production being met:
CASTINGS WITH ‘CAST-IN’ CAST IRON INSERTS
Fabrication rejects cost money and every foundry puts in place systems designed to reduce them. When dealing with components with ‘cast-in’ cast iron inserts and sleeves, an additional problem has to be overcome: separate out the iron without polluting the aluminium.
Even though such rejects
represent a small percentage of overall production they exist and are a
source of metal to be exploited. Recycling such components internally
raises a number of issues that require resolving: choice of technique and
installation running costs. This paper presents the different techniques
on the market today and examines the reasons for the choice made by a
major European automobile company (PSA) who opted for a system based
around a tilting ‘converter’ furnace designed and built by Dross
Engineering. We present the equipment itself and the results experienced
by the PSA foundry division.
The Peugeot SA (PSA) high-pressure
aluminium foundry is part of the Mulhouse fabrication complex. It is
located in the eastern most part of France, in Southern Alsace, on the
frontier with Germany and Switzerland. Part of the PSA group (Peugeot –
Citroen), the integrated foundry facility employs some 290 people and
produces daily 84 tonnes of engine blocks for the group’s 4 cylinder
diesel and petrol engines. The components are produced using the ‘cast-in’
insert technique. The foundry consumes 90 tonnes of aluminium per day
either melted on site or bought-in in liquid form. The HP foundry is
equipped with 8 x 2500t and 5 x 2000t diecasting presses.
Process rejects, although a small percentage of overall production remain an inevitable part of the fabrication process. Several years ago, as part of their forward planning, the foundry management gave themselves the objective of processing the reject parts in-house rather than continue the practice of selling the rejects to a secondary smelter, particularly as they were sold at a loss mainly due to transport costs. The brief, issued by the management team, placed a strong emphasis on:
- Ergonomics - minimal
They looked at a number of
• Classic fixed axis rotary:
Heat transfer – Homogeneous Melt
All these combined in the TRF.
Before proceeding with the project, PSA carried out melt trials at an existing plant equipped with a Dross 80 converter furnace. The trials were conclusive and the project was given the ‘green light’.
Dross engineering size their
furnaces according to the ‘useful capacity’ of the furnace i.e the volume
of liquid metal contained in the chamber. A Dross 100 furnace = 1m3
(35ft3) useful volume, a Dross 200 = 2m3 (70ft3) etc…PSA designed their
recycling plant around a 2m3 (70 ft3) converter furnace, Dross
Engineering’s Dross 200 model. The furnace receives its charge via a
dedicated charge machine, also supplied by Dross engineering. Liquid metal
is transferred to a 35 tonne (77000 lb) bulk holding furnace via transfer
ladles. PSA opted for ladle transfer as opposed to launder transfer for
reasons of versatility and the ability to transfer metal to any of their
other melting furnaces or directly to their machine holding furnaces. The
installation is equipped with an efficient ladle preheat station that has
enabled PSA to reduce pour temperature and to increase savings. Off-gas
from the furnace is extracted via a cyclone and filter baghouse.
The TRF or converter furnace is a ‘dedicated melting furnace’, that can melt a wide range of feed stock and is equally at home processing aluminium, zinc, lead, tin etc…Because the furnace operates as a batch melter, metal pollution by trace elements in the charge (free iron for example) is greatly diminished or even eliminated making it an ideal ‘de-ironing’ unit. The rotary action facilitates charge mixing and promotes melt homogeneity, heat transfer and also promotes self-cleaning. The furnace melts 2 to 3 times faster than equivalent rated static furnaces and offers extremely efficient energy returns. The PSA furnace is fitted with a 2000 kW burner set at 1200 kW, during commissioning, returns of less than 400 kW/t (619 Btu/lb) melted were recorded. Furnace movements are remote controlled by radio link providing a safe, operator friendly working environment.
The furnace is designed more
like a ‘machine tool’ than a furnace. The movements and stresses, even for
smaller models, are unforgiving and punish any misalignment. They demand a
level of precision during assembly that is normally reserved for machine
For the first furnaces of this
type built, Dross Engineering chose to power the rotation via a hydraulic
motor. The results were satisfactory but not without problems. This
initiated design work that resulted in the adoption of an innovative drive
system that is both direct and reliable and that guarantees a positive
movement in both directions. The system is compact and ensures a smooth
start to rotation under charge; it withstands a wide variation of load at
temperature and all the aggression of a foundry environment with a minimum
amount of maintenance. The mechanism is patented and grants to Dross
Engineering’s furnaces a drive system that is unique for this application.
Efforts created by the
movement under load and at temperature require a furnace structure that is
both robust and precise. Dross Engineering uses the finite element method
to provide accurate predictions and evaluations of component response when
subjected to thermal and structural loads. Particular attention is paid to
assembly tolerances and a large number of components are machined. Guide
bands, or tyres, traditionally mounted ‘floating’, are welded to the shell
and machined insitu to tight tolerances and run on ‘elastic’ rollers
assembled to a high level of precision. This ‘elastic roller’ system is
also covered by a recent patent.
A rotary seal system linking
the hygiene hood with the base of the stack promotes efficient extraction
and enables combustion products and fume to be ‘captured’ at source and
channelled to an independent cyclone and filter unit or to the plant’
In itself, the concept of the
converter furnace with its enclosed well allows both the furnace
atmosphere and furnace pressure to be adjusted to give optimum
stoichiometry and to avoid ingress of parasite air, thus promoting reduced
energy consumption and minimal metal losses and the possibility of running
completely salt free or with greatly reduced quantities of fluxing agents.
With the Dross Engineering
converter furnace PSA have an installation that is: