From Novelis an high-efficiency concept with low investment

The Jetcleaner concept is based on static injection of the process gas at high speed, using submerged “jet” nozzles. It reaches the required degassing efficiency and inclusion removal in a very inexpensive way. It is also appreiated for its reduced downtimes, compact size, low static capacity (a must for short campaigns) and compliance with emission standards (1). The concept, patented in 1996, and years of development along with industrial characterization have proven that the level of efficiency of the Jetcleaner approaches the results obtained using conventional degassers available on the market (1,2,3), yet for a remarkably lower investment and lower operating costs. There is a steadily growing interest for this innovative in-line degasser, may it be for ingot, billet, wire or slab casting, in scrap recycling processes, with frequent alloy changes, variable in-level contents, medium performance targets and absolute need for competitive costs.
Equipment description
The Jetcleaner consists in a transfer trough portion, with the same shape, which becomes the treatment chamber once sealed from outer atmosphere. This chamber is slightly taller than the inlet and outlet trough, only to increase the reaction time. It is equipped with side nozzles, a lid with its handling jib and a gas circuit to supply the nozzles. Space requirement is drastically reduced as show on, with a treatment capacity up to 25 tons/hr, the only other requirement being access for operators for cleaning: The overall unit is thus compact, and fits easily, even in existing casting lines, in place of a section of the trough. The sealing of the reactor from ambient air and humidity is obtained by a seal between lid and vessel and the use of under-pour inlet and outlet spouts.
Principle of operation
The main original design lies in the improved gas distribution system: the inert gas, argon or nitrogen, is injected at high velocity from the side by (patented) static and submerged nozzles. The gas is blown at high Reynolds number, above 8,000, to distribute a higher bubble density and generate a higher turbulence (over the whole volume of liquid metal flowing past the nozzles), compared with conventional reactors using spinning nozzles or porous plugs. Degassing is achieved over a far reduced metal residence time, between 15 and 30 seconds compared with a few minutes. The comparison with spinning rotors and porous plugs has been quantified (3): the bubbles are smaller (around 1 mm in size compared with 6 mm, and density increased by a factor of ten), together with the beneficial effect of turbulence: the relative velocity of bubbles to melt leads to a higher hydrogen capture efficiency and prevents them from reaching the surface too rapidly in a small-volume reactor. Still, the overall upward motion leads to a significant filtration effect (2, 4). A bubble-free volume is provided on the exit side, so that all bubbles have time to reach the surface before the metal exits the treatment chamber. In one word, the large density of bubbles together with the turbulent regime compensate for the smaller reaction volume and time.
Degassing efficiency
An Alscan testing campaign (2) has been conducted in a billet casthouse on quite a variety of alloys, typical of extrusion casthouses short campaigns, recycling the plant’s scrap.
Degassing efficiency was found to range between 47 and 63%, depending on the in-coming
level, while the metal flow rate remained around 14 t/hour:
Flexibility
The number of nozzles can be adapted to the actual metal flow rate, to optimize gas consumption for a given degassing target on the low flow rate side, or increase the operation range towards higher flow rates. In the standard version, four nozzles are installed on each side, closely spaced but not facing each other for optimum bubble generation, without a dead zone outside some volume on exit which is left still. To help optimizing, each pair of nozzles can treat around 6 tons/hour of molten metal and the nozzle flow rates are checked individually for proper functioning. Experiments on water models and on actual production show that most of the bubble generation and thus the hydrogen removal takes place in the first 20 seconds in the standard configuration with 8 nozzles. Increasing the residence time brings diminishing returns. To be more precise, the gas residence time is less than about 2 seconds, and the liquid surface in the bubble-filled zones raises at least 15%, with optimum results. This can be exploited in practice: at reduced flow rates, only 6 or 4 nozzles need to be activated, starting from the inlet side, without impairing the degassing performance, corrected by the inlet H2 level. One only needs to check that the metal surface at the outlet spout is still, without bubbles bursts, meaning the full desorption of hydrogen took place. One can also monitor the quality, choosing to run with more nozzles at a higher gas consumption level for higher degassing. In case of higher flow rates, two Jetcleaner® can be used in series or in parallel, this time adding in number of activated nozzles. The parallel configuration doubles the metal residence time with still limited space requirement. Both turn out to be more efficient than a single unit with double length (16 jet nozzles), due to the decreasing efficiency over treatment zone length explained above.
Energy efficiency and environment
The benefits of Jetcleaner can be appreciated in two usually energy-consuming areas:
Heating power:
There is little need as:
• the vessel is rapidly preheated prior to casting just like the casting troughs along the line, using gas burners or an electrical blower, • the metal temperature loss in passing through the Jetcleaner is only a few degrees,
• there is no need for heating to maintain the liquid metal temperature between two casts, as there is no metal retention at the end of casting.
Metal loss or remelt:
The absence of metal retention at the end of the cast (around 15 liters) facilitates alloy change over, both physically and economically:
• reduced waste of time or metal, trimming the cost of traditional and bigger units,
• reduced cost of remelting the drained volume.
In addition, the following benefits result from the Jetcleaner sound design:
Gas savings:
The neutral gas is fed only a few minutes before the cast to purge the system and is maintained after the end until the remaining skull is frozen, then switched off (no holding time), while a minimum gas flow rate is required between casts to avoid clogging the holes of conventional of conventional rotors.
Fumes emission:
The treatment gases are collected as follows:
• owing to the good sealing between treatment trough and lid, the hot fumes are easily collected from a hole in the lid into a hood,
• the fumes are diluted with outer air in order to get a high velocity in the piping and thus avoid any deposit of particles and reduce the temperature of the fumes, ensuring a good lifetime of the fan (arrow),
• the diluted fumes are exhausted through the fan to the furnace stacks or the outside of the building, at roof or ground level, suppressing emissions in the work area.
Acceptance by operators
This has been presented in earlier publications such as (1) and (2), illustration the various benefits of Jetcleaner due to its user-friendly design: smooth operation, most easy change out of the nozzles, accessible interior for cleaning in a matter of minutes, few wear parts (limited to the nozzles and refractory lining), low maintenance need, combined with a low operating cost and controls suitable for automation.
Conclusions
The Jetcleaner is an attractive degassing unit, which clearly meets the aluminium producer’s demand for effective and low cost degassing equipment:
• good metallurgical results, close enough to that of traditional in-line metal treatments, with some flexibility (number of activated nozzles),
• practically no loss of metal due to metal retention or dross formation, saving on metal and the energy required to remelt the drain bowl content,
• ease of operation and maintenance, explaining its good acceptance by operators,
• operating costs savings, by adjusting the number of nozzles (and neutral gas consumption) to the actual flow rate.
Its low capital cost fits price-competitive markets, as encountered in aluminium recycling.

This paper was presented at “Alusil-21/Recycling”, Saint Petersburg, Russia, October 12-14 2010.

Ghislain Le Roy, ghislain.leroy@novelis.com