Of late there have been a number of informed articles that cover how Industry 4.0 will affect the foundry industry, covering the advantages and opportunities that this can be expected to bring. A nice symmetry considering that foundries were instrumental in the first Industrial revolution. Whilst we should all look to the future, it is also useful to take time to re-examine some of the basics.
And you can’t get much more basic than a foundry ladle.
The in-ladle production of ductile (nodular) iron uses a number of long established processes and there is much information available on the subject. However, this information approaches the subject from a mostly academic angle, concentrating on aspects such as:
- Commercial advantages and disadvantages of the various processes
- Best magnesium recovery rate
- Optimum treatment process for particular mixes of alloys and choice of size of particles etc
All of which are obviously important.
The design of the actual type of treatment ladle to be used also tends to follow this academic approach, listing what features a type of treatment ladle should have for the best results but they seldom look at the ladle design from a practical point of view—how these features can be achieved and what impact they have on the overall ladle design.
It is this aspect that I’d like to explore. Acetarc have been manufacturing treatment ladles, of all types, for decades and the following is based on a mixture of my experience, what I’ve seen in foundries and, most importantly, the feedback received from foundries. It has been a long and evolving process and, over the years, we’ve adapted our ladle designs to make them better suited to their purpose. As always, the feedback from the foundries has been critical in this ongoing design development.
It’s not my position to say which process or choice of additives are the best, as there are others better qualified to give an opinion on this, but I think I can point out some ladle features that will help a foundry to have a better treatment ladle.
The best in-ladle treatment process is the one that works best for your foundry. If you are happy with the results then it works for you.
I’m going to cover the following treatment ladle designs; open top, for the over-pour or sandwich process, (Acetarc calls ‘deep treatment’) and the various tun-dish treatment ladles including the teapot spout type.
These ladles often have different names but hopefully you will understand which type of ladle I’m discussing.
Open Top/over pour (Sandwich) deep treatment ladle
The open top ladle is a good place to start as the main design features will also be found, to some extent, on the other types of treatment ladle.
This ladle doesn’t have a cover. There is a pocket in the base that is charged with additives and a covering, usually a metal stamping but sometimes a thin cast plate is placed over the top. The metal is poured into the ladle and the covering is intended to delay the reaction long enough for the ladle to be filled with enough metal to fully cover the additives. It is the most basic type of treatment ladle and the least efficient but, possibly due to the ease of use is still popular.
You will usually see it stated that the ladle should have a shell with a height to diameter ratio of 2:1 or greater. I’ve even seen articles that even suggest 3:1. While this may be true from improving the treatment process efficiency, and is something to be aimed for, it can quickly hit several practical constraints.
Do you have the necessary distance under the furnace spout to fill such a tall ladle?
Then there is always the question of stability with small open top deep treatment ladles. Having a tall narrow shell, can make an empty ladle unstable, especially when you consider the typical foundry floor. We’ve often added stabilizer feet to small ladles for this reason. A 1200mm tall ladle with a top diameter of less than 600mm (not including the lining) or a ladle that stands over 3 metres high can present challenges to access the additives pocket for cleaning and repair.
Detachable bases
Detachable base sections on ladles go a long way to helping with the refractory maintenance and greatly help when it comes to the additives pocket. A lot of the academic information covers the design of the additives pocket, what size and shape it needs to be to achieve optimum efficiency. This information seldom considers how the ladle manufacturer will actually fabricate such a pocket or how the foundry will be able to maintain and charge it. We have frequently discussed with foundries what works for them and then how we can make it. It often seems to be a trial and error process to achieve the optimum size.
Therefore, having a detachable base section allows adjustment to the additives pocket if practical results from the foundry suggest that changes need be made, and without the need to carry out major modifications on the complete ladle.
With respect to the refractory, the additives pocket is likely to get the most wear so a detachable base also enables the foundry to have spare, pre-prepared base sections, that can be quickly exchanged, without the need to wreck out the complete ladle lining.
I’ve also found that, especially with small deep treatment ladles having a shaped base pocket helps reduce the volume of refractory in the base. If you simply create the pocket in the refractory, you can end up with a large amount of refractory that is doing nothing other than be a pain to remove and require a lot of pre-heating.
However, other compromises may have to be made, as I’ve said height can be an issue for example if your furnace spout height is only 1100mm from the floor and a 2:1 ratio ladle has a height of 1200mm.
With regard to the open top (sandwich ) type treatment ladles, we try to make the ladle shell as close to the 2:1 ratio as is practical but we’ve often had to compromises and design ladles with smaller ratios, say 1.5:1 so that the ladle can fit under the furnace spout.
At the other end of the scale a 20 ton capacity open top treatment ladle, with a 2:1 H: D ratio is going to be a very tall ladle and brings its own handling problems. Feedback from foundries show that acceptable results can still be achieved with a reduced ratio.
It’s also my experience that there is more flexibility with the shell ratio when it comes to the covered treatment ladles; the tun-dish and teapot spout type. Foundries have achieved good consistent treatment results with covered treatment ladles that have a shell ratio of 1.5:1 or lower.
Freeboard
It is standard practice to design treatment ladles with an extended freeboard (the distance from the top of the metal to the rim of the ladle). This is to contain the treatment reaction which can be quite violent. The violence of the reaction is dependent on the choice of additives and the treatment reactions that I witness today do seem a lot less violent than they used to be, but we still think that it is advisable to have the extended freeboard. There are no hard and fast rule about this, but it does seem to be the case that the freeboard can be reduced over what we used to have.
However, it can still give rise to the opportunity for the ladle to be overfilled. So, I would always recommend that the metal is weighed into the ladle. The ladle design should have a safety factor such that that this additional weight will not compromise the ladle but it’s not good practice if you have charged the ladle with additives for 1000 Kg of metal and then fill the ladle with 1100 Kg. This could push the crane to its capacity. It’s also going to mess up the additives to metal ratio.
Before I move onto the tun-dish ladles, I’d also like to mention wire feed treatment ladles. I’ve found that what makes a good deep treatment ladle is usually applicable to the wire feed ladle design. With wire feed ladles, the constraints are defined by the wire feed machine so the ladle has to be designed to suit this.
If the open top deep treatment ladle is the easiest to live with then the loose and sliding tun-dish are the next easiest. The Tun-dish cover being clear of the ladle after the treatment process has been completed.
Access to inside the fixed and teapot spout ladles is extremely limited so these ladles work best when the treatment cycle is constant with the through put of metal washing out any build up inside of the ladle.
However, when it comes to Mg recovery, the fixed tun-dish and teapot spout ladles give much better results and the open top treatment ladle the least. With the covered ladles effectively being sealed, the Mg fume is also greatly reduced.
Tun-dish ladles
All tun-dish types of ladles have a shared principle that the molten metal enters the ladle through a restricted opening and the metal seals the ladle so that when the reaction takes place no more air can get into the ladle.
These ladles don’t usually have additives pocket in the base of the ladle; instead they have a divider (a weir) in the base of the ladle to form a reaction chamber.
Loose tun-dish
The loose tun-dish is basically an open top treatment ladle with a tun-dish. The cover is removed by an independent method (hand, forklift truck or crane) after the treatment cycle has finished but before the ladle is rotated.
The additives are placed in the ladle with the tun-dish removed. Sometimes a sandwich layer is placed over the additives and the tun-dish is then placed on the ladle and the ladle filled via the tun-dish. When the reaction has stopped the tun-dish cover is simply removed. The ladle treatment process can be completed and the metal de-slagged before, usually, being poured into casting ladles.
The problem here is the handing of the tun-dish. It will have significant weight and obviously be very hot, possibly with bits of slag dripping from the underside. Small treatment ladles would often have the loose tun-dish lifted off by operatives but, again for obvious reasons, this is best avoided with a forklift truck or crane being a better option. The ladle design must take this into account, with extended sidearms to give the necessary clearance, suitable lifting points on the tun-dish etc.
It’s not good practice to have the ladle on the ground and to rotate the lifting bail clear of the top of the ladle, as it puts a reverse load on the ladle gearbox, especially if the sidearms are extended. If this has to be done on a regular basis, the side arms might need counter weights fitting to reduce wear on the gearbox.
Sliding tun-dish
The sliding tun-dish has the tun-dish mounted on a frame within the lifting bail, so that the tun-dish can be raised and lowered by the crane that picks up the ladle.
Basically, as the loose tun-dish but here the tun-dish cover is part of the ladle assembly and doesn’t require an independent means of handling. We were quite impressed with ourselves when we first came up with this design as it solved a lot of handling problems. However, we quickly realised that there is very little that is new and that variations on this type of ladle were already in use in foundries around the world. If anything, it showed that when faced with the same problems, the same solutions will often present themselves.
There is a simple option where the tun-dish moves up and down and a “locking” option, where the tun-dish can be either locked down, on the top of the ladle, or locked in the up position clear of the ladle. This helps if the ladle needs to be filled while suspended in the air and then prepared for the next treatment cycle whilst on the ground.
Acetarc has manufactured loose tun-dish ladles in capacities from 100 Kg up to 12,000 Kg and sliding tun-dish up to 6000 Kg. However practical factors do start to kick in. With the locking sliding tun-dish cover, there is a need to remove the locking pins and this gets more difficult as the ladle gets taller. The locking pins are located in the ladle lifting bail and you don’t really want to be climbing up a ladder to remove the locking pins when the ladle is in operation.
Conversely, with small ladles there can be a stability problem as the ladle has a lot of weight high up, when the sliding tun-dish cover is locked in the up position. These are points that need to be considered and discussed with the foundry.
The tun-dish does add significant weight and the crane SWL needs to be checked so that it isn’t exceeded.
In both cases the seal is dependent on the underside of the tun-dish cover sitting flush on the top of the ladle. As the ladle is used this “face to face” seal is likely to become affected by a build-up of detritus and the ladle will become less efficient., but it still should give better results than the open top deep treatment process.
There is another trick that can be done with the loose and sliding tun-dish ladles.
That is to make the tun-dish the full length of the cover and divide it into two, effectively creating two tun-dishes. From the ladle manufacturer’s perspective this helps keep the tun-dish cover balanced. From the foundry’s perspective it allows the additives to be put into the other side of the ladle halfway through the working shift so that the original reaction chamber can be “washed out” with metal.
Fixed
Where the cover remains on the treatment ladle throughout the time the ladle is in operation, only being removed at the end of the working shift. The metal is poured into the ladle via the tun-dish box and back out of the tun-dish once the treatment process has taken place.
Consequently, the tun-dish is shaped to form a spout and additives are introduced via a separate hatch.
Teapot spout treatment ladle
The data sheets usually classify teapot sprout ladles as a variation of the fixed tun-dish ladle but most foundry men tend to refer to these ladles as a separate process. The ladle has a fixed cover and the metal is poured into the ladle through a large teapot spout that is throated down as it enters the ladle body at the base. After the treatment the metal is then poured out through the teapot spout. Additives are introduced via a separate hatch in the cover.
The fixed tun-dish and the teapot spout treatment ladle have several design similarities; the position of the trunnions must consider the tun-dish with one and the large teapot spout with the other. The design of the additives hatch is often very specific to each foundry, so we always try to make sure that what we offer meets with customer’s requirements.
Fixed Tun-dish and Teapot Spout treatment ladle additive hatch
There are two basic types of additives hatch; weighted and locked. The weighted one, as the name implies, relies on the weight of the hatch to keep it closed whereas the locked type has a locking mechanism to keep it shut. Both types of ladles can achieve a good seal and pressure can build up inside the ladle. There seems to be a difference of opinion as to whether this is good or bad. The weighted cover will act as a pressure vent.
However, I know that several foundries prefer a locked down cover to maintain the pressure inside the ladle and have said that this gives a better Mg recovery rate.
Pre-cast liners
The largest impact on ladle design is the increasing use of pre-cast linings, especially in regards to the teapot spout treatment ladle. Acetarc manufacture re-usable lining formers for all our ladle types and to be perfectly honest the lining formers for the teapot spout treatment ladles are a pain to manufacture (it’s the connecting part plus the spout forms that is challenging) and the whole kit is expensive.
I understand that the actual lining of the ladle is not easy either. A pre-cast liner gets around these problems. I must admit I don’t know the cost of a pre-cast liner, but I am seeing more foundries make use of them. There a number a design considerations with regards to the ladle. Firstly, it’s easy for us to adjust the shell size to suit an existing pre-cast liner. Secondly, regarding the teapot spout, a significant part of the ladle shell is removed, so we must make sure that the rigidly of the lade shell is maintained.
Conclusion
The best type of in-ladle treatment process may not be the process that on paper gives the best results, but the one that can be best integrated into a foundry’s system. This can be evaluated by evaluating your foundry’s working practices, speaking to your additives supplier and speaking to your ladle supplier. Although sometimes there can be an inclination to use an existing casting ladle, buying a specifically designed treatment ladle is a relatively small capital investment, and it can offer significant advantages.