To make a fire assay method totally automated required a method of automatically separating the lead collector and slag. Particularly for spark analysis the lead button produced needed to be completely slag free and rapidly cooled during casting to ensure homogeneity.
5.4.1 DECANTING
The first slag separation was performed manually. The operator would carefully pour the slag out of the crucible. Once the slag had drained, the lead left in the base could be rapidly poured into a mould. This was fairly successful but had some serious flaws:
• The method was operator dependent and not easily automated.
• The lead collector rapidly oxidised once the slag was removed.
• Residual slag always remained in the crucible and sometimes reported with the lead.
5.4.2 VOLUMETRIC SLAG SEPARATION
The volumetric separator was a more controlled form of decanting. With the higher density of lead it settled and displaced slag. It was possible, if the volume of lead was known, to pour the melt across a receptacle that would fill with the lead while the slag was decanted away. By inverting the receptacle the lead could then be drained and selectively cast. The first volumetric slag separator was manufactured by Hiprom in the prototype Immafuse machine discussed earlier in Section 5.3.1. The operation of this slag separator is shown Figure 5.9.
Some of the issues with slag separation using this method were that the separator needed to be heated to retain the fluidity of the slag so that it could be decanted. So the separator needed to be contained in a furnace of some sort and rotated. This was extremely difficult as the elements in the furnace embrittled and broke easily when the furnace was moved. The slag was corrosive and highly conductive, this caused numerous separator failures. The separator needed to be replaced regularly.
Figure 5.9 Operation of the volumetric slag separator
This slag separator was combined with the melting head for the first production Immafuse induction fusion machine. This was a novel idea as it eliminated the requirement of two separate devices and is shown in Figure 5.10.
Figure 5.10 Melting head and separator combination
Fusion Slag removal Lead casting
Slag
Lead
Lead
Fusion Slag removal Lead casting
Slag
Lead
Lead
Fusion Slag removal Lead casting
Slag
Lead
Lead
Fusion Slag removal Lead casting
Slag
Lead
Lead
Fusion Slag removal Lead casting
Slag
Lead
Lead
Fusion Slag removal Lead casting
Slag
Lead
Lead
Stainless steel casing Resistive heating elements Cast ceramic
Thermocouple
Ceramic lid and separator pot
Slag Lead
Chilled mould with ejector
Stainless steel casing Resistive heating elements Cast ceramic
Thermocouple
Ceramic lid and separator pot
Slag Lead
Chilled mould with ejector
Stainless steel casing Resistive heating elements Cast ceramic
Thermocouple
Ceramic lid and separator pot
Slag Lead
Chilled mould with ejector
Stainless steel casing Resistive heating elements Cast ceramic
Thermocouple
Ceramic lid and separator pot
Slag Lead
Chilled mould with ejector
The disadvantages of the volumetric separator were similar to manual decanting:
• The lead collector rapidly oxidised once the slag was removed.
• The molten lead oxide that formed was highly reactive and corrosive causing the separator to deteriorate rapidly.
• Lead losses were common as the lead was highly fluid and easily spilled, especially if the separator was moved too quickly.
• Residual slag always remained in the crucible and sometimes reported with the lead. This effect worsened as the separator deteriorated.
5.4.3 COWAN SEPARATOR
The Cowan Separator was developed by Mr. George Cowan in collaboration with Anglo Platinum in the early stages and then later with IMP.
The original idea that was proposed to Mr. Cowan was a method to flatten lead buttons into strips that would be suitable for digestion using the lead dissolution technique. Initial efforts involved the flattening of lead with a hydraulic press, though this required mechanical flattening first. Later the re-melting and centrifugal casting of the lead was investigated. This did not work too well as it resulted in lead leaks but what was significant was the separation of the lead from the slag during casting.
The Cowan separator was born by drilling a hole into the base of a standard cast iron mould and casting the lead into the mould directly. The lead was able to exit through the hole before the slag, but with a cold mould the lead had a tendency to freeze. This required the separator to be heated and a cast iron mould no longer became feasible and a ceramic cone was used as shown in Figure 5.11.
The cone separator made use of the difference in density between the slag and lead. The lead sank to the base of the cone displacing the slag above it and rapidly drained through the hole. Once the lead passed through, the slag with its higher viscosity would slowly move through the hole. With its lower surface tension compared to the slag it wetted the cone and collected along the base. By adjusting the diameter of the hole in the base, the rate at which the slag drained could be controlled. The optimum hole diameter was between 0.8-1.2mm as this allowed a 3-5s delay for the slag to drain after the lead. This was sufficient time to remove the mould before it was contaminated with slag.
The original design was entirely manually operated and the cones were loaded in the furnace during the fusion. Prior to the casting the cone was removed and placed in a holder above the mould. The cone was allowed to cool slightly to a dull orange colour and the melt was then cast and the cone moved away from the mould once the lead had passed through. The operation was slow and operator dependent but worked fairly
reliably. The cone was used only once and was disposed of. The drainage time for the lead was 15-30s. Its biggest problem was the oxidation of the fine stream of lead during casting.
Figure 5.11 The original ceramic cone separator
To automate this concept required the cone to be heated in a furnace under controlled conditions. A small furnace was built to enclose a cone that was loaded in the top. The furnace maintained the temperature of the separator at 900-1100°C. The furnace was stationary and the melt was cast into the separator.
The next incremental improvement was to purge the furnace with nitrogen. This was effective in preventing oxidation of the lead during casting. The lead was cast into a steel water-cooled mould with a graphite pad. The graphite was eventually substituted with copper for more efficient cooling.
The first designs used cones with drains made from alumina tube to remove the slag so that they could be re-used. These cones were affectionately called sputniks and were difficult to make. One of the key developments was that multiple holes were used at the base for fast drainage of the lead and this shortened the separation time.
Once the principle of re-using the separator cone several times proved successful the shape was changed to be similar to a conventional crucible so that the robot could manipulate it. The upper walls were extended and given a lip so that it could be inserted into a furnace.
Lead Slag Ceramic
Cone
Mould Slag adheres to cone
Lead Slag Ceramic
Cone
Mould Slag adheres to cone
Lead Slag Ceramic
Cone
Mould Slag adheres to cone Lead
Slag Ceramic
Cone
Considerable work was done on the base of the crucible. The shape was changed to control the direction of slag drainage. The final optimisation was done at Anglo Platinum during hot commissioning trials.
The separator crucible was a critical component for the system and it requires some detail. The size and shape of the crucible had to be consistent as this could affect the alignment and loading thereof. The key points to this technology are highlighted in Figure 5.12. The support rim had to be whole and undamaged. It also had to be a consistent thickness and the vertical height from the base of the rim to the bottom of the crucible had to be within 5mm.
In the older design labelled A in Figure 5.12, the slag was drained from the side. The hole was quite large and slag drained quickly. The slag ran down the side of the crucible and due to the angle, inertia and viscosity of the slag was deviated inwards into the mould. Unfortunately physical limitations in the automated system prevented the mould from being located closer to the separator. The problem had to be resolved by changing the initial design of the separator.
Figure 5.12 Final separator design
Casing Support rim Weir Old drain hole φ8mm New drain hole φ4mm Separator insert Hole φ0.8mm Alumina A B C D Mould Fusion pour Casing Support rim Weir Old drain hole φ8mm New drain hole φ4mm Separator insert Hole φ0.8mm Alumina A B C D Mould Fusion pour Casing Support rim Weir Old drain hole φ8mm New drain hole φ4mm Separator insert Hole φ0.8mm Alumina A A B B C C DD Mould Fusion pour Fusion pour Casing Support rim Weir Old drain hole φ8mm New drain hole φ4mm Separator insert Hole φ0.8mm Alumina A B C D Mould Fusion pour Casing Support rim Weir Old drain hole φ8mm New drain hole φ4mm Separator insert Hole φ0.8mm Alumina A B C D Mould Fusion pour Casing Support rim Weir Old drain hole φ8mm New drain hole φ4mm Separator insert Hole φ0.8mm Alumina A A B B C C DD Mould Fusion pour Fusion pour
The other problem with this older separator design was that a large quantity of slag was retained. This large head of slag created extra pressure forcing it through the insert. The reactive slag also caused the separator to deteriorate.
The final design (B in Figure 5.12) used a smaller drain hole of only 4mm in diameter. The hole allowed the slag to fill the crucible and broke the fall of the lead during casting. The lead with its higher density sank through the melt and collected in the weir. The slag ran slowly out through the drain hole. This left a large head of slag to apply pressure to the lead collector and it was forced quickly through the ceramic insert.
The lead, due to its high fluidity and high interfacial tension flowed directly out of the separator insert and fell into the mould (labelled D). The insert was concave at the top and this allowed the lead to drain more evenly.
Once the lead was through, the slag continued to drain until only a small quantity remained. The slag, due to its higher viscosity, slowly flowed out of the holes in the insert. Due to its lower interfacial tension it wetted and adhered to the separator crucible. It ran down the concave sides of the separator at the base and fell past the mould into the slag bin (C in Figure 5.12).
Advantages of the Cowan separator:
• A static separator, no moving during casting.
• Cheap and disposable.
• Compatible with automation and robot gripper.
• Nitrogen purge of lead during casting prevented oxidation of lead.
• Fast separation.