Olivine sand is an ideal mineral for the production of manganese steel castings. It is however difficult to use successfully in a foundry environment for many well-known reasons. This article by Bob Simpson of Foseco UK describes how Metso Steel Foundry in Czech Republic succeeded in applying this material in combination with the FENOTEC* binder process to achieve very high sand reclamation levels.
Metso Steel Foundry, Prerov based in the Czech Republic, was established in 2008 for the production of manganese and chrome molybdenum steel castings for the mining and construction industries. The company produces finished castings ranging from 300kg to 5 tonnes in weight.
Mould and core production initially consisted of a combination of traditional greensand and CO2 cured sodium silicate binder processes. In 2009 a decision was taken to extend the production capability of the foundry from 5,000 to 16,000 tonnes. To achieve this goal a complete process change was required to attain the required productivity levels and to improve the quality of the castings produced. The decision was taken to install an IMF fast loop moulding system and an IMF sand reclamation system utilising a chemically bonded binder process. The sand reclamation plant was built in two stages, initially a primary and secondary attrition unit and finally the addition of a thermal reclamation system.
Sand disposal is becoming increasingly difficult due to tighter environmental legislation and higher transportation costs and taxation. In addition, the purchase of new sand is becoming more costly due to rising energy prices and the decreasing availability of locally sourced high quality material.
To reduce the impact of these sand costs, a requisite for the new moulding system was that high levels of reclamation were required to minimise new sand input and waste sand output from the process.
Metso decided that to achieve these goals a unit sand system based on olivine sand bonded with the FENOTEC binder process would be the best option.
The use of olivine sand and the FENOTEC binder process in combination would create some challenging technical obstacles if high levels of reclamation were to be achieved. This article shows what methods were employed to ensure that these obstacles were overcome.
Olivine sand
Olivine sand is an ideal mineral to use for the production of manganese steel castings as it is chemically less reactive than silica sand. SiO2 reacts with MnO in the steel to form a MnSiO3 slag resulting in serious burn on defects. A further advantage is that olivine sand has a low thermal expansion rate compared to silica sand and its high thermal shock resistance furthermore reduces sand expansion problems.
Olivine sand is, however, a difficult sand to use successfully in a foundry environment for the following reasons:
• High alkali content (typical pH range 9 to 10, thus restricting its use with certain chemical binder processes)
• Angular grain shape and poor grain size distribution leading to a high binder requirement and poor sand compaction
• Not easily sourced and quality can be variable depending on location
• Expensive compared with silica sand
MgO and SiO2 are not free but combined as forsterite Mg2SiO4 and fayalite Fe2SiO4 (approximate ratio 90/10).
The FENOTEC binder process
Introduced to the foundry market 35 years ago, this alkaline phenolic ester cured resin binder process has proved very popular in all casting segments. One of the main markets for this process is in the steel foundry as the excellent hot strength properties greatly reduce hot tearing defects on critical steel castings. The surface finish on all ferrous and non-ferrous castings is also improved with this binder process.
The FENOTEC binder process is an alkali system and as such is one of the few chemical binder processes that is compatible with olivine sand.
Other processes such as furan and PUNB are not compatible for use with olivine sand due to acid neutralisation with the former and uncontrollable rapid curing with the latter process.
A further benefit is that the curing speed can be easily controlled by using PLC controlled ester blending to give constant levels of productivity over a variable temperature range.
However, one limitation of this binder process is the low reclaim levels achieved using conventional dry attrition techniques due to the build-up of alkali salts on the sand grain, which ultimately cause a reduction in re-bonding properties. To overcome this constant additions of new sand are required.
FENOTEC binder optimisation for use with olivine sand
When the new plant was commissioned it was decided that the best in class steel foundry resin and ester combination would be used.
This would be taken as a benchmark for future development with an aim to optimise the process in respect to productivity, mould strength, addition rates and process costs.
FENOTEC 3000 resin was chosen in combination with FENOTEC HX 35 medium cure and FENOTEC HX 75 slow cure esters. FENOTEC 3000 is based on a high solids/alkali containing recipe designed to give very good strength properties in boxless mould applications.
In 2012 it was decided to evaluate a modified resin recipe having a lower alkali content and at the same time introduce a combination of a potassium and sodium hydroxide alkali base.
The basis of this recipe development was to reduce the residual alkali in the reclaimed sand and thus improve the re-bond properties of the sand, allowing for reduced binder addition rates to be used and greater use of attrited reclaimed sand.
This was a successful introduction and this grade of resin, FENOTEC 280ES, is now fully established.
The moulding process at Metso
A 50 tonne/hr continuous mixer is used to prepare sand mixtures for the production of boxed moulds. This mixer has the capability of running multiple sand recipes including variable resin binder addition rates. Chemical addition rates are monitored and adjusted using flow controllers. In addition the resin is maintained at a constant temperature of 25°C to reduce viscosity variation and thus improve mixing efficiency.
A mould strip time of 45 minutes is required and can be achieved consistently using a PLC controlled hardener blender process based on medium and slow curing esters.
By using a multi-program mixer binder addition rates can be optimised so that the minimum level of resin and hardener can be added to give the required properties.
Binder addition rates are important in respect to achieving the desired strength necessary for mould handling, the casting process and to ensure the lowest amount of residual binder in preparation for sand reclamation.
The residual binder in the sand influences the re-bonding properties of the sand. If the residual binder levels are too high the strength development is significantly reduced and furthermore it will also have a detrimental effect on casting performance.
The normal way of measuring this residual binder is to determine the loss of ignition and percentage of alkali in the reclaimed sand. These tests indicate the level of organic and inorganic binder left in the sand and as such can be used to determine how much clean sand is required to maintain equilibrium in the sand system.
Loss on ignition is a common test method used by foundries, however, the percentage of alkali in the sand is normally conducted by the binder supplier or a specialist analytical laboratory.
One method of determining the alkali content is to use a simple titration method. This involves boiling the sand together with a known quantity of acid and then titrating with a standard alkali solution to pH 7. Unfortunately as olivine sand is already alkali then this method is not suitable. As such, in conjunction with Ostrava University, a method based on electrical conductivity was developed.
Electrical conductivity is the measure of the total concentration of dissolved salts in water. When salts dissolve in water they give off electrically charged ions that conduct electricity, the more ions in the water the higher the conductivity. It can be seen from this that a relationship between residual alkali salts in the sand and electrical conductivity can be established as a means of controlling this alkali residue.
This test method is fully established at Metso as it gives accurate results and it is a very quick and easy test procedure.
Sand reclamation
The function of a sand reclamation system is to take lumps of used foundry sand and process it back to sand grains that are suitable for re-bonding. Sand that has been reclaimed must have similar properties to that of new sand.
To achieve the maximum level of sand reuse, Metso installed a three-part system based on conventional primary dry attrition, secondary attrition and thermal reclamation processes.
Primary dry attrition uses high intensity vibration to reduce the sand lumps down to grain size. This vibration helps to break the bond between each sand grain but only removes a small amount of the binder film coating the sand grain. Vibrating screens help to control the grain size distribution by removing large agglomerates and metallic contaminants.
Sand from this process alone, however, will only allow low levels of reuse due to the high levels of residual binder present in the sand.
To overcome this a more vigorous secondary attrition process is used to remove additional binder film from the sand grain. This is achieved by sand from the dry attrition process being scrubbed between high-speed rollers and the sidewalls of the attrition chamber causing the sand grains to impact with each other.
Sand taken form the secondary attrition unit is then fluidised with cold air to remove dust particles and cool the sand to ambient temperature. This sand is suitable for re-bonding at the moulding station.
Whilst this secondary attrited sand is suitable for moulding, the system still requires a low level input of clean sand to ensure that residual binder levels in the sand remain consistent.
To achieve this clean sand a thermal plant is used to remove all of the organic binder from the sand.
Sand is taken from the primary dry attrition stage and fed into a gas fired fluidised thermal plant. The thermal bed temperature is maintained at 600°C to ensure total combustion of the organic components in the sand. The calorific value of the binder helps to reduce the energy required to maintain temperature and also helps to ensure that the exhaust gases are clean. Sand is cooled and fines extracted prior to transferring into the clean sand hopper.
It is standard practice when thermally treating reclaimed sand from an alkali phenolic resin process to use a special additive to prevent sand fusion in the thermal bed. This fusion occurs due to the residual inorganic component of the binder melting at low temperature and bonding the sand grains together. The additive can also help to reduce the alkali content of the sand by ion exchange between the sand and the additive, the additive is then removed by fines extraction.
As experience of thermal reclamation using olivine sand is very limited a thorough laboratory study was initiated to develop/optimise the use of these special additives.
The final outcome from this study was that an additive was not required as fusion did not occur at the operating temperatures required to run the thermal bed. In addition the residual alkali was reduced significantly by combustion alone.
Current moulding recipes
The majority of moulds are prepared using a standard recipe based on 85 per cent reclaimed sand + 15 per cent thermally reclaimed sand. Special recipes can be used for special applications such as the production of large grinding ring castings, or indeed on occasions when the thermal plant is switched off for maintenance.
Summary
By careful control of the sand system and by utilising the FENOTEC binder process, Metso has shown it is possible to produce quality manganese steel castings using very high levels of reclaimed sand based on olivine sand.
Since the introduction of the new moulding process, four years ago, Metso has significantly improved productivity and casting quality and reduced process costs dramatically.
The level of new olivine sand used in the foundry is extremely low, indicating total reclaim levels consistently above 95 per cent.
Acknowledgements
The author would like to acknowledge: Ivo Dolezal, general manager Metso Czech Republic sro
Ostrava University, foundry division; IMF Group; Jiri Duda, Michal Szotkowski and Tomas Stepan (Foseco Czech Republic); Hengelo binders team.
To access all the tables and images with this report please refer to the printed version in the October 2015 issue of Foundry Trade Journal.
Contact: Paul Jeffs, UK technical manager, Vesuvius UK Limited – Foseco Foundry Division, Tamworth, Staffordshire B78 3TL UK. Tel: +44 (0) 1827 289999, email: [email protected] web: www.foseco.com
*FENOTEC is a Trade Mark of the Vesuvius Group, registered in certain countries, used under licence.