David Parkinson* and Phillip Toperesu** discuss the development and purpose of two new materials for feeder expendable refractories.
The year 2020 marks 50 years since Parkinson-Spencer Refractories (PSR) commenced the manufacture of feeder expendable refractories and is therefore an appropriate time for the company to release two new advanced refractory materials to the market.
Both materials were jointly researched and developed with the assistance of the University of Leeds, School of Chemical and Process Engineering, the first one being carried out under a UK government sponsored KTP (Knowledge Transfer Partnership) scheme and the second carried out by direct sponsorship of a University of Leeds PhD student.
The first material, PSR-925, is a niche product originally conceived as a spout insert material.
The spout insert is a refractory component installed as part of the spout bowl in the tube seating area so as to extend the life of the spout bowl by replacing the highest wearing area with a more corrosion-resistant refractory.
At one time it would have been manufactured from fused cast AZS material but in recent years materials based upon chrome oxide have become popular.
PSR’s approach to developing a competitive material was to avoid the use of chrome oxide for three reasons:
1. It is costly to produce.
2. Chrome-based materials have the potential to release hazardous chromium (VI) and are harmful to the environment and to health. Special consideration is required for their manufacture and disposal.
3. It can add a green tint to molten glass and can cause cross-contamination in the plant.
As this was to be a niche product, PSR decided to dispense with the traditional approach of basing it upon an AZS formulation such as is used in its standard materials, PSR-333 and PSR-315.
These materials rely upon the addition of Zircon (ZrO2.SiO2), more accurately known as zirconium-silicate, where the silica dissociates from the zircon at high-temperature and reacts with alumina in the composition to form mullite.
Instead, the company used zirconia (ZrO2), the oxide of zirconium, where the silica has been removed and then stabilised by the addition of a small amount of CaO.
Without the presence of silica, zirconia is one of the most refractory materials available and is not easily wetted by siliceous glass.
Used as the base material for PSR-925 it therefore had the potential to achieve much greater corrosion resistance in normal soda-lime container glass.
The only other additions to the mix were alumina grains and powders, added in the correct proportions to optimise the packing density, and a small amount of deflocculant to assist the flow properties of what was essentially a slip-cast product.
Using predictive process control (PPC) techniques, with many adjustments made over the two-year development period, and with repeated testing in its own in-house glass corrosion testing rig, PSR achieved a highly refractory, dense, material containing only zirconia and alumina, that very nearly matched the corrosion characteristics of chrome-oxide based materials for the temperature and application intended.
Improved fracture toughness and thermal shock properties of the refractory were also observed.
And then, a ‘light bulb’ moment. If the new material can be cast easily and successfully into the shape of a spout insert, why not extend its application to the entire single piece ‘uninserted’ spout bowl.
Not only would this remedy the issues identified above with chrome oxide inserts but it would also remove the joint between two different refractory materials in a highly sensitive area right before the forming operation.
It would also extend the life of the main body of the spout, a common problem with inserted spouts where the spout corrodes faster than the insert.
PSR now offer a spout insert in PSR-925 if that is the customer preference or PSR’s recommended option of a complete one-piece spout in PSR-925.
Based upon feedback from customers over the past four years, PSR is confident that for a single piece spout in PSR-925 a life in excess of three years can be achieved in most typical soda-lime container glass applications.
The development of PSR-930 was a response to the perception in the industry that higher zirconia contents lead to greater glass corrosion resistance.
At PSR, its traditional feeder expendable materials PSR-333 (11% ZrO2) and PSR-315 (18% ZrO2) continue to perform well and are often the products against which competitive materials are evaluated.
Its first line of research therefore was to evaluate whether or not it is true that more zirconia = greater corrosion resistance.
Unlike with the development of PSR-925 described above, from the outset with PSR-930, PSR decided to remain with the familiar AZS group of minerals.
As with PSR-925, the company also decided to remain with its traditional slip casting process, a process that relies upon the constituent raw materials being suspended in a ‘slip’ with flow characteristics that enable it to be poured into plaster moulds.
Starting with PSR-333 and PSR-315 as being the standards needed to exceed, PSR used a design of experiment (DOE) approach to develop a series of different compositions with varying proportions of zircon.
PSR examined their microstructure, chemical analysis and mineralogical phases and evaluated them for porosity, density, glass corrosion resistance and thermal shock resistance.
It compared compositions containing 35% granular weight of zircon with compositions containing 40% and 45% granular weight of zircon and reached the conclusion that in fact the lower (35% granular weight) zircon containing material showed to be an optimum level and had greater potential for development than the higher zircon containing materials.
However, an equally significant characteristic that was found to have an equivalent influence on the corrosion resistance of these compositions was their particle packing density.
Literature shows that glass corrosion increases linearly with the percentage of apparent porosity, and is shown to be true within a limited range of apparent porosity of around 12-16%. Above this range corrosion rates were reported to be non-linear.
It is for this reason that refractory research and development is directed toward achieving higher densities in refractories.
To put it simply, the more solids that can be got into a ‘slip’ the greater its density, the lower its porosity, and therefore the greater its resistance to glass corrosion.
Getting the right packing density, however, is not easy as it depends upon the available grain sizes and shapes of the constituent raw materials and the proportions in which the various types of raw materials need to be added.
Zircon for instance, although abundant as a raw material, is only found as sand and therefore, in its natural form, cannot provide any of the coarse material required to produce a good packing density.
In this case, the coarse material is provided by the alumina materials added to the composition.
Establishing the ideal particle packing density was traditionally done using a semi-empirical model known as the Andreassen Model, subsequently modified by Dinger and Funk.
Further particle packing development of PSR-930 was aided using DigiPac modelling software.
This uses a digital representation of the particle shapes and sizes in the composition which are then simulated to create a digital packing environment in the form of voxels (pixels as lattice grid cells) in a three-dimensional grid, and enabled us to achieve low porosity with consequent high density.
A study of competitive feeder expendable refractory materials with high zirconia contents revealed that most had porosities within the 18-20% range, with the lowest being 16%.
Similarly, their densities were typically in the range 2900-3000 kg m-3 with the highest being 3200 kg m-3.
With a porosity as low as 13% and a density as high as 3380 kg m-3, PSR-930 exceeds the porosity and density characteristics of all equivalent competitive materials that PSR could find by a comfortable margin.
A further key feature of PSR-930 is the microstructure of inter-locked zirconia-alumina-mullite grains.
This is analogous to that seen in the microstructure of fused cast AZS materials and goes toward explaining why PSR-930 has improved glass corrosion resistance and improved thermal shock properties.
But theoretical resistance to glass corrosion needs verification and using PSR’s own in-house glass corrosion testing rig we carried out rigorous testing against its own ‘standard’ materials and available competitive materials.
Resistance to thermal shock is also a vital characteristic for feeder expendable refractories and in spite of its low porosity and high density PSR-930 outperformed our own standard materials and available competitive materials in repeated standard water quench tests.
PSR-930 is now available for all feeder expendable parts, although a variation with finer grain materials is used for orifice rings to allow the casting of small complex shapes.
Based upon feedback from field trials over the past two years, PSR is confident that the PSR-930 will increase the life of feeder expendable parts by between 30-50% in most standard soda lime container glass operations.
*Chairman, Parkinson-Spencer Refractories
**Research & Development Manager, Parkinson-Spencer Refractories Halifax, United Kingdom