RAW MATERIALS FOR NON-COMPLEX HIGH PERFORMANCE LOW CEMENT CASTABLES

Robert McConnell, Almatis Inc., Bauxite, Arkansas 72011, USA, [email protected] Frank Drnevich, Almatis Inc., Leetsdale, Pennsylvania 15056, USA, [email protected] Andreas Buhr, Almatis GmbH, 60439 Frankfurt/Main, Germany, [email protected]

ABSTRACT Most newly designed castables are low cement formulations. The conversion from high cement to low cement castables started using silica fume, simple calcined aluminas, 70% alumina cement, and dispersing additives. These castables require less mixing water vs. conventional castables, however, they still provide only limited performance with regard to installation properties and service life under severe conditions such as in steel ladles. The demand for easier installation and for improved performance during use requires more advanced raw material concepts. These end-user demands result in modern low cement castables (soft vibration or self flow) that exhibit higher refractoriness and wear resistance as compared to the simple low cement castables mentioned above. The challenge is to design high performance castables while avoiding highly complex formulations. Since a single castable cannot be the appropriate solution for all requests, the paper discusses a number of improvement options for moving from simple low cement castables towards high performance, soft vibration or self-flow castables. Highly efficient dispersing systems are utilized to achieve high performance properties in non-complex formulations. INTRODUCTION With the significant increase in worldwide steel demand, capacity utilization, and steel prices the refractory industry is seeing a renewed emphasis on refractory performance. Steel manufacturers are selling all the steel they can produce so process uptime has again become very important. Additionally, the higher quality steel being produced results in aggressive process conditions that shortens refractory life. Refractory service life and refractory failure prevention often are as important as refractory cost in this environment.

This environment is leading to worldwide changes in unshaped refractories. It appears there is (1) more high value refractory products being sold, (2) more use of synthetic mineral raw materials, (3) more use of specialty minerals – hydrateable alumina binders and spinel, (4) more use of easy-to-place rheology castables – self flow and soft vibration, (5) use of pumpable products to shorten installation time, and (5) renewed interest in faster dry-out / heat-up schedules. One worldwide trend has been to refractory concretes that minimize installation related shortened service life or catastrophic failure. All refractory manufacturers have their archives of installer caused failures such as: Short casting life or long wet-out time was “fixed” by adding excess mixing water that resulted in lower strength and higher porosity => short life. Inability to cast a complex shape being “fixed” by adding more water to be able to place – similar to above. Or there were voids that required repair after demolding. Poor vibrator practices with heavy vibration castables did not eliminate hidden casting voids resulted in catastrophic failure in service. Explosive steam spalling failures from fast or un-controlled dry-out schedules And the list goes on………… To eliminate or at least minimize the installation related failures refractory manufacturers are moving to refractory concretes that are easy to place. These products have self-flow or soft vibration placement rheology at very low casting water levels - 3.5 to 5.5% with high density aggregate. There is a strong interest in products that are fast to wet-out and are not strongly dilatant in the mixer. Not only must refractories be designed to routinely survive severe service conditions, they must also be designed to routinely survive severe installation conditions. Fast turn-around is a requirement and the properties as installed are what will determine the service life – not the data sheet results. But never do the end-users quit looking at purchase price. All of this must be accomplished while minimizing the total cost (some unit measure of “value”) to the end-user. Raw Materials Selection Trends In this environment the typical raw materials selection decisions include: Is the mineralogy appropriate for the application’s service conditions? What will the service life be? Is there value in greater process uptime? The cost of the various materials options ($$$$ vs $$$ vs $$ / MT) The availability of the various materials options (local or weeks to import) The cost to inventory the selected materials o 8 components @$$/MT or 12 components @$$/MT? • Will installer caused failures be minimized? • • • • •

As the decisions are considered for aggressive service conditions, it can generally be assumed that: • Natural minerals will be lower cost to purchase, be more variable, contain more impurities and provide shorter service life • Synthetic minerals will be higher cost, have lower impurities, be more consistent and will provide longer service life

Achieving the required castable properties typically results from: • Selecting matrix materials with closely controlled particle size distribution • Selecting controlled sizing synthetic aggregate • Selecting matrix fines and aggregate with appropriate mineralogy • Selecting highly effective water reducing and set controlling additives • Minimizing oxides that can form low melting point phases in-use • Ensuring easy placement rheology at low casting water levels • Using as few components as possible DESIGN OF TEST CASTABLES This paper discusses raw materials options for how these needs are met. It looks primarily at the matrix materials required to move from traditional low moisture castable technology to the high performance refractories that are growing rapidly. To demonstrate the ability to produce higher performance low cement castables with a minimum number of materials, each formulation uses only 3 aggregate sizes. The design differences are in the matrix materials. The aggregate used is tabular alumina (corundum) that provides excellent resistance to thermal cycle failures1 and abrasion / erosion resistance. Mono-Modal A20SG and Bi-Modal Reactive A17NE Aluminas

A20SG

A17NE

16.0

14.0

12.0

% In Class

10.0

8.0

6.0

4.0

2.0

0.0 0.1

1.0

10.0 Microns

100.0

In one formulation option, no silica fume is used. The bi-modal A17NE reactive alumina is utilized to provide both the