PRODUCTION AND EQUIPMENT

FORMATION OF Si3N4-BONDED SiC SIDEWALL BRICKS

BY DYNAMIC IMPACT PRESSING

Kassim Al-Joubory1

Translated from Novye Ogneupory, No. 1, pp. 14 – 18, January, 2014.

Original article submitted September 10, 2013

The efficient operation of an aluminum smelter requires the use of an inexpensive method of making the

refractories for the cathode of the aluminum reduction cell. In contrast to methods based on uniaxial hydraulic

pressing, dynamic impact pressing does not require the use of a high-strength mold or a powerful press, i.e. it

is significantly less expensive to make the refractories employed in this method. Careful selection of the

granulometric composition of the SiC grit and the silicon powder and maintenance of the necessary moisture

content in the granular mix increase particle packing density and green density. Dynamic impact pressing

makes it possible to produce high-performance sidewall bricks that will have excellent service properties and

a long service life.

Keywords: silicon carbide products, Si3N4 binder, dynamic impact pressing, nitriding.

INTRODUCTION

This research was performed out at Pyrotek Products

Ltd. and was funded by the Organization for Research, Sci-

ence, and Technology as part of the GRIF Program (1999).

The methods most commonly used to form engineering

ceramics and refractories are isostatic pressing, injection

molding, pressure die casting, tape casting, and uniaxial

pressing. Uniaxial pressing is characterized by high produc-

tivity but also has certain limitations. For example, this

method can be used only to form parts of simple geometry

and green density is quite nonuniform — particularly in the

direction of the pressing force. This can lead to delamination

or crack formation due to entrapped air and can reduce di-

mensional accuracy. High pressure acts on granulated pow-

der inside a mold in the uniaxial pressing operation. The

pressure expels air out of the powder, which is then com-

pacted as the individual granules are forced closer together.

The German company Burton Refractories GmbH uses

1600-ton presses and high-strength metal molds to form SiC

blocks with a density of up to 2.62 – 2.67 g/cm3. The very

high pressure exerts a wedging effect on the walls of the

mold, since each granule “collides” with two or more other

granules. This imparts lateral components to the applied

force and to the motion, and those components ultimately

have an effect on the mold walls. The initial force in the ver-

tical direction is quickly dissipated into side thrusts and fric-

tion against the walls, which requires an extremely strong de-

sign of mold. The air that remains in the pressed product

leads to its delamination if the air pressure is greater than the

green strength of the newly pressed piece [1].

The technology of dynamic impact pressing has already

been used for many years to form refractory blocks. The

mechanization and automation that were eventually intro-

duced into the original process of manual ramming stimu-

lated the ceramics industry to begin researching this method

and investing in it rather than in the more expensive technol-

ogy of uniaxial hydraulic pressing. In dynamic impact press-

ing, the powder in the metal mold is subjected to a dual ac-

tion. A high-energy vibrational force acts on the bottom plate

of the die, which expels air from the low-density material

and causes the material to undergo compaction. Then power-

ful pneumatic hammers mounted on the crosshead of the

press apply a cumulative compressing force to the upper part

of the material. The hammers transmit their energy through

the top plate. The magnitude and duration of the force ap-

plied to the material filling the mold is controlled by care-

fully regulating the duration of the vibrations and the force of

the hammers and, to a lesser extent, by changing the vibra-

Refractories and Industrial Ceramics Vol. 55, No. 1, May, 2014

10

1083-4877�14�05501-0010 © 2014 Springer Science+Business Media New York

1Pyrotek Products Ltd. (Auckland, New Zealand);

E-mail: aljoubory@gmail.com.

tion cushion support pressure and the down-thrust pressure

on the top plate during the hammers’ operation.

It has been observed that the effectiveness with which

the powder being pressed is compacted depends on the parti-

cles’ granulometric composition, shape, and surface rough-

ness, the friction between the particles, the chemical compo-

sition of their surface, and the types of additives and binders

that are used [2, 3]. The main prerequisite for increasing the

density of the compacted powder is carefully choosing a par-

ticle size such that the voids between the coarse particles will

be filled by successively finer particles. In connection with

this, we used a continuous size-grading model proposed by

Andreasen and refined by Funk and Dinger [4]. It has the

form:

CPFT/100 = [(D – Ds)/(D

L– D

s)],

where CPFT is the cumulative percent of particles finer than

the diameter D; Ds

is the diameter of the fine particles; DL

is

the diameter of the coarsest particles; q is the distribution

modulus. The maximum theoretical density is attained at q =

0.37, but any deviation from this index changes the optimum

value. By deviation, we mean a deviation in the shape of the

particles from a sphere, an incorrect number of particles in

each size class, etc. Todd Sander used the model to write a

computer program [5] in which the model is used to calculate

the percentage composition of the different charge compo-

nents that have a certain granulometric composition. That in

fact served as the starting point for our investigation; the ac-

tual granulometric composition of all of the components was

then further refined in production trials.

Silicon carbide refractories on a silicon nitride binder are

excellent materials for lining walls. These products are

highly resistant to oxidation and have good resistance to cor-

rosion and erosion, good resistivity and thermal conductivity,

very high strength, and low open porosity. Silicon carbide

blocks with a close dimensional tolerance were fired in a

high-temperature furnace with a controlled N2 atmosphere.

Conditions were created in the furnace for the occurrence of

a solid-phase reaction between the initial components of the

charge and ground metallic Si. The reaction resulted in the

formation of a matrix of SiN3 crystals that encapsulated the

SiC grains. Diffusion of nitrogen into the voids between the

particles and the subsequent reaction with Si particles caused

the particles to expand by 21.6%. This filled the material’s

pores and increased its weight by 66.5%, which in turn in-

creased the density of the block during the nitriding opera-

tion.

PREPARATION OF BLENDS

The study was conducted using grade-EC6 SiC from

Alcoa Inc. (U.S.) and Wacker-Chemie GmbH (Germany). Its

granulometric composition (continuous particle size distribu-

tion mesh) ranged from 8 to 1000 mesh. We also used

dust-sized metallic silicon with d50 = 1.7 �m and a coarser,

220-mesh fraction from the company “Simcoa” in Australia.

After the data on these components was entered into the

computer program, we obtained the initial composition of

each component of the blend and the determined the relative

amount of that component which the blend should contain in

order to maximize particle packing density. The model of

granulometric composition was chosen by using a Waker

grid with an 8 – 10-mesh. This grid replaced the Alcoa

6 – 10-mesh grid that had been used previously. The Waker

grid is more compatible with the shape of SiC particles, al-

though it is more expensive.

The physical characteristics required of the material were

not obtained after mixing, forming, drying, and firing, which

made further studies necessary. More than 50 blending

batches with different constituent proportions were prepared

for these investigations. The compositions were used to form

blocks of a certain size in order to realistically assess the

density of the blocks and the fired products and determine

their physical properties. The density index of the raw mate-

rial for all of the compositions in two of the mixes (Table 1)

turned out to be high enough to make it possible to predict

that the density of the fired products would be greater than

2.6 g/cm3. This is the index value that is used by the research

and engineering divisions of Comalco Aluminium Ltd.. in

Australia. It guarantees that the large blocks of the Bell Bay

series will have the desired properties. These blocks are used

in Comalco’s smelter.

Choosing the method for preparing the blends and using

additives, plasticizers, and binders affect the final choice of

initial components. This was the most important parts of the

production process. The dry batch of each composition was

placed in a “Cumflow” mixer, which is a pan mixer with a

turbo mixing device. A mixer made by the German company

‘Muller” was also used to evaluate the effect of mixing rate

on the final particle packing density. To obtain a uniform

blend, the coarser SiC fractions were charged first and then

the finer fractions were added while the mixer was operating.

Another batch of coarse-grained SiC was added last and mix-

ing was continued for 10 min before the addition of pow-

dered metallic Si. After a uniform blend was formed, we

slowly added PVA binder to the mixer followed by a PEG400

solution; the mixer was left on when this was done. In order

to break up the coarse agglomerates and obtain a freely flow-

ing powder, each blend was passed through a screen with

3 – 5-mm openings before it was pressed.

FORMING

A mold made of a lightweight structural metal and hav-

ing hardened surfaces was used to obtain 409 � 485 � 75 mm

blocks. Products down to 10 mm thickness were formed in

molds with four 194 � 234 � 170 mm cavities from semi-dry

blends of granulated powder. Powder of a certain density was

charged into the mold and pressed on a “Butler” impact press

Formation of Si3N4-Bonded SiC Sidewall Bricks by Dynamic Impact Pressing 11

(Fig. 1a ). The parameters of the pressing operation: vibra-

tion time (VT); vibration cushion pressure (VC); hammer

operating time (HT); down-thrust (DT). We studied the pa-

rameters of the press and all of the variable quantities in or-

der to maximize the density of the raw material. It was found

that an increase in the time of vibration is accompanied by

segregation of the coarse particles. This interferes with fur-

ther compaction of the material and results in density being

nonuniform through the thickness of the product. Excess

force from the hammer produced high-density blocks. In this

case, the bulk density of the pressed product tended to in-

crease with an increase in the moisture content of the blend.

It was observed that increasing the blend’s moisture content

made it possible to obtain a high-density raw material while

using less energy. A high moisture content results in weak

bonding of the particles and lamination cracking perpendicu-

lar to the pressing direction. The cracks first appeared either

after the block was extracted from the mold or during the

drying stage. The tensile forces created by the air that re-

mains in the product adversely affects the PVA binder and

the bonding forces, which in turn leads to crack formation.

The DT and VC indices of powder having a certain density

changed from 55 to 90 psi (1 psi = 6894.76 Pa) with fixed

values of VT and HT, although the difference in green den-

sity was no more than 1% in this instance. We should point

out that a balance needs to be struck between the moisture

content and different parameters of the pressing operation in

order to obtain the desired green density and produce fully

nitrided blocks. Composition (A) had a density of 2.61 g/cm3

and a moisture content of 4.0%, while composition (B) had a

density of 2.54 g/cm3, a moisture content of 3.6%, and a high

content of the desirable fine fraction under the same pressing

conditions.

NITRIDING AND PHYSICAL PROPERTIES

To prevent the formation of cracks and possible breakup

of the blocks during the nitriding operation, we carefully

dried the pressed blocks at room temperature and in a dryer

with a temperature of 110°C. The dried blocks were then

place in a closed hermetic muffle and fired in a “NiBek” kiln

(Fig. 1b ) under excess N2 pressure. The nitriding regime was

chosen so as to realize the complete reaction throughout the

thickness of the large block. The holding time was increased

with an increase in the nitriding temperature in order to in-

crease the rate of diffusion of N2 through the thickness of the

product. Raising the initial temperature too rapidly resulted

in fusion and caking of the silica-bearing powder (melting

point 1410°C), which made it impossible to fully nitride the

products [6]. Figure 2 shows the nitriding regime, including

such parameters as the linear change in temperature, holding

time, and nitrogen flow rate.

RESULTS AND DISCUSSION

Although the selection of the initial components based

on their shape and size and their concentration in the blend is

an important problem whose solution theoretically guaran-

tees maximum particle packing density, preparation of the fi-

nal semi-dry blend with the binders and plasticizers is also

very important. Mixing and the homogenization of the blend

are key factors that affect the final pressing density. Al-

though a higher final value of powder pressing density was

obtained after mixing in the Muller mixer compared to the

Cumflow mixer, the intensive turbulent mixing led to the for-

mation of a powder that was saturated with air. It is easy to

work with such a powder and fill the mold with it, but the

pressing operation itself is difficult and it is difficult to attain

the high green density required.

In dynamic impact pressing, high pressure is applied

over intervals separated by short periods of time. The initial

vibration under load is followed by pressing in intervals with

a high impact pressure. Such a method easily expels the air in

the charge and alters the positions of the granules relative to

one another; a high particle packing density is achieved as a

result. We obtained high-density blocks with the dimensions

12 Kassim Al-Joubory

TABLE 1. Optimized Composition of the Blends that Allows the Production of Raw Materials and Fired Products of the Desired Density, %

Batch*SiC

(8 – 10 mesh)

SiC

(10 – 18 mesh)

SiC

(18 – 35 mesh)

SiC

(80 mesh)

SiC

(36 mesh)

SiC

(220 mesh)

SiC

(400 mesh)

SiC

(1000 mesh)

Si

(dust)

Si

(220 mesh)

A 9.0 32.6 5.9 17.2 14.5 1.7 3.1 2.8 7.6 5.4

B 13 20 20 20 0 7 10 0 7.78 2.22

* 1% solid PVA (6.25 kg, 16% solution) + 0.5% PEG400 were added to each mix.

Fig. 1. ”Butler” DL4X duplex press made by Butler Impact Tech-

nology Ltd., Great Britain (a) and the gas-operated “Nibek” press

made by Nibek Ltd., Great Britain (b ).

75 � 409 � 489 and 194 � 234 � 170 mm (Fig. 3). One of the

blocks was cut through its center; it turned out to be of uni-

form color and structure from the middle to the edges, with

no traces of lamination or coring. Pieces of the material were

cut out from the central part of the block and subjected to po-

rosity and density measurements and x-ray diffraction analy-

sis at Industrial Research Ltd. in New Zealand. The ranges of

compressive and flexural strength were determined in accor-

dance with the standard ASTM C133. The indices of the

specimens are shown below:

Density, g/cm3:

apparent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.65

true . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.17

Porosity, %:

open . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5

closed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9

Ultimate strength, MPa:

in compression . . . . . . . . . . . . . . . . . . . . . . . . . . 197

in bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25.4

Elastic modulus, GPa . . . . . . . . . . . . . . . . . . . . . . . . 131

Mean linear thermal expansion coefficient

(100 – 1000°C), 10–6

°C–1

. . . . . . . . . . . . . . . . . . . . . . 4.2

Thermal conductivity, W/(m·K),

at an average temperature (°C) of:

300 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4

540 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.1

720 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2

Chemical composition %:

Si3N4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . <22

SiC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . >78

Resistance to thermal shock is an index that combines all

of the mechanical and thermal properties of a refractory. Not

a single crack was found on control specimens subjected to

5 cycles of rapid heating to 1100°C and quenching in room-

temperature water. The specimens also showed good resis-

tance to molten aluminum and cryolite attack, with no wet-

ting. The control specimens were dipped in molten alumi-

num and rotated at a speed of 23 rpm at 800°C for 72 h. The

solidified aluminum and dross on the surfaces were easily

peeled. We also performed a test with cryolite: the specimens

were dipped in cryolite and rotated at 1000°C for 72 h. The

minimal physical changes seen in the specimens indicated

that they had good resistance to the effects of cryolite. Resis-

tance to oxidation was tested on 25 � 25 � 100 mm speci-

mens over 12 h at different temperatures up to 1450°C. The

increase in the specimens’ weight was determined after the

tests (Fig. 4).

Formation of Si3N4-Bonded SiC Sidewall Bricks by Dynamic Impact Pressing 13

Fig. 2. Nitriding regime in the firing of large SNBSC blocks.

Fig. 3. Fully nitrided SNBSN blocks.

Fig. 4. Weight gain of SNBSC blocks at different temperatures.

Fig. 5. X-ray diffraction pattern of a cross section of a fired block.

It is apparent from the x-ray diffraction patterns that the

block contains SiC and Si3N4 only where the main Si3N4

phase is �-Si3N4 (Fig. 5). The acicular form of the �-Si3N4

particles imparts greater strength to the fired material. How-

ever, its low ultimate strength in bending can be attributed to

surface flows which develop when specimens are bent at

three points.

CONCLUSION

The program created to study the technology used by the

company Pyrotek for dynamic impact pressing as a new

method of producing high-quality refractories was success-

fully completed and achieved its stated objective. Very good

indices were obtained for the high-quality silicon-carbide

sidewall refractories with an Si3N4 binder that have been

developed for aluminum-smelting furnaces and other appli-

cations. The program has uncovered new opportunities in

overseas markets for Pyrotek’s high-quality non-oxide re-

fractories.

REFERENCES

1. A. Kaiser and R. Kremer, “Fast acting vacuum device - guaran-

teed quality for pressed refractories,” in: Refractories Manual.

InterCeram (2003), pp. 28 – 33.

2. J. Zheng, W. Carlson, and J. Reed, “Reed dependence of compac-

tion efficiency in dry pressing on particle size distribution,”

J. Am. Ceram. Soc., 78(9), 2527 (1995).

3. F. S. Ortega, R. G. Pileggi, P. Sepulvede, and V. C. Pandolfelli,

“Optimizing particle packing in powder consolidation,” Amer.

Ceram. Soc. Bull., 78(8), 106 (1999).

4. J. E. Funk and D. R. Dinger, “Particle packing. Part IV. Applica-

tion of particle size distribution concepts,” InterCeram, 43(5),

350 – 353 (1994).

5. T. Sander, Department of Ceramic Engineering. T. Sander. Uni-

versity of Missouri-Rolla (U.S.) (1999).

6. F. L. Riley, “Reaction bonded silicon nitride,” Material Science

Forum, 47, 70 – 83 (1989).

14 Kassim Al-Joubory