Refractories - Pneumatic Placement (Gunning) of Plastic Refractories

Use of pneumatic placement as an installation technique for plastic refractories adds another placement method for use by industry. This method has been found to result in quality installations at considerable savings in time and consequently at lower cost than for conventional ramming practice. Many gunned plastic installations have been made with both the clay-air bond and phosphate bond systems. These plastic refractory materials can be manufactured in both extruded and granular form. Extruded materials are used principally in the U.S. and Canada due to the type of equipment available, while the granular materials are used in Europe and Japan.

Compositions and Performance

The plastics used for gunning applications are based on conventional plastic compositions, but have been modified for gunning use. While it is possible to ram a plastic designed for gunning, the reverse does not hold true and it is not possible to gun off the shelf plastic materials. A conventional material will cause buildup in the gunning system and clog the hose. While gunning plastic mixes are designed for minimum rebound losses, in a typical installation these losses may be as high as 15% to 20%. However, unlike the rebound associated with a conventional castable gunning installation, the rebound from plastics may be recycled, thus reducing the actual rebound loss to 5% to 15%. Gunned plastic installations are performing as well as those of rammed ones in many applications around the world.

Properties of Gunned Plastic Refractories

The compaction and density of gunned plastic refractories is good. Table 1 compares the physical properties of gunned and rammed samples of a superduty plastic and of a 70% alumina phosphate bonded plastic. As may be noted from this table, the physical properties are very similar for the gunned and the rammed plastic materials of the same composition.

Table 1. Comparison of properties for gunned and field rammed plastics.

Installing Gunned Plastic Refractories

This new installation process consists of utilising specially prepared plastic refractory, which had been packaged in a protective wrapping to prevent moisture loss, and introducing the unwrapped slugs into a rotary shredder or granulator. This converts the material into a loosely arranged granular form which is conveyed directly into the gun-feed system. The pieces or granules from the shredder are typically ½” to ¾” (1.25 to 2.0 cm.) in diameter. The conveyer has a variable speed control and regulates the amount of feed to the gun. The plastic is immediately moved by high pressure air through a 2” diameter hose to distances up to two hundred fifty feet and blown to the point of installation on walls, floors or ceiling. Several types of rotary guns have been successfully used, but they require modification to drive the feed mechanism to the hopper arrangement to make them suitable for gunning plastics.

The actual gunning installation of plastics is similar to that of conventional gunned castable except there is essentially no dust. The nozzleman does not have to be concerned with controlling water additions, but instead may concentrate on the placement. Lining design and anchoring systems are very similar to those of rammed installations. Gunned plastic roof installations may be made with anchor tile on 14” (30 cm.) centres. Sufficient air volume and pressure is necessary for a good installation. The momentum of the refractory entering the nozzle section of the hose is very important to obtain good consolidation and placement density.

Summary

In summary, the pneumatic placement of plastic refractories is a viable alternative to the conventional ramming technique. Besides the big advantage of pneumatic placement to lower installation time and cost, the refractory plastic can be transported considerable distances through the hose to the actual installation site and placed without the creation of dust. Many such installations have now been made worldwide.

Note – A complete list of references can be obtained by referring to the original text.

Primary author: Edwin Ruh

Source: Abstracted from International Ceramic Monographs, Vol. 1, no. 2, pp. 772-93, 1994.