High-performance facilities and production methods incorporating the latest technological developments are required to produce high-grade refractory bodies. Unfortunately, many refractory companies are running decades-old production plants that do not meet today's standards. Common problems include quality variations in production batches; high reject rates, personnel expenditures and maintenance costs; and frequent bottlenecks in material preparation plants.

As the manufacturing processes in consuming industries such as steel, cement and glass have improved, refractory consumption has declined. To remain profitable in this environment, refractory producers must supply their customers with products of the highest quality at reasonable, competitive prices-and meeting this objective requires modern, highly efficient production technologies. Taking steps such as trouble-proofing batch compositions through exact silo discharge and precise weighing technologies, using optimized mixing technologies, and incorporating high-performance control technologies are some basic ways companies can optimize their preparation plants and maintain a competitive edge in the global market.

Feeding Equipment

In refractory plants, raw materials are often fed from the silos through screw feeders for powder and fine-grained components, and through vibratory conveyors for coarse-grained bulk material. In older plants, the feeding is often done by hand via pushbuttons or operating levers. The screw feeders used for the discharge of the ensilaged raw materials are equipped with pole-changing motors, which usually results in two different feeding speeds. Neither the screw feeders nor the vibratory conveyors are usually equipped with shut-off flaps. These systems tend to cause several disadvantages, including:

• Inaccurate feeding
• Uncontrolled material flow at the end of the process
• Weighed-in quantity fluctuations from one mixture to the other
• Inconsistent feeding with different kinds of raw materials

Additionally, in the case of manual feeding, reaching the desired values depends on the operator and is not reproducible. These older processes should be updated to automatic, formula-controlled feeding. The screw feeders can be modified so that they can operate with a frequency inverter, and flap gates should be installed at the discharge end of the feeding equipment. Such modifications will ensure that the raw material is fed accurately, and that the afterflow of material is effectively stopped. They also allow manufacturers to maintain compositions with high reproducibility and adjust the material feed to the individual flow properties of different materials.

Scales

Mechanical scales are still used in many older refractory plants. These outdated scales often suffer from low weighing accuracy, are prone to dust layers, and are subject to changes in weather conditions. They also typically involve high maintenance costs due to wear, and the supply of spare parts can be scarce. Additionally, when using mobile scales, the operator has to move along with the scale, and the operation of the discharge gate is done by hand. The gate can be closed too early or too late, and these mistakes are difficult to track.

Replacing these machines with electro-mechanical weighing systems (with fixed or mobile scales) can provide a number of advantages. The weighing accuracy of these systems is considerably higher than mechanical scales, and their hermetically encapsulated, measured-value sensors are not impaired by dust or weather conditions. Today's mobile scales are also fully automatic, including an optimized travelling distance, which helps reduce errors. The newer scales are also almost maintenance-free.

Mixing Technologies

The refractory industry has accepted the intensive mixer as the standard preferred mixer for stone press bodies. These types of mixers enable users to simply and economically prepare various refractory products in all conceivable consistencies. Intensive mixers are characterized by a quickly rotating rotor; the drive rating of the rotor is variable to a great degree and is designed according to the specific energy consumption of the mixture.

Today, many producers are still working with older intensive mixers that are equipped with a horizontal rotating mixing pan and rotating mixing tools (i.e., a slowly rotating blade tool and a faster rotating rotor). These mixers, some of which have been in operation for decades, still often produce sufficient mixing quality and are being built for other sectors, such as the glass industry. In many cases, however, the mixers cannot cope with the refractory industry's current high technical demands.

When these mixers were originally developed and introduced to the market around 1964, the composition of refractory products was still relatively simple. Light powders like microsilica had not been invented. Today, however, light powders have to be added in many cases, and very small amounts of additives are often included as well. In these instances, mixers with horizontal mixing pans can become overstressed.

Mixers with inclined mixing pans were invented in 1979 and have been successfully introduced in a variety of sectors. This mixing system makes it possible to prepare mixes of any consistency in a short amount of time (see Figure 1). Compared with intensive mixers with horizontal rotating mixing pans, additional advantages include:

• Only one movable mixing tool (for volumes below 5000 l [1321 gal]), which results in less caking, easier cleaning and fewer tool adjustments.
• Relatively narrow, adjustable grain size range.
• Fast and complete discharge, up to 98% within 30 seconds.
• Optimally adjusted construction for individual mixing tasks.
• Vacuum technology available.

These mixers, which are available in a variety of sizes up to 7 m3 (247 ft3), are regularly used in new refractory plants. In the last five years, more than 95% of the mixers sold in the refractory sector were intensive mixers with inclined mixing pans. In many cases, they can even replace muller mixers, with a few adjustments.

About 20 years ago, when refractory concretes still contained 15-20% cement, many production sites acquired simple units like tube mixers for processing dry mixes. These mixers are generally not capable of mixing today's amounts of additives, of up to 50 g/t, homogeneously. Furthermore, according to many publications, segregation takes place during the mixing process; portions of coarse grain are thrown farther to the outside than the fine-grained portions. Mixers with inclined mixing pans, however, are capable of mixing quantities in the parts per million range homogeneously, and they can prepare mixes without segregation. Due to the rotating mixing pan, all of the material is circulated at the wall scraper approximately 15 times per minute, independently from the quickly rotating rotor.

Prefabricated parts are mostly made of refractory concrete that has to be prepared correctly. The screed mixers still used for this application in many companies are increasingly being exchanged for mixers with inclined mixing pans. Due to these mixers' intensive disintegration and treatment of the fine material, 15% less water is necessary, and the porosity of the material is lower.

In the precast sector, small mixers are increasingly being used. Among them are constructions where the tools can be swivelled to the top. Thus, the whole mixing chamber, as well as the tools, can easily be cleaned with a water hose. As an alternative, the whole interior can be cleaned with water by using a high-pressure atomizing unit. The water needed for the next batch is then partly already there.

Apart from the preparation of refractory concrete, the effective discharge out of the mixer and the transport on site is also important (see Figure 2). Conveying systems are available that can carry the whole mixer batch. The material is then unloaded according to a time schedule into a hopper that is situated above a roller conveyor with the molds, and adjustments to the customer's specifications can easily be carried out. In addition, self-flowing refractory concretes can easily be prepared and made into prefabricated elements with this technology.

For oxide ceramic products, granulated material is often produced through spray drying. However, outdated equipment in older plants can produce uneven grain size distributions, which leads to problems with volumetric mold filling and unacceptable rejects. A tighter granule-and therefore less size variation-can be achieved by producing granules in an inclined mixer through build-up granulation, with a subsequent redrying from granulating moisture to press moisture. In addition, one mixer can be serviced while the other maintains production. While batch processing times vary according to material, 10 minutes is sufficient in many cases.

Special materials, such as alumina powders, sometimes require resin-bonded mixes that are prepared with organic solvents. To ensure safety and reduce the risk of explosion, it is necessary to work in a closed system and with inert gas. For this type of application, the inclined mixing pan is built into a vacuum-tight casing. By recirculating nitrogen or solvent vapors, the mixtures can be redried at a slightly lower pressure (e.g., from 6% granulating moisture to 2% pressing moisture), which further improves operating safety.

Process Control

All of the different sectors in the plant have to be able to communicate with each other to ensure reliable and reproducible processes. Achieving this objective requires a control system that provides the following functions:

• Instrumentation on a plant-wide scale (sensors and actuators)
• Power section (motor control center, etc.)
• Control section (stored program control)
• Feeding control
• Operating and display unit (computer-aided visualization system)
• Data acquisition and evaluation

State-of-the-art control systems are mostly structured hierarchically. The different levels are connected with data lines to allow a continuous data transfer. The production process is always transparent, and operating data are available in "real time." Since the preparation processes are, to a great extent, carried out automatically, reproducibility is high and many operating errors can be avoided. Orders can be entered and processed automatically, and all plant processes are registered, which means that even manual operations can be monitored and reproduced.

The production process can be carried out within the plant's specified tolerances. In case of a fault alarm, the diagnosis is described in detail. Since no time is wasted in trying to find the error, standstills are kept short and the plant's productivity is increased. Additionally, any weak points can be detected early and dealt with during scheduled maintenance times.

Production data can be stored for one year, or longer if required, so that a company needing to identify the raw materials used for a certain mix many months after a product went through the plant could easily retrieve this information. And because of the system's central data processing, all data are available to authorized individuals.

In today's highly competitive refractories industry, manufacturers must look to the latest processing innovations to remain a step ahead of the competition. Improvements in feeding equipment, scales, mixing technology and process controls can help companies produce higher-quality products more efficiently, positioning them for success today and in the future.

Peter Nold, Ph.D., is technology manager, Ralf Lobe is manager test center, and Klaus Jean Frey is project engineer for Maschinenfabrik Gustav Eirich, Hardheim, Germany.

For more information about advances in material preparation technologies, contact:

• Eirich Machines Inc., 4033 Ryan Rd., Gurnee, IL 60031; (847) 406-1381; fax (847) 336-0914.
• Maschinenfabrik Gustav Eirich GmbH & Co. KG, Postfach 11 60, D-74732 Hardheim, Germany; (49) 062-83/51-0; fax (49) 062-83/51-325.

SIDEBAR: UNDERSTANDING DEFECTS

Defects cost money. For example, in a refractory plant producing 60,000 t of brick per year with a scrap rate of 5% of the total production, 3000 t of material are defective. This would amount to annual reject costs of $1.5 million, only a small portion of which could be saved by recycling the raw materials back into the process. Some possible causes for scrap include revised raw materials and/or formulas, inaccurate weighing, insufficient or uneven mixing processes, and maladjusted pressing moisture. In many older plants, manual operations such as the addition of additives are often not controlled and documented, bonding agents are proportioned inexactly, mixing periods are not observed, and formulas are not maintained properly.

Upgrading to automated, repeatable, efficient technologies can help plants avoid many of these types of errors and significantly reduce defects.

Links

• Eirich Machines Inc. e-mail
• Maschinenfabrik Gustav Eirich GmbH & Co. KG e-mail
• Eirich Machines Inc. website
• Maschinenfabrik Gustav Eirich GmbH & Co. KG website