Rolls are one of the key elements in nearly every mill throughout the world. Many facilities are taking advantage of modern material developments to improve mill performance and reduce costs. This article traces the development of the roll chill and examines some of the innovative materials now being produced.
The processing of wheat using cast iron rolls in place of millstones was first reported in central Europe in 1823.
Porcelain rolls were seen for a short time, though reports indicate use in some commercial mills up to about 1930. At the time, porcelain rolls were considered to be more favorable than cast iron rolls.
However, they tended to wear quickly and required frequent grinding to keep truly cylindrical. Porcelain rolls were also found to produce more bran powder, so a gradual return to cast iron rolls occurred.
In the 1870s, rolls were strikingly short by modern standards. Most rolls were only about 9 inches long. Eventually, Ganz and Company in Hungary developed rolls ranging in length from 12.5 to 15.5 inches. These longer rolls were soon introduced to the Unites States by Nordyke and Marmon and by Henry Simon to Great Britain.
Steel rolls at that time would soon wear out and crack. Development work therefore concentrated on improving the cast iron roll.
In 1874, three engineers, Robins, Wood and Allbut, devised a method of improving the surface chill of cast iron, and these methods soon came into general and continued use for break and reduction work.
The now traditional static casting foundry techniques were developed to devise a method of rapidly cooling the outside of the roll to create a hard skin of white iron (iron carbide and Pearlite) with a softer core of grey iron (flake graphite).
CENTRIFUGAL CASTING. In 1953, Henry Simon licensed the production of centrifugally cast dual metal milling rolls from the U.S. inventors and patent holders. The rolls were known as "Evenchill."
During casting, the mold is rotated at a high speed, great enough that the molten metal becomes evenly distributed around the cast. The molten metal is thrown onto the cooled mold, casting the metal from the outer surface inwards.
Consequently, a finer crystalline structure with greater strength is formed without the slight flaws sometimes found in conventional casting. Dimensional precision, dynamic balancing and machining properties are also improved.
In situations where a roll is required to be suitable for both break and reduction applications, the outer surface of the roll needs to be heterogeneous — composed of different elements — to meet the demands of both fluted/corrugated and smooth roll applications.
For fluted/corrugated rolls, the outer surface must be resistant to abrasive wear in order to retain the flute/corrugation profile. The surface must be hard, yet not brittle, to avoid chipping. In addition, the chill roll must remain "machinable" with conventional fluting tools.
Smooth rolls require the outer surface to be capable of being "frosted" and be resistant to abrasive wear. These rolls should maintain a consistent matte, non-glazed finish to grip the product.
ALTERNATIVES. Flour milling is a traditional industry and is sometimes seen as being reluctant to change. However, developments in metallurgy and a demand for improved roll performance led to further progress being made with roll materials.
Several unsuccessful attempts were made from 1960 to 1980 to improve flute/corrugation life by using various grades of steel and cast alloy iron rolls.
In the 1980s, a breakthrough occurred when MIS Engineering — a leading foundry in South Africa that already produced a range of high-alloy wear parts for the brick, quarry and mining industries — produced some trial high-alloy rolls in conjunction with Premier Milling Company in South Africa. The rolls could be machined using conventional equipment, yet claimed a corrugation/flute life two to three times greater than the chilled cast iron roll on first break applications.
Seeking a European milling engineering company to conduct further trials prior to introducing the high-alloy roll to the flour milling industry, MIS in 1990 approached Robinson Milling Systems Ltd. — a company formed from the merger of Thomas Robinson and Henry Simon, now part of the Satake Corporation of Japan.
Long-term trials in both hard and soft grist mills in the United Kingdom verified that the service life of rolls could be extended two to three times. On a price per tonne basis, the extended roll life significantly reduces cost. Refluting is required less often, reducing shut downs, roll changing labor and machining costs.
Satake now supplies these high-alloy rolls, called "Mitak."
Roll materials continue to be improved. Satake has developed what it calls the "Super Roll," a longer-life roll that is being used in the Asian market. The increased life is a result of the alloy’s crystalline structure, giving greater toughness to the roll corrugations.
Other European roll manufacturers have introduced similar high-alloy iron products. Spanish roll manufacturer Fundiciones Balaguer produces the "K12 Long Life" roll, suitable for flour milling and as a cracking roll for oilseeds and feeds.
German roll manufacturer Breitenbach produces the "Marathon" roll for flour milling, oilseed milling and the feed industry.
U.K. roll manufacturer Turner Chilled Rolls Ltd. produces the "Ni-Chrome" roll for cereals, rice and oilseed extraction.
Developments in roll materials have not been limited to the break process. In the 1980s, research work was conducted to extend the life of the frosted/matte surface finish of reduction rolls. The surface finish is known to contribute to the grinding performance, particularly on head end reduction passages.
Breitenbach introduced the self-surfacing, indefinite chill cast iron roll. Breitenbach claims that for each individual passage, optimum surface roughness is self-generated and maintained, allowing for an optimum grind to be achieved.
The performance improvement this roll offers can result in either lower energy consumption for the same feed rate or a higher feed rate at the same energy consumption. Tests have also shown higher flour releases, lower roll surface temperatures and reduced evaporation losses.
Fundiciones Balaguer is producing similar "self-matting" rolls for reduction applications. In conjunction with Satake, these rolls are being successfully tested in several U.K. mills. Early indications show improved water absorption and particle size reduction.
CHOOSING THE RIGHT ROLL. Because the milling roll can significantly affect mill performance and economic efficiency, millers should carefully consider all available material types.
Roll suppliers can provide technical data about the different roll types to allow an informed decision.
For example, although their initial purchase cost may be higher, long-life break rolls can actually save a significant amount of money in the long term. Recent analysis in the U.K. has shown that for each half million tonnes of wheat milled, the cost per tonne using high-alloy rolls is less than half of that for standard chill cast iron.
Satake Corporation, U.K. Division, in association with MIS Engineering, South Africa, and Fundiciones Balaguer, Spain, contributed to this article written by Satake’s Roger N. Cook.