The correct operation of an entire milling plant is determined by many factors, including some related to the design, the performance of all the machines, as well as correct and timely maintenance. One of those factors is the grinding rolls, which are used to crush wheat grain to obtain flour and semolina.
Cremona, Italy-based Ocrim S.p.A. has studied and designed the way to optimize and enhance the mill’s operation, linked to the performance of the roller mills, to optimize both time and maintenance costs. This led the company to focus on the grinding rolls, designing a solution that would incorporate as many benefits as possible for the end user. For this market request, the solution presented involved including titanium-coated rolls for the fluted passages in the grinding process, ensuring a much longer life of the rolls currently used by the milling industry.
Ocrim’s Research and Development department, following this new engineering strategy, carefully studied the results related to the titanium coating of the rolls in order to share with its customers the important opportunities that this product can offer.
In flour milling, wheat grains are crushed to obtain flour and semolina due to a combined action involving grain cutting and its compression. The diagrammatic study to obtain the best possible yield from the plant is strongly affected by the state of the fluting of the mill rolls. With new fluting it is possible to obtain the nominal yield of the plant because it is in the best conditions to achieve what is envisaged by the diagram, therefore it is crucial that the profile of the roll fluting is maintained for as long as possible
The grinding rolls are all made of chilled-iron cast alloy with different hardness, according to the requirements or the technological diagram. The duration of the rolls is thus directly dependent on its hardness. The fluting of a cast iron roll is preserved and, therefore, maintains its characteristics for longer when the roll has high hardness values (e.g. 530 HB).
In this regard, Ocrim’s Research and Development department has conducted several research studies and trials, achieving tangible results through the application of a special titanium coating for the corrugated rolls. This coating causes a considerable increase in the surface hardness values and consequently increases the duration of the fluting over time, to keep the ideal configuration of the plant as unaltered as possible.
The titanium coating of the rolls is carried out using cutting-edge technology, called Plasma Assisted Chemical Vapor Deposition (PACVD), which ensures the best possible results. This sophisticated operation consists in creating a multilayer coating, for a total thickness of about 3 microns of titanium nitride (TiN) and titanium boride (TiB). This ensures the combination of the best characteristics of the two materials: the high hardness of the TiB and the low coefficient of friction of the TiN.
Given the low thickness of the coating, one cannot talk about the hardness of the roll, but of the surface hardness of the roll (microhardness), which reaches the significant value of 2200 HV, four times higher than the hardness of the commonly used traditional rolls, which can reach up to 560 HV (in the best cases).
The titanium coating, protecting the points of greater operation/pressure, ensures greater resistance to wear, while in the other points it maintains its characteristics, providing protection against oxidation and reducing the surface friction of the product. All this leads to considerable savings in energy and maintenance costs.
To scientifically demonstrate the validity of this solution and before application on working machines, the wear was simulated by means of two cylindrical specimens, rotating at two different speeds, placed in contact under a controlled load. The test showed how the uncoated cast iron specimen started to wear out (to lose material) from the beginning, while the titanium-coated specimen suffered negligible wear during operation. Once the layer of coating material is worn, the wear continues at the same speed as the uncoated specimen.
Downstream of the laboratory test, a field test was conducted at a mill in Italy. After installing the rolls and adjusting them, it was requested not to make further adjustments until the next check. The passage under examination showed an average granulometry of the incoming product of 1,368 microns. With new rolls, the average granulometry of the outgoing product was 885 microns. After 100,000 tonnes of processed product, the granulometry shifted to 950 microns. Despite the high production, the average outgoing product granulometry value has not changed much, synonymous with an insignificant wear of the rolls, unlike what would have happened with traditional cast iron rollers.
To support this study, Ocrim has used a contactless electronic device, developed by the company’s Research and Development department, which quickly assesses the state of wear of the corrugated cylinder. With this device, the technologist can decide whether or not to renew the fluting, because the (portable) sensor rests on the cylinder and can instantly detect the percentage of wear compared to the new cylinder.
Results and conclusions
Based on the field results obtained, the rolls with titanium coating are economically more advantageous than the traditional ones, since they last longer. The longer life of the titanium-coated rolls, although with a higher starting cost, results in the following:
Savings in terms of time and maintenance costs, because, with this technology, Ocrim guarantees, in a B1 passage, an average duration of about three years with no maintenance
Fewer plant stops to change rolls, which involve loss of productivity
Lower costs related to savings for the renewal of fluting and/or the purchase of new rolls
Reduced plant yield losses, which instead inevitably occur with traditional rolls due to faster loss of their fluting profiles.
From the analyses carried out already with a production of 7,200 tonnes per year, considerable savings are obtained by using titanium-coated rollers, confirming the validity of this new titanium product.
The Ocrim titanium-coated rolls already are being used by customers in their plants with results that exceeded expectations.