Technology in the mill: let’s discover the purifier

In this article dedicated to in-depth information about the technology used in a mill, we will tell you about one of the main components of a milling plant, the purifier: its purpose, its history, and how it works.

In one of our previous articles describing the merits of the various stages of processing, the process of milling grain with a cylinder mill, we specified that rigorous procedures must be followed to obtain excellent flour. Having been carefully cleaned the grains of wheat are subjected to a milling process that can be divided into 3 phases:

1. Each grain is broken open, and the endosperm, the “body” of the grain, is extracted: this phase is carried out by machines called roller mills;
2. The product of this “breaking open” is sieved to separate the already finished flour from other intermediate products that require further processing: this phase is carried out by machines called  plansifters;
3. By again using the roller mills, the intermediate products obtained by sieving with the plansifer are re-milled to transform them into flour.

This process is performed in a number of “steps” which are defined by the milling diagram, a flow-chart showing processes to which the various products are subjected inside the mill. The process can be carried out in its entirety to produce type “0” or “00” flours, or only part carried out, when extracting flour intended for particular uses, or wholemeal flour, for example.

The importance of the purifier in a soft wheat mill

Let’s now focus on the milling diagram. In the image that you can find at this link, we have provided a simplified diagram of the various processes to which the wheat grain is subjected, in sequence. As you will see, some intermediate products selected by the plansifters are not sent directly to the roller mill responsible for re-milling, but are processed by other machines called purifiers. Based on different mechanical principles, the purifiers carry out an even more accurate granulometric selection than the plansifters.

These machines, essential for the production of middlings in durum wheat mills, are also used in those soft wheat mills that wish to obtain excellent flour for fresh pasta. These machines are in fact used on middlings which, unlike flour, has a more lively and angular consistency. Purifiers eliminate any flecks or traces of bran from the middlings and semolina coming from the plansifter, sending as pure a product as possible to the subsequent re-milling stages. This scrupulous selection takes on a particular relevance in the production of excellent flours for the preparation of fresh pasta, which, as stated in our in-depth article, must have a very low ash content and no flecks in order to guarantee the maximum aesthetic result.

The history of the purifier

Purifiers have been used in cylinder mills since they first evolved in the second half of the nineteenth century. We already find the fundamental machinery still used by modern plants today in the diagrams of the mills of this time. It should be noted that unlike roller mills and plansifters, whose main mechanical evolutions were patented and the materials used in their manufacture recorded, purifiers underwent real conceptual evolutions. The purifiers of the time were, in fact, called “semolina cleaners” and the only thing they had in common with today’s machines was the use of air to remove flecks from the flour. Among these semolina cleaners you could find machines with the most disparate designs.

Some worked vertically whilst others were based on the use of centrifugal force, some were equipped with veils for selecting the flours, whilst others had planks on which the product slid in very thin layers to facilitate the elimination of lightweight flecks by the use of direct air flow.

Only with the arrival of the so-called “Marseillaise cleaner” was it possible to define the principles that would later be used in the development of the most modern purifiers.

In the figure below we illustrate one of the first examples of a Marseillaise cleaner that was very popular for milling both durum and soft wheat in Italian mills of the late 19th century.

The Marseillaise cleaner consisted of a long sifting table, which had a lengthwise movement to ensure a regular and delicate flow of the product onto the veiling. The machine was equipped with a fan, which blew over the product from bottom to top and air flow regulation devices which removed the lighter particles, such as small flecks of bran. The heavier particles, on the other hand, which resisted the action of the fan, crossed the veiling and could be collected in the drains placed under the sifting table itself.

This machine, while adopting principles similar to those of modern-day purifiers, still had considerable weaknesses. To function correctly, the surface of the sieves had to be exposed to the environment in order to guarantee adequate air flow for the removal of unwanted particles. It is not difficult to imagine the poor cleanliness and dusty conditions in which these machines were installed. It was precisely obvious weaknesses like this that prompted the designers to come up with the Marseillaise, enclosed by a box placed over the sifting table and a second suction fan, capable of processing and diverting the lightweight particles into special settling chambers.

We can thus trace the birth of middling cleaning machines based on the 2 principles that still characterize modern-day purifiers back to the end of the 19th century:

• Particle size selection carried out by sieving units;
• Selection based on specific weight and different aerodynamic penetration using a fluid bed system.

The operation and mechanical characteristics of the modern-day purifier

As we have just specified, the purifiers receive some of the intermediate sized particles selected by the plansifters. Unlike the latter, which receive the milled products from the roller mills and therefore have a very wide grain size spectrum, the purifiers are fed with already partially selected products. Because they work on a narrow granulometric spectrum, the purifiers are able to be very accurate in their selection, due to the sieving action of the frames, which we will describe later, and the use of a “fluid bed” system. This feature, which is based on the suction of air currents, allows purifiers to also select flours according to their specific weights and not only their granulometry, which is the case with plansifters.

Purifiers are able to distinguish the “clean” middlings, which can be classified as a finished product, from the “impure” middlings, which requires further processing of the bran particles in the roller mills.

(1) Sieving unit

(2) Frame

(3) Elastic supports

(4) Layers of sieves

(5) Suction chambers

(6) Suction

(7) Supply feed

(8) Collection channels

(9) Outlet

(10) Air regulation valves

To better understand the operation of this important mill machine, its main mechanical characteristics are described below:

• Modern-day purifiers, unlike the Marseillaise cleaner, are double-sided machines. Each side of the machine has a sieving chamber (1) which vibrates lengthwise and rests on a frame (2) held up by elastic supports (3).
• The sieving units, generally made up of three layers of superimposed sieves (4), are housed in closed self-cleaning chambers (5), and pressed down by an air suction system (6). The air flow that is generated by the suction hits the frames from bottom to top creating a “fluid bed” system.
• The purifier is fed by a self-adjusting system (7), which allows uniform distribution of the product over the entire surface of the sieve.
• Once classified, the purer heavy flours are collected in collection channels (8) and sent for further milling, or, if finished, sent to the flour silos. The lighter flours are sent for further milling in the roller mills by means of the outlets (9) placed at the end of the machine, and connected to the drop pipe.
• The air regulation valves (10), fundamental for the correct operation of the machine, are positioned on the top of the suction chambers. Thanks to these it is possible to ensure a constant air flow over the various sections of the sieves.

At this point it is easier to understand how the purifier works:

1. The unrefined flour is loaded uniformly onto the first level of the sieves (4) The holes in the mesh of the vibrating sieves increase in size from the front of the machine to the back.
2. Simultaneously, thanks to the suction system (6), a flow of air passes through the sieves from bottom to top.
3. The joint action of vibration and suction causes the flour to stratify: the bran fragments are immediately sucked up, the “impure” middlings and the lighter middlings (rejected by the sieves) are sent to the outlets at the end of the purifier (9), the purest and heaviest middlings pass to the sieves of the underlying levels, where they are further purified and, finally, sent to the collecting channels (8).

As in the case of the plansifter, this very careful selection is carried out by an apparently simple element, the sieve, which in turn is composed of 3 fundamental components:

• An aluminum frame equipped with lengthwise guides.
• A gasket which varies the size of the mesh openings according to the selection to be made.
• A horsehair brush, which cleans by sliding back and forth on the lengthwise guides of the frame, thanks to the vibrating motion to which the entire frame is subjected.

To make these concepts even clearer, we refer you to the video below, which shows what happens inside a working purifier.

In summary: the history, characteristics and strengths of the purifier

In conclusion by summarizing the information we have provided in this article, we can say that:

• The purifier is perhaps the mill machine that has most recently come of age: today’s versions are totally different from those of the late 19th century, not only because of the materials used in their manufacture but also because of the basic operating concept.
• Modern purifiers are totally enclosed machines, subjected to controlled aspiration and self-cleaning to guarantee the highest standards of food safety.
• They are capable of the most precise selection ever by taking the specific weight of each flour into account: in this way they are able to separate the top quality middlings, which can be classified as finished products, from the “impure” middlings and particles of bran.
• Thanks to their selection capability, purifiers are essential in a soft wheat mill that wishes to obtain top quality flours for fresh pasta. Soft wheat mills without purifiers, or equipped with only a small number of these machines, are forced to limit their production to flours intended only for baking.