Chapter Six: Some Operating Guidelines - Vertical Roller Mill

Many ask me about the procedures for starting and operating the vertical roller mill, a grinding machine used in the cement industry.

I have to confess that answering that question is complicated.?

I do not mean that it is complicated to control the vertical mill, although some operators report that starting up the vertical mill is quite a challenge. It also took me some time to learn how to operate this type of mill well. I mean the difficulty due to the number of models and projects of vertical mills available in the industrial market. In addition, the vertical mill has the option of operating in initial grinding (raw materials) or final grinding (cement), which involves detailing some specific procedures.

An interesting historical fact is that a vertical roller mill uses the same operating principle as the pistrium or pistrinum, an antique Roman grain mill. The largest grain mills used worked a millstone rolling on a circular table or stone, powered by wind, water, human or animal power. The grains (cereals) are dumped on the table, being crushed (ground) with the weight and constant rotation of the millstones.

We leave aside the history to deal with the present...!!

Let's clarify a few things...

Before mentioning some operational guidelines for the vertical mill, I consider it appropriate to clarify the following:

?The operational recommendations mentioned are generic. It is the responsibility of the mill operator (CRO) to comply with the procedures (step-by-step) determined by the manufacturer and the cement company.

? In this article, we will not mention brands, trade names, models, or technical names of the various projects of vertical roller mills.

? In this article, we will not deal with the commissioning mill or the first operation.

? To shorten the mention of the mill, we will refer to the vertical roller mill as VRM.

Let's go?

Generally, in automated plants, the different equipment's in a plant is interlinked or grouped to execute a start and shutdown sequence. Therefore, before starting VRM operation, certain startup conditions have to be met.

What conditions are these?

1. The auxiliary groups, hydraulic system, transport system (does not include feeding transport), dust removal system, draft (minimum rotation), and separator, working.
2. If the mill has a pneumatic seal system, this system must also be working.
3. Hot gas dampers open. Preheated mill according to the operating parameters established by the manufacturer. If the mill has a hot gas generator, this must also be operational and remain operational to maintain the necessary temperature.

What is the ideal internal temperature of the VRM in operation?

Answer: Depends!!?

We refer to the thermal exchange between the fresh material and the gases. There are materials with high humidity. Understandably, wet materials need more temperature and volume of gases. However, specialists recommend that the temperature after the VRM (outlet) be between 90oC and 105oC.

What about water injection?

Feed material from the mill that is dry and fine; The water injection contributes to helping form the material bed on the grinding table. The spray of the water spear is adjusted depending on the operational behavior of the mill. However, the water injection generally points towards the front of the rollers.

So, what is the starting sequence for the VRM?

Starting Sequence:

1. Start finished product transport system;
2. Start the hydraulic system of the table in operation;
3. Starting of auxiliary groups (separator, rotary feeder, recirculation bucket elevator, vibratory conveyor, roller lubrication system, etc.);
4. Start filter cleaner (bag filter);
5. Gas line system in working (fan, regulation dampers, hot gas generator, etc.);
6. Main motor starting;
7. Main drives of the grinding rollers in operation (rollers up);
8. Water injection and dosage of grinding aid (if applicable) ready to work;
9. Raw material conveyors and feed hoppers are stocked with sufficient raw materials (fresh material) and ready to work.

Observations

• The increase or decrease of motor power is influenced by material grindability. The feed material dosage must be set so that the motor power is constant. This setting is mainly important when the material hardness varies a lot.
• In the VRM, you will see that the differential pressure (ΔP)?is perhaps the most important operational parameter after the motor power.

Parenthesis - Delta P

Differential pressure is controlled with a constant feed flow.

The constant flow of raw materials fed to the VRM, is the operational stability basis. You will see that in the following paragraphs...!!

Continue...

• The outlet gas temperature is controlled by the setting closed-open of the gas damper and the temperature exchange with the fresh materials fed. As mentioned above, it is expected that the outlet temperature will not exceed 105oC.
• It is worth mentioning that the outlet temperature control (not exceeding 105oC) protects the bearings, the seals, and the mesh of the bag filter, among others, from damage due to overheating, as well as the grinding conditions inside the mill and the temperature of the product (gypsum dehydration) during normal operating conditions.
• The gas constant flow circulating inside the mill has an immediate effect on the draft, material recirculation, and particle size distribution of the final product. Therefore, gas flow is a parameter that deserves constant monitoring.
• Product granulometry is determined by the separator speed (RPM). High-speed, fineness product. Low-speed, thickness product.
• Before starting the mill feed, check the mill motor power, mill inlet/outlet gas temperature, gas flow, and mill spring system hydraulic pressure according to the operation handbook.
• During the first operation (start-up), the material quantity for the bed to generate friction between the rollers and the table is set. However, the differential pressure is a relevant parameter to knowing how much fresh material is inside the mill and the exact time to increase the hydraulic friction pressure between rollers and the table.

Of course, there are others information about the operation of the VRM. We do not want to deal with all this information in a single publication. We'll cover other guidelines and formulas for VRM in a future article.

You will find additional information (formulas) on the operation of the VRM grinding plant by clicking here.

I wait for you in the next article The Cement Newsletter...!!

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This article was sponsored by ICC - Independent Cement Consultants.?

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• Juan Ortega - [email protected]
• Jo?o Butkus - [email protected]

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