Method of Observing a Change of Mass Inside a Grinding Unit TECHNICAL FIELD

The invention relates to the field of grinding processes, and in particular to a method of observing a change of mass inside a grinding unit like a ball mill or semi- autogenous grinding (SAG) mill.

BACKGROUND OF THE INVENTION

Grinding processes are commonly used in industrial processes, particularly in the cement and minerals produc ^¬ tion and in food processing. The purpose is to grind raw material to an adequate grain size. Thus the surface is increased so that subsequent chemical reactions or mechan ^¬ ical separation processes take place more efficiently.

A large amount of electrical energy is consumed in the grinding processes. For example electrical energy cost is the second largest cost position in the cement production process. For that reason grinding optimization systems are widely used in industry. These optimization systems rely on the amount of mass inside the grinding unit which is related to a filling level or filling degree. This quanti ^¬ ty changes continuously for instance due to the variable hardness of the material to be grinded. Measuring inside a grinding unit is impossible during operation because of the grinding work and a resulting dust. An additional problem is that the output from the grinding unit can not be measured directly because the mass transport is done by convection either with water or with air.

A measurement principle outside of the grinding unit, which is also known as a "electric ear" and which is wide ^¬ ly used especially for ball mills, detects noise or vibra- tion of a mill shell with a microphone. The idea is that when the mill is becoming empty, the noise or the vibra ^¬ tion level is higher than when the mill is filling up. The results however are unreliable and often useless because of a poor accuracy of the measurements. Performing a spec ^¬ tral analysis of the noise and identifying a relative share of selected frequencies slightly improves the accu ^¬ racy. But an air gap between the mill shell and the micro ^¬ phone which makes the method very prone to interferences from neighboring sources of noise and the thick walls of the shell which unpredictably influence different frequen ^¬ cies still lead to an insufficient accuracy.

DE 19933995 discloses a measurement system for observing a mass inside a ball mill using microphones directly at- tached to the wall of the mill shell. The system analyzes intensity and the spectra of the noise and combines it with measurements of the phase angle of the mill to gain additional information about the movement of balls inside the mill. But the proposed solution still suffers from the unpredictable influence and the insufficient accuracy.

SUMMARY OF THE INVENTION

It is an objective of the invention to provide a more re ^¬ liable and accurate method for observing a change of mass inside a grinding unit as part of a grinding process with a storing unit.

This objective is achieved by a method for observing a change of mass inside a grinding unit according to claim 1. Preferred embodiments are evident from the dependent patent claims.

According to the invention the change of mass inside the grinding unit is observed in the sense of a state observer in contrast to a measurement of said quantity. A storing unit like a silo is connected with the grinding unit via a mill output path and is located in a flow direction of the material downstream of the grinding unit. The change of mass inside the grinding unit is derived from a previously established mass balance for the grinding unit and a pre ^¬ viously established mass balance for the storing unit. A mass flow of a process input path, a mass flow of a process output path, and a change of mass of the grinded material inside the storing unit as parts of the mass bal ^¬ ances are repeatedly determined. Determining comprises measuring and deriving wherein deriving involves modeling.

In a first preferred embodiment the grinding process is controlled using the estimated change of mass inside the grinding unit as an input for a controller. Thus an energy consumption of the grinding process can be reduced or op ^¬ timized.

In a further preferred embodiment the change of mass in- side the grinding unit is derived using state estimation techniques like a Kalman filter or a moving horizon estimation. Thus complex grinding processes with undetermined process models that, e.g. due to the existence of unknown process variables, are not amenable to a direct analytical solution like grinding process with determined process models can also be observed.

In a further preferred embodiment the grinding process comprises a closed circuit with a sizing unit which can also be referred to as a separating unit or screening unit like a sieve, a screen, a separator cyclone, or a filter. The sizing unit is located in the flow direction of the material upstream of the storing unit separating coarse and fine material using a cutoff size. A return path re ^¬ turns coarse material from the sizing unit back to the grinding unit. The change of mass inside the grinding unit is derived from the previously established mass balance of the grinding unit, the previously established mass balance of the storing unit, a previously established mass balance of the sizing unit, and a previously established mass bal- ance of the return path. Thus the method for observing the change of mass inside a grinding unit can be applied to systems with closed circuits grinding processes. In a further preferred embodiment a change of mass inside the grinding unit return path is derived. This value is used to derive the change of mass inside the grinding unit. Thus more detailed aspects like a transport delay in the return path can be modeled and lead to a higher accu ^¬ racy of the observation.

In a further preferred embodiment the change of mass of the grinded material inside the storing unit is determined by optical or acoustic sensing of the surface of the mass inside the storing unit. Thus an accurate determination of the change of mass inside the storing unit can be pro ^¬ vided .

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail in the following paragraph with reference to preferred exemplary embodiments which are illustrated in the attached drawings, which schematically show:

Figure 1 an open circuit grinding process with a mill and a silo; and

Figure 2 a closed circuit grinding process with a mill, a separator cyclone, and a silo.

The reference symbols used in the drawings, and their mean ^¬ ings, are listed in summary form in the list of designa ^¬ tions. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows an open circuit grinding process with a mill 1 and a silo 4. Material that needs to be grinded enters the mill 1 through a process input path. In the mill 1 the material is grinded. After the material has been grinded the material leaves the mill 1 through a mill output path connecting the mill 1 and the silo 4 and enters the silo 4 where the material is stored. Material can be taken from the silo 4 via a process output path. There are repeated measurements of a mass flow of the process input path mi and a mass flow of the process output path m ₅ using weight sensors. In addition a fill volume V ₄ inside the silo 4 is measured using a optical surface sensor 5. A mass balance for the mill 1,

d/dt Mi = mi - m ₂

a mass balance of the silo 4, a known average material density p in the silo 4,

d/dt M ₄ = m ₂ ' - m ₅ = d/dt V ₄ p

and an assumption that is no transport delay in the mill output path

m ₂ ~ m ₂ '

lead to a following model equation for the change of mass inside the mill 1:

d/dt Mi = mi - m ₅ - d/dt V ₄ p

This model equation is repeatedly solved using the repeat ^¬ edly measured mass flow of the process input path mi, re ^¬ peatedly measured mass flow of the process output path m ₅ , and the repeatedly measured fill volume inside the silo.

Fig. 2 shows a closed circuit grinding process. In addi ^¬ tion to Fig. 1 a separator cyclone 3 is located between the mill 1 and the silo 4. The mill is connected with the separator cyclone through the mill output path. The grinded material leaving the mill 1, is lifted by an ele ^¬ vator 2, and enters the separator cyclone 3. In the sepa ^¬ rator cyclone 3 the grinded material is divided into coarse and fine material according to a cutoff size. The fine material flows through a separator output path into the silo 4. The coarse material is dropped back into the mill 1 through a mill return path. A mass balance for the mill 1,

d/dt Mi = mi + ιτ^' - m ₂

a mass balance of the separator cyclone, an assumption that the change of mass inside the separator cyclone is negligible, d/dt M3 = m ₂ ' - η¾ - m ₄ ¾ 0

a mass balance of the silo 4, a known average material density p in the silo 4,

d/dt M ₄ = m ₂ ' - m ₅ = d/dt V ₄ p

and the assumptions that there are no transport delays in the mill output path, the mill return path and the separa ^¬ tor output path

m ₂ ~ m ₂ '

m ₃ ~ m ₃ '

m ₄ ¾ m ₄ '

lead to a following model equation for the change of mass inside the mill 1:

d/dt Mi = mi - m ₅ - d/dt V ₄ p

Instead of the configurations shown in Fig. 1 and 2 any open or closed circuit grinding configuration with one, two or more grinding stages are possible like for example a combination of a SAG mill and a ball mill which comprise any form of grinding unit, separating unit, storing unit and in the case of an closed circuit arrangement any form of return loop.

It is also possible to make other assumptions for the change of mass inside the return path and the separator cyclone 3 and/or to provide further measurements instead. If, contrary to the above assumption, a change of mass in the return path is accounted for, further measurements of additional process quantities may be needed. On the other hand, the model of the grinding process may become unde ^¬ termined due to missing measurements, and a model and es ^¬ timator approach like a Kalman filter or a moving horizon estimation as described in EP 2169483 can be used to de ^¬ termine the change of mass inside the grinding unit. This is particular advantageous if the grinding process cannot be treated as time invariant. The measurement of the mass flow of the process input path mi and the mass flow of the process output path m ₅ can be done using other measuring principle such as optical measurement. For the measurement of the fill volume V ₄ inside the silo 4 acustic principles can be used.

LIST OF REFERENCE NUMERALS

1 mill

2 elevator

3 separator cyclone

4 silo

5 surface sensor

mi mass flow of the process input path m ₂ mass flow of the mill output path m ₃ mass flow of the mill return path m ₄ mass flow of the separator output path m ₅ mass flow of the process output path

Mi mass inside the mill

M ₂ mass in the return path

M ₃ mass inside the separator cyclone

M ₄ mass inside the silo

v ₄ fill volume inside the silo