EP0399453A2

EP0399453A2 - Improvements to vertical microsphere mills, particularly for dispersing pigments in a fluid vehicle

Info

Publication number: EP0399453A2
Application number: EP90109681A
Authority: EP
Inventors: Antonio Pelizza
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-05-24
Filing date: 1990-05-22
Publication date: 1990-11-28
Also published as: EP0399453A3; IT1232100B; IT8967390A0

Abstract

The mill comprises a milling chamber (12) with circular cross section, a rotor (20) which is rotatable in the chamber with a plurality of disk-like wheels (22) which are superimposed and separated by spacers (23), and in order to allow the processing of products with high viscosity and high solid content the diameter of the wheels (22) increases from below upward, the milling chamber (12) delimits, together with the wheels of the rotor, a milling compartment with annular cross sections which decrease from the bottom upward, and the wheels are separated by spacers (23) the axial extensions whereof also decrease from below upward.

Description

The present invention relates to improvements to vertical microsphere mills of the type comprising a milling chamber and a rotor which is rotatably mounted in said chamber so as to disperse a pigment in a fluid vehicle, by means of a milling means constituted by said microspheres, contained in said chamber.
The prior patent application no. 67313-A/87, in the name of the same Applicant, discloses a mill of this type with a truncated cone-shaped milling chamber with circular cross section, a rotor which comprises a plurality of disk-like wheels which have differentiated diameters and thicknesses which decrease along the axis of the milling chamber and generate a truncated cone-shaped rotor profile which is orientated concordantly with the truncated cone profile of the milling chamber, wherein the wheels are mutually separated by spacers which have an active profile, co-operate with the wheels as regards milling and prevent the accumulation of materials.
The taper of the milling chamber and the concordant taper of the rotor give a high efficiency to the mill which is essentially based on the pattern of the centrifugal acceleration gradient, which is maximum at the base of the chamber and decreases upward so as to ensure the correct distribution of the material being processed in said chamber, and constant shearing forces which are a function of viscosity and of the peripheral speed gradient.
Nowadays, as known, due to ecological and environmental safety reasons, the trend is to formulate, produce and use paints with a low solvent content, and this is achieved by using resins with a high percentage of solid content (SC) and small amounts of solvent.
A consequence of this trend is that current mixtures of pigment plus fluid vehicle processed in vertical mills as specified have a high true viscosity which is due to the "tackiness" of the resin and a high solid content.
A typical formulation of said paints is, for example, the following:
Pigment(s) powders with a granulometry comprised between 5 and 25 microns
Binder ( s ) solution of resin and solvent with high viscosity- 40,000-50,000 CPs at 25°, solid content equal to 50-80% and specific weight equal to 1 kg/dm³
Solvent(s) liquids with very low viscosity (1-10 CPs) and zero solid content
Additive(s) liquids in a percentage which can vary between 0.5 and 1.5%.
Under equal conditions (machine, power absorbed in processing, temperature, degree of grinding), products with high true viscosity require significantly lower feed flow-rates with respect to products with low viscosities, since the greater effect of entrainment of the milling means (microspheres) toward the discharge system produces the clogging of said means for an equal flow-rate.
This is a severe disadvantage which occurs in known mills of the specified type, when high-viscosity paints formulated as in the preceding example are used.
The aim of the present invention is to eliminate this severe disadvantage by improving known vertical mills of the specified type so as to allow them to operate correctly, in particular without clogging, with high-viscosity paints formulated as specified above.
According to the present invention, this important aim, and other objects which will become apparent from the following detailed description, are achieved by virtue of the fact that the rotor of the mill is provided with a plurality of wheels the diameter whereof increases from below upward. that the milling chamber delimits, together with the wheels of the rotor, a milling compartment with annular cross sections which decrease from below upward and that the wheels are separated by spacers which have axial extensions which also decrease from below upward.
According to an embodiment of the invention, the milling chamber is a truncated cone, with a taper which is the reverse of that of the rotor, and delimits a milling compartment which has cross sections which rapidly decrease upward and is suitable for products with particularly high viscosity.
According to a variated embodiment, suitable for products with lower viscosity, the milling chamber is cylindrical.
The decreasing axial extension of the spacers of the wheels is a function of the diameters of the milling chamber and of the rotor in the cross section being considered and corresponds to the difference between said diameters to less than a constant the value whereof is comprised between 0.95 and 1.25.
Further purposes, characteristics and advantages will become apparent from the following detailed description and with reference to the accompanying drawings which are given by way of non-limitative example and wherein:

figure 1 is a schematic axial sectional view of the improved mill according to an embodiment of the invention;
figure 2 is a sectional view, similar to figure 1, of a variated embodiment of the invention.

With reference to figure 1, the reference numeral 10 indicates the shell of the milling chamber in which an interspace 11 is defined for a cooling fluid which circulates from below upward in the direction of the arrows f-f.
The shell 10 delimits the milling chamber 12 which, in the illustrated example, is a truncated cone and tapers upward with a taper angle comprised between 5° and 9° and is advantageously chosen equal to 6°.
The shell 10 is closed below by a bottom 13 at which said shell has a port 14 which is used to continuously feed the mixture of pigment and liquid vehicle into the milling chamber 12.
A further inlet port which is not illustrated and is provided in a known manner approximately in the median portion 11 of the shell, allows to load the milling means, for example constituted by silica microspheres, into the chamber 12.
A rotor 20 is rotatably mounted in the chamber 12, and its axis of rotation x-x coincides with the axis of the shell 10. In a per se known manner, the lower end of the rotor 20 is free and arranged at a short distance from the bottom 13, and the upper end of said rotor is axially and torsionally rigidly coupled to a motor shaft 21 which is rotatably supported by a guide dome (not illustrated) by means of conventional bearings as described and illustrated in detail in the above mentioned prior patent application no. 67313-A/87.
The rotor 20 has a plurality of wheels 22 which are separated by spacers 23; said wheels and said spacers are packed together, in a per se known manner, by axial tension elements which are not illustrated. The upper and terminal portion 20a of the rotor has no wheels and is a truncated cone with a taper which is concordant with the taper defined by the assembly of the wheels so as to delimit an expansion volume 12a which is connected to a discharge duct through an annular port 24 delimited between the jacket 10 and a bladed wheel 25 which is rigidly associated with the rotor.
According to the present invention, the diameter of the wheels 22 is variable and increases in a linear manner or in steps by groups of constant-diameter wheels from the bottom to the upper portion of the milling chamber, so that the rotor is also substantially a truncated cone but its taper is the reverse of that of the shell 10.
The milling chamber 12 consequently delimits, together with the wheels 22 of the motor, a milling compartment with annular cross sections which freely decrease from the bottom upward.
The axial extension δ of the spacers 23 similarly decreases from below upward according to a linear variation rule which is expressed by the following equation:
δ = K (Dc-dg)
where K ls a constant the value whereof is comprised between 0.95 and 1.25, Dc is the internal diameter of the milling chamber and dg is the diameter of the wheel in the cross section being considered.
The described arrangement achieves the stated aim and objects since the viscosity of the vehicle plus pigment mixture is very high in the lower portion of the milling chamber, as the temperatures of the loaded product and of the cooling fluid are low.
The shearing stresses on the product being processed are consequently high, since:
τ=η $\frac{dV}{dX}$

where η is the viscosity and dV/dX is the gradient of the peripheral speed of the wheels which, for an equal efficiency of the mill, is reduced in inverse proportion to the increase in the viscosity η, with a consequent reduction in the diameters of said wheels and increase in their mutual distance.
As the product rises in the milling chamber, the temperatures of said product and of the cooling fluid increase, with a consequent decrease in viscosity which is compensated, in terms of the constancy of the shearing stresses, by a corresponding increase in the gradient dV/dX and therefore by a corresponding increase in the diameters of the wheels and a corresponding decrease in their mutual distance.
Numerous practical tests have shown that the described arrangement prevents the product from clogging even with high flow-rates, since the temperature at the discharge is kept high, with consequent significant fluidity of the product itself in this step of the processing cycle.
The variated embodiment of figure 2, which is suitable for the processing of products with a lower viscosity, for example 30,000 CPs at 25°C, differs from the previously described embodiment due to the fact that the shell 10 is cylindrical, so that the cross sections of the milling compartment decrease more moderately from the bottom upward and the distance between the wheels 22 also decreases more moderately in the same axial direction, as well as due to the fact that the variation in the diameter of the wheels is stepwise in three groups A-B-C of wheels with a constant diameter which increases from one group to the next.
The details of execution and the embodiments may naturally be extensively changed, with respect to what is described and illustrated by way of non-limitative example, without thereby abandoning the scope of the invention and without altering the concept thereof.
In particular, the reverse arrangement with respect to that of the variated embodiment of figure 2, wherein the rotor is cylindrical and the milling chamber is truncated cone-shaped, as described with reference to figure 1, is within the scope of the invention.
Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims

1. Vertical microsphere mill, particularly for dispersing pigments in a fluid vehicle with high viscosity and high solid content as specified, of the type which comprises a milling chamber with circular cross section and a rotor which is rotatable in said chamber with a plurality of disk-like wheels which are superimposed and spaced by spacers, characterized in that the diameter of the wheels increases from the bottom upward, that the milling chamber delimits, with the wheels of the rotor, a milling compartment with annular cross sections which decrease from the bottom upward and in that the wheels are separated by spacers the axial extensions whereof also decrease from the bottom upward.

2. Vertical mill according to claim 1, characterized in that said milling chamber is a truncated cone with a taper which is the reverse of the taper defined by the wheels of the rotor.

3. Vertical mill according to claim 1, characterized in that said milling chamber is cylindrical and the rotor is a truncated cone or vice versa.

4. Vertical mill according to claims 1 and 2 or 3, characterized in that said milling chamber is delimited by a shell in which an interspace is defined for a cooling fluid which circulates from below upward.

5. Vertical mill according to claim 1, characterized in that the decreasing axial extension of the spacers of the wheels is a function of the diameters of the milling chamber and of the rotor in the cross section being considered and corresponds, to less than a constant the value whereof is comprised between 0.95 and 1.25, to the difference between said diameters.

6. Vertical mill according to claim 1, wherein the diameter of the wheels is variable in a linear manner.

7. Vertical mill according to claim 1, wherein the diameter of the wheels is variable in a stepwise manner by groups of constant-diameter wheels.

8. Vertical mill according to claim 1 and any one of claims 2 to 7, characterized in that the upper terminal portion of the rotor has no wheels and is a truncated cone with a taper which is concordant with the taper defined by the assembly of wheels; said upper portion delimiting, in the milling chamber, an expansion volume which is connected to a discharge duct through an annular port delimited between the chamber shell and a bladed wheel which is rigidly associated with the rotor.