Dispersing device

Details Dispersing device Dispersing device

2001-05-02

2003-05-20

Rosenbaum, Mark (Department: 3725)

Solid material comminution or disintegration

Apparatus

Loose grinding body comminutor

C241S172000

Reexamination Certificate

active

06565024

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a dispersing device, particularly a ball or bead mill for use as a submersible mill, comprising a container that receives and processes a product to be dispersed, a grinding device with a housing that contains grinding bodies, where said housing has openings enabling the product to be dispersed to pass through, an agitating tool arranged in the housing and a first flow-producing device, where the housing and the agitating tool can move relative to one another and at least one shaft protrudes into the housing via a through-opening that allows the product to be dispersed to enter the housing.
A device of this kind disperses fine to very fine, solid particulate constituents in the liquid phase.
Three sub-steps occur simultaneously during the dispersion process:
1. Wetting of the surface of the solid material to be incorporated by the liquid constituents of the product to be dispersed,
2. Mechanical separation of agglomerates into smaller agglomerates and primary particles, and
3. Stabilization of primary particles, agglomerates and aggregates to prevent renewed clumping (=flocculation).
Although the following description primarily relates to the dispersion of paints and coatings, this processing technique can also be applied in a similar manner in other fields (e.g. biology, food processing technology, pharmacy, agrochemistry, ceramics industry and the like).
A grinding device of this kind is known from U.S. Pat. No. 5,194,783. This patent discloses an agitating submersible mill that disperses according to the circulation process. It essentially consists of a wear-resistant basket filled with grinding media designed as grinding balls, which is submerged in a double-walled container. A cylindrical drive shaft runs through the center of the basket. This drive shaft drives the bar-type agitator mounted inside the basket. The walls of the basket exhibit sieve-like perforations.
When dispersing paints, for example, it is of economic interest to minimize the use of relatively expensive primary colorant particles. The better the dispersion is, the more intense the color effect and gloss are. Thus, good dispersion can, for example, reduce the use of expensive primary colorant particles by permitting the use of cheaper secondary particles. In the ideal situation, each primary particle is wetted separately.
In order to enable circulation of the product to be dispersed through the basket, the drive shaft drives a flow-producing device in addition to the agitator. This flow-producing device must be positioned outside the basket in order to ensure adequate flow. Thus, the drive shaft penetrates the basket. A separating and sealing system is fitted at the point of penetration to prevent the grinding balls from escaping from the basket. The central position of the flow-producing device has definite advantages in terms of fluid mechanics, because it ensures uniform circulation throughout the container.
However, in order to carry out an economical dispersion process using the dispersing device known from the prior art, the product to be dispersed must be pre-dispersed. Pre-dispersion is preferably performed using a dissolver disk due to the fact that optimum pre-dispersion is indispensable from an economic standpoint, particularly in the case of agglomerates that are difficult to disperse and require the use of the grinding device during subsequent processing. An inadequately pre-dispersed product not only necessitates longer running times of the grinding devices known from the prior art, but it also frequently happens that the desired fineness is not attained. As a rule, omissions or errors in pre-dispersion cannot be compensated for by other systems, particularly because inadequately pre-dispersed products clog the holes in the basket during subsequent use of the grinding device, this hindering, or even completely stopping, circulation through the basket.
Although very satisfactory grinding results are achieved with the devices known from the prior art, they are—like virtually all agitating ball mills—subject to the disadvantage that the point where the shaft penetrates the grinding basket is provided with a dynamic friction gap or other similar means, through which the grinding balls can escape from the housing or the grinding basket into the container. Furthermore, dynamic friction gaps require relatively narrow tolerances in order to function properly, meaning that their manufacture is complex and expensive. However, even with the highest precision manufacturing and faultless operation, the problem still occurs that the grinding balls are unintentionally crushed in the friction gap, thereby destroying the friction gap and contaminating the product to be dispersed.
Another problem occurs at the annular through-opening between the shaft and the housing. During operation, these components move relative to one another. The product to be dispersed flows through this annular gap into the housing, as is required for the grinding process.
On the other hand, this through-opening is associated with the considerable disadvantage that grinding balls uncontrollably and unintentionally escape from the housing during the grinding process. This is disadvantageous in two respects. On the one hand, the product to be dispersed must be filtered again after the grinding process and prior to further processing in order to filter out the beads, thus necessitating an additional, time-consuming processing step. On the other hand, the loss of beads must be compensated for at regular intervals, as the grinding performance would otherwise decline.
A basket mill is known from EP 0 546 715, whose upper housing cover has a cylindrical collar on top, in which an impeller provided on the shaft runs. This device also does not prevent the grinding media from unintentionally leaving the housing.
BRIEF SUMMARY OF THE INVENTION
Consequently, the technical object of the invention is to further develop a dispersing device of the kind specified at the outset, such that the grinding balls are reliably prevented from escaping from the housing or the grinding basket.
According to the invention, the object is solved in that a second flow-producing device, designed as an impeller, is provided in the region of the through-opening between the shaft and the housing, and in that the housing in the region of the through-opening is designed as a pump housing in the shape of a half-shell to accommodate the impeller, in order to prevent the escape of the grinding balls.
As a result of the design according to the invention, the relative movement generates a flow into the inside of the housing of the grinding device. This flow is so strong that it reliably prevents the grinding balls from escaping through the through-opening and into the product to be dispersed inside the container. In addition, the second flow-producing device results in more thorough mixing and draws the product to be dispersed into the housing of the grinding device more rapidly, thus increasing the throughput.
Finally, the flow-producing device deflects the grinding balls that escape from the grinding basket through the through-opening. Should this deflection be inadequate for preventing the grinding balls from getting into the through-opening in any manner whatsoever, the flow generated by the flow-producing device is sufficient to suck any balls that still escape back into the grinding basket or grinding device. Moreover, the design of the grinding device according to the invention is associated with the special advantage that the flow generated by the flow-producing device holds even considerably lighter and cheaper grinding balls (e.g. designed as glass beads) inside the grinding device, so that heavier and more expensive grinding balls, such as those made of zirconium oxide, which are usually required for numerous grinding processes due to their greater density and resultant weight, can be dispensed with. The use of cheaper grinding balls made of glass substantially reduces grinding costs.
The flow-producing de

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