Pigment Flow Characteristics

Posted in: , on 16. Dec. 2005 - 18:37

Does anyone know enough about pigment flow characteristics to advise me how best to address a process problem?

I work in a grinding/milling unit where a particular pigment is internittently refusing to flow.

The equipment line consists of a conical hopper fitted with a rotating mixer arm, which feeds a helical feedscrew which in turn feeds a hammer mill then an extraction filter (under negative pressure). From here the pigment passes into a small tank then through a densifier (2 rotating drums, one of which operates under vacuum to squeeze the air from the milled powder), and out of a volumetric filling point.

Until recently the pigment - mostly in granular form before feeding to the mixer - flowed perfectly well through the process line. Recently we noticed that the discharge rate had dropped off, and this seems to be exacerbated by reducing levels of pigment in the tank.

Then the pigment stopped flowing from the tank. It was bridging badly in the tank. We reduced the negative pressure in the system and forced it to flow, albeit with an increased number of line blockages. We have also been running with the tank full to help matters.

Now the pigment has stopped flowing from the mixer - its bridging terribly.

So far the only direct correlation I've found in terms of physical properties is between tamped volume (density) and discharge rate, with an r-squared of 0.92.

I'm not sure whether I should be looking at shear strength, tensile strength/cohesion or sticking with bulk density as a measure of static flowability. Can someone please advise?

What is the best procedure to determine the critical parameters and their limits in this system? What equipment will I need?

Shear Measurements

Posted on 19. Dec. 2005 - 09:27

Dear Sir,

we have a wide range of characterisation for powders and pigments and we are able to measure the flow ability of your pigments. So please feel free to contact us.

Kind Regards

Robert Waggeling

Robert.waggeling@ana-tec.com

AnaTec

RPD - Invista (UK) Ltd., U.K.
(not verified)

Re: Pigment Flow Characteristics

Posted on 19. Dec. 2005 - 10:49

The conventionally accepted measure flowability of powders is shear strength and the most widely used machine to test this parameter is the Jenike Shear cell tester. this is the machine which most use for hopper design, etc. I am sure some will argue that other systems are technically better for various reasons but the Jenike tester is the most widely accepted method of obtaining quantatitive data for use in calculations. However to operate the Jenike shear tester is a specialist job, it needs skill and experience to get good consistent results. I think what you are looking for is a simpler, easier to use device which gives more qualitiative data to try to analyse the problem. There are a number of lower cost easier to use devices which would give you a result to allow you to develop results which you can analyse relative to each other. I think many of the manufacturers of these devices claim they are comparable / better than the Jenike tester anyway but I will leave it to others to comment on that. Lyn Bates' company Ajax, (Lyn is a regular contributor to this web site) used to and as far as I know still do make a tester and there are a number of others.

However I am not sure what use this testing will be to you, you will develop a graph of shear and possibly get a number for flow / no flow. But I am not sure what you can do with the information. Would you not be better getting some samples which flow and don't flow and analysing them for characteristics which could affect flowability (particle size distribution, moisture content, etc) both in the raw materials and the finished product.

Re: Pigment Flow Characteristics

Posted on 20. Dec. 2005 - 11:19

From your description, you may be dealing with a case of limiting flow rate. Many other causes are also possible. Without a material flow properties characterization of your blend, a geometrical discription of your tank and process information, e.g., required flow rate, temperature, etc., it is hard to say. Please visit our site at www.diamondbacktechnology.com and click on "Request a Proposal". Fill out the information that you can and we will respond with a proposal to help you you.

Limiting flow rate occurs when a counter flow of air occurs at the outlet of your tank as a result of a vacuum formed in the interstitual gas (air) above the outlet. The inrushing air creates a "body force" the manifests as an apparent arch. It is not a true arch because material sloughs off the arch free surface at a rate dependent on the permeability of the material. You saw an excellent correlation between bulk density (tapped) and problems. The density/flow rate is related to particle size. The finer the particle size, the more of a problem.

Please call/e-mail if you would like to discuss further.

Regards,

Lee Dudley

President

Diamondbacktechnology, Inc.

805-544-3775 x15

805-549-8282 FAX

415-793-1255 mobile

ldudley@diamondbacktechnology.com

Lee Dudley [email]ldudley@diamondbacktechnology.com[/email] [email]lvdudley@onebox.com[/email] 805-544-3775 x15 805-549-8282 FAX [url]www.diamondbacktechnology.com[/url]