Stephenb 16/01/2013 15:30:58

limestone and viscosity

Good day.

In our quarry we came across 2 types of limestone (C&G). Our target viscosity is 2.5% (a wet process plant and therefore we have to pump), however, with stone C we find that we can use about 3% less moisture than stone G to get the same viscosity.

Below is the chemistry:

SAMPLE	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O
C	19.10	4.05	3.02	39.31	0.62	0.23	0.16
G	19.00	6.54	4.45	35.93	0.67	0.23	0.12

Is there some kind of rule from the chemistry that will tell us which stone will require more moisture or is there any scientific evidence that links one or more of the above analytes with moisture and viscosity?

Thanks.

Ted Krapkat 17/01/2013 04:04:12

Hello Stephen,

One of the most important factors affecting the viscosity of limestone/clay slurries is the type and quantity of clay(s) in the mix. Simply increasing the percentage of one type of clay over another can significantly change the rheological properties of slurry while hardly changing the chemical composition at all.

Back in the 80's our first kiln line was a semi wet process with the raw mill located right at the limestone mine and the slurry was pumped 25km in an underground pipeline to the kiln, located near the coast. The inlet pressure at the mainline pumps was 6-8MPa with an outlet pressure of only 100-200kPa. Consequently, every batch of slurry pumped to the kiln not only had to be within chemical targets, but within rheological targets (viscosity and yield stress) as well.

This initially posed some problems for us since there were many types of clays in our deposit. All of them were chemically suitable, however some of them had strikingly different rehological behavior and caused excessively high viscosity at quite low levels in the slurry.

I remember doing a research project on this problem, the aim of which was to find an economical additive to counteract these viscosity changes and keep moisture content as low as possible while still staying below the maximum viscosity and yield stress required for pumping.

We found that clay viscosity was strongly affected by both pH and the presence of soluble cations such as Na+ , K+ and Ca+2. If both pH and cationic concentrations could be changed at the same time, viscosity could be changed over a very large range for quite small dosages of additives. As pH and cation concentrations were increased, the slurry viscosity first rapidly dropped to a minimum (thinning effect) and then rose dramatically (flocculation). The best and most economical viscosity modifier we found was soda ash (Na2CO3).

We subsequently installed a tank and adjustable dosing system to add soda ash solution at a rate of ~10-15 g/t dry solids, added into the raw mill feed chute. At first we only used this thinner whenever the slurry viscosity increased due to clay changes from the mine, then later we started using the soda ash virtually all the time to optimise slurry moisture content and reduce water consumption.

Hopefully this gives you some ideas on how to manage your own situation, which I believe is also due to viscosity changes as a result of clay variability.

Regards,
Ted.

Stephenb 18/01/2013 13:41:32

This is very interesting. However, chemically can i be able to determine this high viscosity clay in the limestone or is there some quick test i can do to determine this?

Do you have this paper that you wrote available that i can read?

Thank you very much for your response.

Ted Krapkat 21/01/2013 05:09:02

Hello Stephen,

Since many clays found in a limestone deposit are both chemically and mineralogically very similar, there is no easy method of chemically differentiating clay types. You could use XRD analysis but this is costly and time consuming.

The only practical way to determine which of the clays in your deposit are the problem is by testing their viscosity in a slurry. Ideally, this slurry should also contain the same amount of limestone that is present in your plant slurry, since CaCO3 contributes Ca+2 ions as well as affecting the slurry pH (~8.5). Both these factors can influence the rheology of the clay.

I suggest that you perform some laboratory trials (or better still; plant trials) with slurry made from powdered limestone and dry clays in the same wt/wt proportion as your plant slurry and having the same moisture content. Measure and compare the viscosity of each to identify those that would be a problem for your process. Or, if that is not possible, there may be an external laboratory who can do this type of testing for you.

Sorry, but I don't have an electronic copy of our R&D work on limestone/clay slurry thinners. After nearly 30 years, I doubt if even a hard-copy exists any more.

Regards,

Ted.