Crushing road gravel with wet clay.

[jmant180]

My company has a portable crushing plant that consists of a jaw crusher feeding a cone crusher. We are currently in a set that requires the final product (MDOT 23A road gravel)to have 9-16% loss by wash. We have been getting a lot more rain than usual this year and we are having trouble with the clay plugging up the cone because it is too wet. Does anyone have any suggestions about how to dry the clay or get it through the machine, or a machine that might work better? Also, I have heard of using lime to dry out the material. If this was used, how would it affect the final road gravel product when it is placed on the road? 23A is typically used for roads that will not be paved. Would the lime decrease the quality of the gravel for this purpose?

 

[iandig]

In reply to your enquiry as to whether adding lime wil dry out the clay, then yes it will. There have been a few threads regarding the use of lime to dry overly wet soils and it may be wrth looking at these to get a more 'global' view on the subject.
The way in which the lime 'dries' the clay takes various forms. Firstly it is typical to use quick lime, which has a very strong reaction, and as the lime re-hydrates (slakes), it releases large amounts of heat, causing some of the water to be driven off. Also, to hydrate, the lime uses up some of the water (approx 1:4 water to lime, i.e. 0.25% water per 1% lime added). Lime also can effect the structure of the clay, and changes some of the physical properties, in simple terms it becomes stiffer at the same moisture content. All of these effects can be beneficial to the handling characteristics, and the clay can very quickly change form a sticky mess, into a crumbly friable structure.
As to the effect of adding lime to a combination of crushed rock and clay, there can be a big improvement in the quality of the final product, over and above crushed rock by itself. The use of hydraulically bound sub-base material is increasing all the time, in France all their sub-bases are a combination of materials like this, a binder is added (8% cementitiuos content - but not all OPC) and the material is pre-mixed and blended at plants before delivery to sites for road construction. The resultant material has improved stiffness over traditionally unbound materials, and is one of the reasons why the roads in France are so much better than in the UK (just don't tell anyone I said that!)
However, indiscriminate adding of lime to a material is not a cure all, there are a range of tests which can show what the effect will be and how much you should add. I was involved with a large road job, where the original plan was for the use of borrow pits to provide stone for use as sub-base and capping, unfortunatley, the quality of the stone was very poor (weak oolitic limestone, a weak rock with a high proportion of clay and very water sensitive). To improve the material for use,a combination of lime and cement was used to stabilise, with the result that the quality of the end-product was so high, it could have allowed a reduction in the total construction thickness of the bound pavement, if time had allowed the Highways Agency to approve it - End result was a better road, higher re-use of site won material, and reduced number of off-site vehicle movements.
If you would like some more info, please let me know.

 

[smckennz]

Hi Jmant

a cone is a bad choice for clay unless it is bone dry.
Consider closing the gap on your jaw, bypassing the cone and living with a lower throughput.

Cheers

Steve