Our cement plant has some very old ball mills. One in particular is a 13' x 46' two compartment mill, closed-circuit, with a low-efficiency separator. It is not an air-swept mill. The separator and mill are vented by the same dust collector. We are currently running a high mill outlet ventilation on the mill, greater than -4" W.C. What is an acceptable mill ventilation static pressure range for this type mill circuit?

I don't think it is a question of the static pressure. More important is the speed of the air exiting the mill. 1.5 to 2.0 m/sec (5 to 6.5 ft/sec) would be normal for a mill of the type you describe. If the air velocity is too high you risk pulling material out of the mill in ventilation air before it has been sufficiently ground. The particle size distribution and hence the hydraulic performance of the cement is influenced by the drafting to a surprising degree.

We are carrying out research on blended cements with a view to increase the utilisation of blast furnace slag/fly ash. In this context, we are exploring use of high energy mills (Jet Mill, Vibratory Mill etc) together with traditional milling devices. I wish to ask:
1. How important is the strength of cement? There is considerable amount of literature on high performance cement/energetically modified cement ie cement with compressive strength in the excess of 90 Mpa and concrete with greater than 145 Mpa.
2. What could be the major limitations in using mills such as Jet Mill in cement industry? We understand these mills consumes lots of energy but it is also true that the efficiency of traditional mills decreases as we approach the target size. Should not the combination work better?
3. I am looking addresses of supplier for lab size jet mill and vibratory mill (2-10 kg/h) for the grinding of cement clinker, BF slag and fly ash.

The strength of the cement is important as the hydration of the cement is responsible for all the early strength development of blended cements. The hydration of the cement minerals also releases the calcium hydroxide which activates the pozzolanic reaction of the fly ash or slag. Jet mills are not used in the cement industry due to the operating costs and the throughput capabilities of the mills. Large cement plants employ mills capable of producing tens por hundreds of cement per hour. I don't think jet mills can reach that capacity? For a laboratory mill my first port of call would be ELE (Engineering Laboratory Equipment).