We have to take gypsum out of a rectangular silo and we are not sure if a rotary valve is necessary between the silo and a screw conveyor. At the moment we have problem of ratholing and bridge as well, thanks in advance for any help.

I would suggest installation of an acoustic cleaner and sounding the horn periodically to combat the ratholing problem.

We carry out refractory work with screw jacks, maximum 10m lining at a time (ie 5m maximum simultaneously at two different locations in same kiln). As ours is a small plant of 1000tpd, with single kiln of 64m length, deploying a brick lining machine and dismantling machines are not viable in terms of cost for us. What therefore is the maximum length of refractory work can be done at a time, in how many parts, simultaneously, from the various aspects such as safety, life of refractory etc.

I would recommend using the glueing method rather than screw jacks and then you can replace as long a section as you like without the concern of installing and removing the jacks. My understanding is that the glueing method will have no problems with a kiln of 3.95m diameter. However, you would be better to check this with suppliers of the refractory glue when placing your order.