I never have engineered vertical screw for sticky powders , but I have seen a few of those working in a huge sludge treatment plant. That plant was developped for the evaporation of waste water out of mud residues coming from an adjacent major urban sewagewater treatment plant , to obtain dry organic balls fertiliser sold to the flower industry.
The screws were manufactured at Dinamec (
), diameter 1000 mm , lenght 13 meter vertical , outlet on top , material SS304. Huge beasts. Tel. +32 9 367 94 94 , time GMT +1. Call them for advice , Mr Andre Geeroms , if he is still working there (was general manager).
WAM is also manufacturing those items.
The material was like wet compressed dirty mud , just as you would expect when having sewage residues scrapped from the bottom of the decanter tanks of a sewagewater treatment plant , residues being compressed in horizontal screws with rapidly decreasing thread step distance toward the outlet , for water removal thru top holes made in the tube around the screw , prior gravitary injection into the vertical screw conveyor.
They worked very well , although they told me that it was difficult to maintain them , given the specific mechanical construction and heavy loads applied to the bottom screw bearings , who had to support all the upwardly displaced product mass + screw mass itself. Given the dimensions of the beasts , the loads must have been impressive. They basically had two for each process line , who were put to work alternatively , the one on rest being 'maintenanced'.
I also would make sure there are easily accessible openings foreseen on top of your building roof , with permanent hoisting system foreseen on a steel structure above the outlet of the vertical screw systems , to be able to extract the screw from the screw tube or electrical motor gearbox + bearing housing for maintenance , or if something blocks inside it (sabotage by fired personel , have seen it on other occasions).
I personally would use only SS304 , because I deduce there is no salt or other aggressive caustic product in your mixture. If the SS304 is thoroughly welded and the welds properly passified and polished afterwards, it could go on for ages , given that you take precautions to keep your screw systems out of reach from rusting metallic parts or unprotected steel items during transport , installation and maintenance afterwards. It is not for fun that serious equipment manufacturers have completely separate SS and plain iron work shops , with separate non-mixable rollbanks and saw&cutting machinery for each steel type , and take care to very well wrap they SS equipment in protective bubble plastic sheets before calling in the careless transporters.
And finally , you can foresee a contract clause basically stating 'no cure , no pay' , and supply the italian supplier on your cost a few 2 ton closed big bags of your wet coal products to let him have the opportunity to make realistic inhouse tests on a scale model. They all have that somewhere stashed , a leftover model that was used to conduct test for other shit products , so you always can ask them to supply you a video tape with the proof that your product worked well in their 'labo' , that with the contractual clause should give you the confidence to go for it.
The alternative is probably not in bucket elevators, if your product is as sticky wet as your are stating. You only will have a clogged top outlet , recirculating continuously the same product that can't escape the bucket elevator due to continuous outlet clogging.
A closed chain conveyor would do the job perfectly , but will take much more place and I fear , cost a lot more than a duo vertical screw set. A belt conveyor is another solution , although the cleaning issue with your product makes it not advisable to use open belt conveying.
I personally would try the vertical screw model , if I had a space problem in my production room. Otherwise , I would go for an expensive chain conveyor for certain work reliability and take the arduous maintenance of such a system for granted.
However , it is your money and your time.
