Municipal Waste Stream
Using a wet ball mill will do nothing but make mush out of most all of the waste stream that is organic and a real mess.
You would be much better off using small screeners of the type that are the flat type with the wide screen size like the EZ screeners in batteries of screen trains and mesh sizes with air knives under the decks to continually clear them while operating from smaller to larger sieves to segregate materials for further screening in a set of secondary screeners in trains
The screeners are low in cost with respect to a wet ball mill and will be much less messy to deal with and using rubber screen fabric decks will aid in odor and vermin control with hot water and mild bleach solutions.
I know the folks at EZ screener would love to get in to this and help you as a very small area would be required to set up 4 screen train batteries to do this -as the EZ screeners dont use a lot of power.
probably no more than 50 by 200 feet not including sorting belts pre and post waste stream to establish 4 screen trains and 4 sorting belts with capture hoppers for screened materials in one cut to aid in flow and cleaning.
A clear span steel building with open sides and ends like hay storage buildings would allow for ventilation and washdown for odor and dust control and a a and continous sewer grating in the center to collect washdown water and direct it to a grease trap and catch basin man hole and then to the sanitary sewer for treatment or discharge.
An oxidation ditch in a small building would also work very well as long as two RBC units "Rotating Biological Contacters" are used to move flows of water for aeration and later for discharge to the sewer system. ■
Re: Comminution Of Household Waste
You are starting down a path where many thousands before you have failed miserably and/or lost money.
You are correct that a shear shredder will reduce it to 2" without much problem. Why do you want to go down to .75mm??? That is ambitious bordering on insanity with ductile materials like plastics. Adding water to organics makes sewage very quickly.
Garbage is much easier to sort in larger identifiable pieces. Do not delude yourself that this will be nice homogenous material, some days it will look like a pile of stinking muck.
Clean paper fibre and metals have value, get them out first. If you are composting, get the plastic out or the compost is worthless. If you are going to incinerate it you don't even need to shred it.
Arrange a visit to a municpal waste composting plant to see what organics and waste look like out the back end of a collection truck. I think you will quickly be convinced that fine grinding is not economically feasible.
Sorry to sound negetive, but I have worked on some of these projects and the last job I would want is that of a sorter on the first picking belt. ■
Particle Size And Shearing To A Small Size
Thanks for your reply JD, there are 2 reasons for going down to a small size -
1 - To make the organic portion easier and faster to digest. (I am not sure of what the optimum size for reducing organic material should be ?)
2 - To make the material easier to sort by mechanical means.
I have some experience of waste re-cycling plants, and sorting waste by hand is something that I would like to try and avoid.
Water would be added to the waste to make the mixture flow, I assume that 20% by weight of the mix would be waste. Most digesters have a lower ratio of waste, but because of the extra mass of the aluminum in the mix, my ‘backyard experiments’ seem to show that a 20% mix is easily workable.
Waste is generally in the form of thin sheets of material, and I think this is why shearing seems to be way most waste is comminuted. I understand that shear shredders will reduce to 2”, would it not be possible to use a series of shear shredders to go down a factor of 4 say. Something like 64mm, then 16mm then 4mm, then 1mm or less. ■
Re: Comminution Of Household Waste
Most composting operations don't seem to shred much below 2", they compost, then at the end of the heating cycle, run the material through a trommel screen, process the unders and throw the overs back into the next pile, or get rid of the large overs which are not breaking down. Anything that has to go smaller goes through a hammermill or hog.
Are you trying to compost (aerobic in the open) or digest, (anerobic with liquid in a closed container).
Shredding to a small size before sorting is a mistake in my opinion. Assuming you could shred it that small, plucking out small bits of plastic (or any other material) from a damp organic material is tough.
Film plastic is a difficult material to reduce in size, it stretches rather than tearing, I doubt you will have much success in getting it much below 1/2" if that small. Wood and metals are a bit easier to size reduce. But again, because they have a market value, get the metals out while still in big pieces with a magnet or eddy current. It will be much easier than trying to take them out in small bits.
Doing a manual pre-sort is not a pleasant job, but if you don't do it you have to deal with things you are not expecting, usually after they have bunged up the process or the equipment. ■
Re: Comminution Of Household Waste
Balers and compactors are good for your task. You can buy from any ergonomic and recycling equipment providing industry. ■
Reply
Some products like cardboard boxes are very advantageous in this case, as they are safe, light and most importantly recyclable. So we should prefer those things at first which are recyclable.
Smurfit Kappa Direct ■
Comminution of Household Waste
Hi,
I would like to grind up household waste to about 0.75mm (750micron). As I understand it a industry standard shredder would be able to reduce the waste to particles with a max size of about 50mm wide strips. This process would, I understand consume about 8 kWh of electricity per 1000kg of waste. (source solid waste handbook by WD Robinson)
Waste consists of about 70% organics like -
11 % Food scraps
5 % Wood
35 % Paper and cardboard
12 % Yard trimmings
7 % Rubber leather textiles
The rest is -
8 % Metals
6 % Glass
12 % Plastics
4 % Other
Therefore only about 8% of the material is really hard to grind up metal. The metal is made up of about half aluminum, the rest mostly steel.
The shape of most waste is in the form of sheets rather than cubes. So plastics bags, paper, aluminum and metal cans, and so on are in sheets that very thin.
The purpose of grinding up the waste is to make it easy to sort, and also to make the organic part more to digestible. (more info on www.wasterefinery.com)
Because waste is made up of material that has many different shapes and hardness, I think that it will be difficult to calculate how much energy will needed to grind it up. Nevertheless, I have a few questions, and as I have very little knowledge of comminution, I understand that some of the questions may be inappropriate, but here they are -
A - The idea of our project is grind up 50,000kg of waste per day, so my overall question is what the best way would be to grind up this material, and about how much electricity would be required to process this in kWh?
B - Would it be best to shred the material into 50mm strips first. Companies such as www.ssiworld.com make shredder machines for this purpose.
C - From the little I know, I assume a wet ball mill process would be the next step?
I believe that the organic material in a wet (and hot) mill will would grind up fairly easily. Glass is fairly brittle, so I assume that would grind up easily also. I assume that the plastics would also cut easy in a wet process.
D - If a ball mill be the best way to grind up this material, and if so what diameter, rotation, ball size, etc. would be appropriate? I kind of envisage a mill where water could be circulated thru the mill, to flush out the fine material, a kind of continous process set up.
E - Would a series of ball mills, say something like from the 50mm strips to 20mm sieve in the first mill, then from 20mm to 5mm in the second mill, and then down to 0.75mm in the third mill?
D - Are there any online calculators for Bond, Kick and Rittinger formula? ■