I suggest you address your question to the experts at Micromeritics Analytical Services (link below). I am not an employee or stockholder. Have just found them extremely knowledgable and helpful. http://www.particletesting.com 

Dear Feridun

You could make an estimation, as you propose, if your material is composed of spherical particles.

1) But if you have irregular particles,   elongated  particles (needle shaped, one dimension larger than the screen radius) could pass the screen.

2)Also if you have agglomeration (it is not clear if you have mm or micrometer particles) which is stronger for smaller particles you could have very small particles that cannot pass the screen because they agglomerate in larger particles.

3)Humidity is a probable cause  for agglomeration.

4)If your screen is damaged (pinholes) anything will pass.

Regards

Miguel Prado

Dear Feridun,

assuming all specified above by Miguel and that screening efficiency refers only to particles of 30 mm and critical size (i.e. only for -30+18 mm in your case), you get 40,11% of -10 mm in undersize.

 Dear Feridun, Yes, you can. To be more specific, I need to know how you define the efficiency. However, your problem would be simpler, if you could define your screening operation with two or three parameters instead. These are known as Tromp (Partition) curve parameters and include d50, which is about 30 mm in your case, Imperfection, which would be between 0.12-0.18 for such a coarse screening, and bypass, which would be almost zero for wet screening at this size and 5-10% for dry screening where fine particles stick to the surfaces of coarse particles and get carried into the screen oversize. With 88% efficiency, your case appears to be dry screening. Once you have reasonable estimates of these three parameters, for example 30 mm, 0.15 and 5% respectively, you can Use Rosin Rammler or Logistic equation to calculate Tromp (Partition) number for each size fraction and from the Tromp numbers you can calculate the particle size distributions of the screen oversize and undersize and, if necessary, manipulate these parameters until the calculated efficiency matches 88%.

Dear all,

I would like to thank for your all valuable response and

Make a plott of particle retained versus particle size (or screen size) (Schumann's plot) in a log log plot.

Extend the straight  line for fine particles. This is your particle size distributions for the fines.