Down at 75 um you use cyclones for optimum particle classification.

A hydrocyclone has no moving parts, has a smaller footprint but is less efficient.

a spiral classifier has moving parts, a much larger footprint but is more efficient if correctly set up (feed density) weir height and calming zone.

if you have a spiral classifier, use it, if you do not have one, install hydrocyclones

If you want to do a classification based on grain size, hydrocyclone is the most appropriate. 

If you want to make a classification based on density (separation of two different minerals) spiral classifier is better. 

Derrick screen is the best choice for 75 micron cut as it gives you the highest separation (classification) efficiency because it simply separate by physical size.

Hello Mr. Derrick Screen,

would you please complement your "answer" with good reasons and comparisons with spiral classifiers and cyclones? Am looking for technological advantages and capital cost comparisons. You must have supporting data in separation efficiency improvements &  performance of a Derrick screen to carrying it purchase price?

In addition to operational advantages, cyclones are able to differentiate between equal size particles of varying specific gravity. The high specific gravity middling particles are preferentially classified to the cyclone underflow by the high centrifugal forces developed in the cyclone, and returned back to the mill where the valuable mineral fraction can be liberated. The lighter gangue particles leave the circuit directly as cyclone overflow. This benefit is especially important in low grade porphyry copper ores where the preferential classification of the middlings results in a favourable compaction of mineral values within a very narrow size range. The concentration of copper in a typical cyclone overflow is highest in the 43 x 10 micron fraction, which corresponds to the highest mineral recoveries in the rougher flotation cells. Cyclones in a grinding circuit shape the size distribution of the valuable minerals so a large proportion falls within the range associated with high flotation rates.

The preferential classification benefit of cyclones is further illustrated in Table 2 which compares the metallurgical results of cyclones versus spiral classifiers in a copper grinding circuit. The cyclone produced an overflow product containing more copper in the finer size ranges where it will be subject to higher recovery in flotation. Table 3 summarizes results of cyclones versus classifiers. The copper tails dropped from 0.32% with the mechanical classifiers to 0.28% with cyclones. In addition, the higher circulating load produced by the cyclones resulted in increased circuit capacity while producing the same grind at a higher overflow density.

Hi,

Based on my experience, spirals tend to drop in efficiency at below 150microns.

A friend familiar with this told me the main problem with spiral classifiers is the cut size is directly proportional to retention time (flow rate, Stoke's law, that sort of thing). To get a fine cut size requires flow of almost zero = the finer you want to cut, the lower the flow and density required.

Hitzrot, H. W. wrote a great paper in Society for Mining, Metallurgy & Exploration Mineral Processing Handbook called "Mechanical Classifiers".

The principal use of mechanical classifiers has been in closed circuit wet grinding; however, this application has been widely displaced over the past few decades by the hydrocyclone. The hydrocyclone installation, including its feed pump, offers lower capital costs and requires less floor space.

However, these advantages of the hydrocyclone are being gained at the sacrifice of some of the advantages of the mechanical classifier. The mechanical classifier requires less power and has lower maintenance costs than the hydrocyclone and its feed pump. The underflow product from the mechanical classifier can have a higher solids content and contain less entrapped undersize particles than the comparable product from the hydrocyclone. Consequently, it is capable of operating at a higher classification efficiency. A reduced circulating load results under these conditions in a closed-circuit grinding application.

The proper selection of mechanical classification equipment requires that the properties of the solids and liquid are adequately defined. The following descriptive information regarding the solids is desired: feed rate, chemical and physical composition, density, temperature, size analysis, and desired separation size.

Review https://www.911metallurgist.com/blog/mechanical-classifiers in details.

The 2 videos at the bottom are classifiers with 50 TPH of overflow at 140 um and 35-40% solids.