The criteria I would apply are:

• The lead/zinc component can be liberated from the waste at a size above 6 mm.

Much below 6 mm size heavy media efficiency falls off, below 2 mm probably will not work at all.

• The processing feed rate will be less than 5,000 t/hr (prefer under 2,500 t/hr).

Above this size (and even in the range of 2,500 to 5,000 t/hr) you are reaching the maximum equipment size, plus the complexity becomes excessive.  You also need to run some pilot tests to see if you can even make a final grade with dense media.

It is currently used for Pb/Zn sulphides (galena and sphalerite) to remove light waste particles. It is very efficient for deposit where large dilution occurs. As Mike said, it is a way to remove waste before grinding as heavy media (HM) process works on coarse particles. Generally, the plant has two routes just after crushing: 

• one to the HM circuit when dilution is high, the light particles (reject) are then eliminated before grinding; heavy coarse product reports to grinding circuit and fine product (screened before HM cyclones for example) to the concentrator or to the second grinding stage;
• one directly to the grinding circuit when dilution is low (not enough reject).

It can be seen as sorting before grinding.

Not about HMS or DMS but on a similar topic, Gekko have been performing lots of work around the pre-concentration of Pb/Zn ores using 'continuous gravity'. Normally the mass yield to sink is higher using gravity compared with dense media, however if the ore is amenable, it offers a very efficient 'capital and operating' cost option. We often find that 'massive sulphide' style deposits can liberate at very coarse crush sizes (3-15mm) and so pre-concentration prior to grinding may be an attractive option, especially if there is high dilution or the head grades are low or uneconomic to start with. The finely disseminated style deposits are difficult. Silver Standard are doing this quite successfully at ~270tph on a Zn, Sn and Ag ore in Argentina. 

Heavy media works well where there is typcially a good break between the high density and low density with a small amount of middlings. It does not matter if it is coal, zinc, tungsten, etc. The needs to be a god break. The Tennessee mines ran a heavy media separation at 5 to 10,000 tons per day. There are several heavy media coal plants that run significantly higher than that. Heavy media is all about having a good density break and the ability to have a cheap media that fits in the density break.

Nyrstar in Tennessee has 2 processing plants using Heavy Media Separation on a 2.6% Zn sphalerite hosted in dolomite. 60% of the mass reports to tails of 0.2% Zn while the flotation feed created by the is around 6%

Read more at http://www.911metallurgist.com/blog/dense-heavy-medium-separation-hms-dms

Also, the old Cominco Sullivan concentrator ran for almost 100 years a sink-and-float HMS circuit on a Pb-Zn ore. 

This ore was a SEDEX type.