NCZ con would be a low grade Zn con with some impurities that...

NCZ con would be a low grade Zn con with some impurities that would needed to be blended/managed by the smelter (i.e it would not be a ‘clean’ concentrate that the smelter could directly use).

Typically zinc con have min 50% Zinc content so while it is also off-spec in terms of what they have agreed with their off-take partners, it is also low grade for a typical zinc concentrate. As I said before, in a tighter market for zinc concentrates this may not have been that big a deal but now it is a pretty big reason for smelters to push for higher discounts since they will be missing out on zinc production if they treat NZC con vs a higher grade con. 

NCZ are also playing this game with big boys and don’t have their advantages - MMG (Dugald River) have zinc smelter outlets in China via Minmetals group. Glencore has zinc smelters and have blending and trading options for their zinc concentrates. Teck (Red-Dog) is high grade con around 56% Zn and they also have their Trail smelter in Canada.