You need to understand the difference between spot and benchmark...

You need to understand the difference between spot and benchmark TCs. While NCZ is producing out of specification zinc concentrates (or ‘commissioning grade’) they are selling SPOT!

You also need to read the articles you post properly, from the 2nd article you posted:

But another factor is also looming large in this year's TC/RC talks, China's spot TC/RCs.

"It will be a very important point of reference in the discussions," said the UK analyst. "It has been rising very steeply in recent months. ... The spot TC is now a long way above last year's benchmark, whereas in previous years, the spot market has been below the benchmark when the negotiations have come around."

During 2018 negotiations, he said, China's spot TC was around $20/mt or less. 

So it's not surprising that the 2018 benchmark was reduced," he said. Currently, China's spot TC/RC is trending at around $250/mt