By Sarah Kent And Alex MacDonald 

GENEVA--Trafigura Beheer BV is pushing to exert greater control over one of the world's largest zinc refiners, its latest attempt to solidify influence over a company that produces the commodities it trades.

Trafigura is proposing two directors for the board of Nyrstar NV, a Belgium-based zinc refiner in which it owns a 15% stake. Shareholders vote on the board slate Wednesday. Nyrstar's existing board and management oppose one of the candidates, saying he is too closely associated with Trafigura to hold an independent director's seat.

Last year, Trafigura based in the Netherlands, built up its stake in Nyrstar after a steep drop in the latter's share price following a rights issue. Trafigura has called the stake a "strategic investment." Analysts figure Trafigura wants access to the rights to trade the zinc Nyrstar produces, as well as to supply its refineries with zinc concentrate.

Trafigura is the world's third-largest independent trading house by revenue, behind Glencore PLC and Vitol Holding B.V. These three, and many of their smaller peers, have built up holdings of physical assets like pipelines, terminals, mines and refineries. That gives them more control over things like production and delivery, and more knowledge and information about supply and demand. All that can help decision making when it comes time to buy and sell everything from oil to soybeans.

Glencore has been the most aggressive acquirer of the group, buying up mining giant Xstrata in 2013 for $29.5 billion. Trafigura has tended to focus on logistical assets, like terminals and pipelines, rather than oil fields, refineries or mines.

More recently, though, Trafigura has sought to make strategic investments in producing companies, then aggressively leverage that into influence on the board. Last year, it sought shareholder support to oust the board of Spanish copper mining company EMED Mining Public Ltd., after building a holding of about 19% in the company.

The move came after Trafigura grew frustrated with EMED's slow progress financing and developing its copper project in southern Spain, according to people familiar with the situation. EMED's mine is about 20 miles from Trafigura's Agua TeƱidas copper mine. Trafigura is building a processing plant that would benefit from additional volumes. In the end, four EMED directors--including the CEO and chairman--stepped down as part of a $30 million bridge-loan agreement, funded in part by Trafigura. EMED declined to comment.

In a recent bond prospectus, Trafigura said it was seeking to build on its Spanish assets to create a stand-alone, midsize mining company, without elaborating on its strategy in regards to EMED. Trafigura added that its investments "are focused on identifying and harnessing opportunities that provide complementary volume flow to its trading business, open up new markets and create recurrent, sustainable income sources."

With Nyrstar, Trafigura said it saw an opportunity to buy into a business with an attractive outlook, based on its expectations that zinc prices are heading up in the long term. But it also said it saw the potential for operational improvements, and Trafigura's sizable position in the zinc market could benefit both parties.

"It was a strategic investment...but when we sit back we think we can add value given that we have one of the largest zinc concentrate trading books," Chief Executive Jeremy Weir, told reporters recently.

Nyrstar is opposed to the appointment of Martyn Konig, chief investment officer for T Wealth Management SA, a private investment vehicle used by some of Trafigura's senior management. His ties to Trafigura make him unacceptable as an independent director, Nyrstar says.

"Mr. Konig has clear ties to Trafigura as evidenced by his involvement in the private company which manages Trafigura's senior executives' personal finances," said Julien De Wilde, Nyrstar's chairman, in a statement.

Trafigura says Mr. Konig's relationship with the trading house wouldn't compromise his independence, and that he has years of experience that make him well-qualified to sit on the board.

Write to Sarah Kent at sarah.kent@wsj.com and Alex MacDonald at alex.macdonald@wsj.com

Access Investor Kit for Nyrstar NV

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=BE0003876936

Subscribe to WSJ: http://online.wsj.com?mod=djnwires