Clough & Associates Ltd. Page 13 Union Hill Heritage Management Plan HISTORY, CONTINUED Historical Summary, continued The extraction and refining process also evolved, with recovery rates considerably improved by adoption of the cyanide process in the 1890s. Finely crushed ore was passed through cyanide tanks, the cyanide bringing the gold into solution so that it could be easily drained off, then processed with zinc to create zinc slimes which could then be refined in the melt house. Experiments with the cyanide process began in 1892, and a successful cyanide plant was in operation in 1894. The process was improved in 1909 with the construction of two tall concrete air agitation tanks to replace the original percolating vats, and a further four were built the following year. Large quantities of water were required both for power supply via Pelton wheels and stream engines, and also for processing the ore (washing, wet crushing, slimes, separation, cyanide and other processes). In 1889 a dam was constructed on Ohinemuri River and another on Homunga Creek (Figure 2) and a low-pressure 4.2km water race was constructed to bring the water to the southeast side of the battery, driving two Pelton wheels at the bottom of a 52ft (c.16m) shaft. A high level race was completed by 1891 to bring water from the Waitete, Walmsley and Mataura Streams, arriving at the battery through pressure pipes along Pipe Lane and driving three Pelton wheels. Underground tunnels (tail races) took waste water from the wheels and drainage from the workings to the Ohinemuri River. Transport between and from the mines and battery involved various tramways, initially running horse drawn trams except for the Silverton tramline on the eastern side of Union Hill, which had a locomotive. The Paeroa to Waihi railway, which greatly facilitated the transport of goods to and from the mines, opened in 1905. Waihi Battery operated as a full battery and refinery until 1913, when the battery closed, but the refinery continued to be used to process zinc slimes from the company’s other batteries – in particular the Victoria Battery at Waikino – until 1953. The third and final refinery was built in 1907. Some limited prospecting was carried out on Union Hill in the middle decades of the 20th century, and in the 1970s and 1980s Mineral Resources (NZ) Ltd established a gold recovery plant on Union Hill to reprocess tailings. The company carried out prospecting and limited mining. In 1987 the Martha mine was reopened by Waihi Gold Company and a conveyor was built across the northern end of Union Hill to carry ore from the mine to the ore processing plant at Baxter Road. Continued on next page
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