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The New Cullinan AG Milling Circuit – a Narrative of Progress by L http://dx.doi.org/10.17159/2411-9717/2019/v119n2a3 The new Cullinan AG milling circuit – a narrative of progress by L. Musenwa*, T. Khumalo*, M. Kgaphola*, S. Masemola*, and G. van Wyk† of a modern, fit-for-purpose processing plant at CDM, with a throughput capacity of 6 Mt/a. This has replaced the previous plant at CDM, In 2017, Petra Diamonds completed the construction and commissioning of which was originally commissioned in 1947 a modern, fit-for-purpose diamond processing plant at Cullinan Diamond and which had undergone numerous Mine (CDM). The design of CDM’s milling circuit is unconventional in that it comprises an autogenous (AG) mill with a grate discharge with large refurbishments over the years. Due to its age ports, low-revolution jaw crushers, and high-pressure grinding roll and operational complexity, it had become crushers with large operating gaps. In this paper we review the design to expensive to maintain, requiring significant provide guidance on what is expected from the milling circuit and to stay-in-business capex, and costly to operate, demonstrate how the design aims to address challenges experienced in the particularly given its large 26 ha footprint. The old plant, which was based on staged crushing technology. We assessed plant was also mostly based on old crushing the performance of the CDM AG milling circuit from commissioning and technology, which is known to be inefficient early production stages to examine its impact along multiple dimensions. for the recovery of large stones. The design of In the assessment we sought to understand the lessons from our milling the new plant was therefore aimed at circuit regarding diamond liberation, energy consumption, and the future improving recoveries at a lower cost and of diamond processing as a whole. reducing diamond breakage. The new plant was also designed to use more kimberlite processing, autogenous milling, high-pressure grinding roll, environmentally friendly processes, which is diamond liberation, diamond damage. vital for the long-term sustainable future of the mine. The new plant utilizes gentler processing methods – comminution by attrition and abrasion instead of extensive staged crushing. The new comminution methods used are AG Cullinan Diamond Mine (CDM), previously milling and high-pressure grinding rolls known as Premier Mine, is an underground (HPGRs), which are expected to reduce diamond mine owned by Petra Diamonds. diamond breakage and improve recoveries Petra Diamonds is a leading independent across the full spectrum of diamonds, diamond mining group and a growing supplier including the larger/exceptional stones for of rough diamonds to the international market. which the mine is renowned. The top cut size Petra Diamonds is listed on the Main Market of of 75 mm will cater for diamonds of more than the London Stock Exchange under the ticker 3000 carats, such as the Cullinan Diamond. ‘PDL’. Petra’s total resource of 305 million The plant has a much smaller footprint of just carats ranks third by size after De Beers and 4 ha, and the engineering infrastructure has ALROSA (Petra Diamonds, 2018). been significantly reduced. CDM was established in 1902. The mine rose to prominence in 1905, when the 3106 carat Cullinan Diamond — the largest rough diamond of gem quality ever found — was discovered there. The mine has produced over 750 stones that are larger than 100 carats and more than a quarter of all the world's diamonds that are larger than 400 carats. Cullinan is renowned as a source of large, * Petra Cullinan Diamond Mine, South Africa. high-quality gem diamonds, including Type II † Thyssenkrupp, Johannesburg, South Africa. stones, as well as being the world’s most © The Southern African Institute of Mining and Metallurgy, 2019. ISSN 2225-6253. This paper important source of very rare blue diamonds was first presented at the Diamonds — Source to (Petra Diamonds, 2018). Use 2018 Conference, 11–13 June 2018, In September 2017, MDM Engineering Birchwood Hotel and OR Tambo Conference completed the construction and commissioning Centre, Jet Park, Johannesburg, South Africa. 113 L The new Cullinan AG milling circuit — a narrative of progress "D6B??B=5DB=D9B@6>=9D8<>7C;;B=5 This is an essential step in mineral processing as proper liberation allows for maximum recovery of minerals in the Traditionally, Southern African diamond processing plants downstream processes. Undergrinding leads to inefficient are designed to include various stages of crushing to liberate recoveries (Steyn, 2011) and overgrinding, especially in the diamonds from the host rock. These types of plants have diamond processing context, leads to major value loss due to been operated to process kimberlite deposits in Southern breakage of the final product. Knowledge of the ideal product Africa over a number of years, and include the old CDM plant size is essential in the design of any comminution circuit. which was commissioned in 1947. The crushers in a The new plant has been designed to improve the conventional plant use compression and impact forces efficiency of material flow, thereby significantly lowering between two steel surfaces to break the feed material. operating costs. The new plant’s footprint is significantly less Although the crushers are optimized to operate at the than that of the old CDM processing plant. Some of the maximum possible crushing gap, there is still the potential comparisons to the old plant are summarized in Table I. for breaking diamonds between the steel surfaces. Furthermore, cone crusher products, having steep product &'%"#$' &!!'% distributions, are associated with poor liberation of diamonds The new CDM plant process flow is illustrated in Figure 1. from the host rock (Daniel, 2010). The new mill plant is designed to handle run-of-mine (ROM) In recent years AG milling has been slowly introduced in ore as well as reclaimed tailings. The ROM ore is conveyed diamond processing in Southern Africa. AG milling is an from a 5000 t capacity ROM silo to the mills. The reclaimed established comminution process (van Niekerk, 2013), and and recirculated ore is conveyed from a 3000 t capacity silo to has been used in Russia over the past 40 years. The Russian the mills. The ROM ore from the silos is blended with experience cannot, however, be translated directly to the reclaimed tailings in the desired proportion for optimum processing requirements in Southern Africa. Russian grinding. The plant is designed for 6 Mt/a, and will initially methods have historically focused on maximizing diamond process 4 Mt/a ROM plus 2 Mt/a reclaimed ore. The mill is liberation (Janse, 2007). The Russian government designed for an overall fresh feed rate of 750 t/h feeding two regulations require very strict auditing of operational mills in parallel. This is blended as follows: performance to ensure that the orebody is being completely ® ’exhausted’, typically down to 1 mm. 500 t/h ROM ore ® Catoca mine in Angola was the first significant Southern 250 t/h reclaimed ore. African diamond operation to install AG mills. ALROSA, the Each mill is designed for a fresh feed rate of 375 t/h and Russia-based diamond producer, is a major shareholder in a 152% circulating load. the Catoca mine. The first diamonds were recovered in 1997 The mill discharge slurry (–150mm) flows onto a and the plant was expanded in 2005, raising the combined dedicated scalping vibrating grizzly installed for each mill. processing capacity to over 10 Mt of ore per annum. All of the The grizzly oversize is conveyed to the jaw crushers. The jaw Catoca plants are equipped with AG mills as the main crusher product is conveyed to the recycle ore silo. The jaw comminution devices. The Catoca mine’s main orebody or crushers were designed with a bypass option to return excess pipe consists of volcanogenic-sedimentary rocks, with the load to the mills or bypass the crushers during maintenance. inner ring of the vertical pipe made up of porphyric kimberlite, while the central part is filled with kimberlitic breccias. This type of material can be classified as relatively Table I soft and is therefore ideally suited for AG mill processing (van Niekerk, 2013). '>68@<B;>=D>4DA:CD>?9D@=9D=C0D'+1D8?@=A; In 2012 Karowe mine was successfully commissioned by Lucara in Botswana. The Karowe plant is equipped with an *?9D8?@=A C0D"D6B??B=5D8?@=A AG mill as the major comminution device. The Karowe Throughput per annum 2.8 Mt ROM 6.0 Mt ROM capacity operation has been modified and optimized over a number of 2.5 Mt tailings Initial feed: 4.0 Mt ROM years to process various material fractions through the plant. 2.0 Mt tailings At Karowe the material varies from soft and friable to very Total footprint Approx. 27 ha Approx. 4 ha competent. The operation has been very successful, Major installed equipment especially as the world’s second-largest gem diamond, at Conveyors 151 belts (15 km) 22 belts (3 km) 1111 carats, was recovered in November 2015 (Lucara, Conveyor transfer points 179 32 2018). Screens 88 22 Pumps 121 7 :CD=C0D'+1D6B??B=5D8?@=AD9C;B5= Crushers 18 4 (excluding mills) Feeders 21 14 The CDM comminution circuit was designed to reduce Substations 17 2 diamond breakage and improve recoveries across the full Electrical motors 589 84 spectrum of diamonds; including the larger/exceptional Improved electricity efficiency stones for which the mine is renowned. When applied Power consumption 22.5 MW 25.0 MW correctly, AG milling offers a diamond-friendly comminution Power consumption per ton 4.7 4.2 environment (Daniel, Bellingan, and Rauscher, 2016). The Improved water consumption purpose of any comminution process is to liberate or expose m³ per ton 3.5 1.2 locked-up valuable minerals within the host rock or gangue.
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