Crushed

AS CHINA'S THREE GORGES project announces the topping out of the left bank power house and spillway, the contractors have confirmed record production rates from its crushing lines to meet demand for aggregates. More than 35M tonnes will have been crushed by completion of phase 2 in 2003 with, it is estimated, more than 95% being passed through metso-minerals crushers.

Two companies, China Gezhouba Construction Group Corp (CGGC), the country's largest hydro power dam construction company, and Xiaanxi Manufacture Sand Co, part of the 378 JV, were awarded contracts to produce the crushed sand and aggregates for the project in 1996.

Xiaanxi Manufacture Sand Co purchased HP500 and Barmac VSI crushers and installed its first crushing line in 1996 exclusively for sand production for the project. Opting for three locally manufactured primary gyratory crushers, it completed the line-up with Metso Minerals crushers, including two Nordberg HP500s, one each for the secondary and tertiary sections, and two Barmac VSI quarternary crushers- the first Metso Minerals equipment to be used on a Chinese dam project.

By the start of phase 2 in 1998, with the increased demand for both sand and aggregates, Xiaanxi recognised that additional units would be needed, including two Nordberg primary gyratory crushers and a further two HP500 for the secondary station. Further expansion saw the final Metso line-up comprising two Nordberg 50-65 gyratory crushers, three Nordberg HP500 secondary crushers, three HP500 tertiary crushers and five Barmac B9000's.

Two 100 t capacity feedbins pass the maximum 1m3 Grade 8 granite into both Nordberg 50-65 gyratory crushers to produce an output at -250 mm. Designed to produce 1200 t/hr during peak periods, both crushers have been operating at up to 1750 t/hr.

Dropping 30m through the primary stage, the crushed rock is fed onto a 70m long conveyor belt to the 70,000m3 primary stockpile.

Conveyed to the secondary feed chute, rock is separated into three screens; +150 mm is passed into three Nordberg HP500s, below -150 mm material is conveyed directly to the secondary stockpile some 1500m away on the opposite bank of the Xiaanxi Gorge.

Rated at 580 t/h, the HP500 produces an output of -80 mm, passed on a second conveyor line to the tertiary.

Held in an 110,000m3 stockpile, rock is fed through the pan feeds onto five conveyors, each serving three double screens. These produce five product stockpiles at 0-5; 5-20; 20-40; 40-80 and 80-150 for transportation on demand from the concrete plant.

Aggregates at 40-150mm can also be routed to the three Nordberg HP500 tertiary crushers when required to be crushed at 0-40mm, and routed back to the main stockpile.

Aggregates at 5-40mm can also be routed to the quaternary section and held in a 140,000m3 stockpile.

Five Barmac VSI B9000s at the quaternary station manufacture sand with finess modules of 2.8.

The Barmacs have a production throughput of up to 450 t/hr with screened sand being held in a 400,000 t sand stockpile or fed back to the entry stockpile to be recrushed further by the Barmacs.

In 1999 the operation produced 4.8Mt of sand and aggregates, increasing to a peak of 5.8Mt in 2000, and 4.2Mt in 2001. With peak demand passed and phase 2 nearing completion, a target of 1.3 Mt is predicted for 2002.

According to Li Bing, deputy chief engineer for the Xiaanxi project, the next major peak demand will be 2003-2004 for phase 3, when similar quantities will be needed.

During the peak demand periods in 1999 and 2000, the primary and secondary stations were operating two seven-hour shifts daily, whilst the tertiary and quaternary stations worked three shifts over a 20-hour day.

The majority of the crushed aggregate is being produced for the 378 Joint Venture, comprising the 3rd, 7th and 8th Bureaus of The China National Water Resources and Hydropower Engineering Corporation (CWHEC). Other customers include the Qing Yun JV and China Gezhouba Construction Group, who are both working on the spillway and left side dam construction.

Within the 378 JV, the 7th Bureau has responsibility for concrete production, the 8th Bureau for quarrying and crushing, and the 3rd and 8th Bureaus concrete placement as part of the dam and the power house.

Tertiary and quaternary section head for the Xiaanxi project, Chang Shuhe, describes the Metso Minerals Barmac crusher as being at the heart of the sand production.

China Gezhouba construction

The second crusher installation, with a 2600 t/hr production, is considered to be the world's largest single crushing operation.

Whilst Xiaanxi can be described as a 'horizontal' installation, CGGC Gushuling crushing plant has opted for a more compact 'vertical' secondary and tertiary installation, designed by Metso Minerals engineering team, with the footprint area of just 1200m2 and comprising three lines, two featuring a Nordberg HP300SH and a Nordberg Omni 1560SX.

During the peak period each of the three lines produced 600,000 t/month, but today production is currently nearer to 100,000 t/month.

From the primary, crushed rock at -150 mm is fed through a series of stacked double and triple deck screens and conveyed directly to five product stockpiles - 0-5 (considered as contaminated waste with too high mica content percentage and therefore not suitable for concrete production), 5-20 (making up to 30% of production), 20-40, 40-80 and 80-150.

To meet demand for different sizes, as required, material can be rerouted from the 20-150mm conveyor lines through the Nordberg HP300 tertiary, together with +150mm from the primary to be crushed to -80mm, screened and stockpiled.

Rock at 150 - 80 mm can also be fed through the secondary Omni 1560, screened and stockpiled.

All the granite crushed by CGGC lines and with a 198 MPA compressive strength, have been excavated from the proposed ship lock eliminating the need for any quarry activities. Eventually with a length of 1.6km, more than 40M m3 of granite will have been excavated to prepare for construction of two five-steplocks, each capable of carrying a 3000 t passenger or cargo boat.

The final crushed rock product is moved directly to site by conveyor, a distance of just 4.5km.

CGGC are only providing aggregate with no sand requirement, supplying a number of contractors in addition to their own needs for the shiplock and dam pouring.