Case Study: Thickening Solutions at Taiyuan Iron & Steel (Group), CO., LTD.

The successful installation by Outotec of one 53m diameter high rate thickener at TISCO Jianshan Iron Mine underlines the significant benefits of high rate thickening compared to conventional thickening. This one Outotec thickener has replaced two existing thickeners each of 53m diameter and achieved an increased tailings capacity in just one thickening stage. Since the installation, the resulting overflow liquor is of excellent quality, and easily reusable as process water. This installation by Outotec was the first of its kind in China, with more than 1000 installations globally.

Taiyuan Iron & Steel (Group) Co., Ltd. (TISCO) is a large iron and steel producer in China and an advanced stainless steel producer. As the largest subsidiary company of TISCO Group, the TISCO Mining Company sites include the Jianshan Iron Mine, Ekou Iron Mine, Yuanjiacun Iron Mine (under construction - 2009), Dongshan Limestone Mine and Daguanshan Dolomite Mine.

TISCO’s Jianshan mine lies in Loufan County, Shanxi province, China, 115km from the city of Taiyuan. Since founded in 1992, Jianshan has become the primary iron concentrate supplier for TISCO. The beneficiation process uses three stages of crushing, three stages of grinding, five stages of magnetic separation and anionic reverse flotation.

Previous primary thickening circuit

The old primary tailings thickening circuit at Jianshan consisted of a two-stage thickening process, requiring more time and energy to operate. Four tailings streams, including fine solids from reverse flotation (tailings stream No. 2-5), were split into two 53m diameter conventional thickeners, where they were thickened to an underflow density around 35%w/w solids. The underflow from both thickeners were combined with tailings stream No.1, a coarser solids stream from the magnetic separators, before being split into another two 53m diameter conventional thickeners. The final underflow at 40-45% solids was pumped to the tailings dam, with overflow from each thickener recycled to the plant.

Originally, the raw ore capacity in 2002 at Jianshan was 6Mtpa, with 3.3Mtpa tailings flow. Following the 2nd expansion in 2006, the plant capacity was increased to 9Mtpa, with tailings increased to 4.87Mtpa, necessitating improvements to the tailings system to cope with the increased capacity.

The old Jianshan circuit had a number of key challenges:

• Restricted tailings capacity due to poor performance of existing thickeners (throughput and density)
• A high content of fine solids in the recycled water from the thickeners, adversely affecting the flotation circuit and causing loss of iron in the tailings

Test work

In 2006 TISCO commissioned Outotec in Shanghai to conduct test work for the optimum design for Jianshan’s iron tailings thickening. Comprehensive dynamic test work was carried out, using a 94mm diameter bench-scale test unit. The underflow yield stress was measured with a Haake rheometer to enable modeling of the rake torque requirements.

During the test work, underflow densities ranging from 51.5~58.4%w/w solids were obtained with the 94mm diameter test rig, operating at flux rates ranging from 0.20~0.40t/m 2 hr. Flocculant consumption was approximately 20~40g/t.

Outotec developed the high-rate bench-scale test thickener for use as a dynamic test method, with feed and flocculant continuously fed into the test thickener. The unit is operated at the same process conditions as the full-scale unit, hence the process simulates the performance of the high-rate thickener. This has been verified by more than 1000 cases of test work correlating closely with successful thickener installations that follow. Underflow and overflow are analyzed to provide data to determine the sizing and performance of the thickener.

Optimum Thickener

Outotec’s high rate thickener (HRT) was the optimum thickener solution as the TISCO plant process required higher throughput and a higher underflow density. Alternative choices might have been a high compression thickener (HCT) or paste thickener (PT), both of which produce higher underflow density, but with a slurry yield stress of just 113Pa at 58.4% underflow density, these choices were not required. The key to a successful high-rate thickener installation is the effective use of flocculant with correct feed dilution to achieve a consistently high throughput (tonnes per hour per square meter) at high underflow density.

High rate thickening benefits

The heart of an Outotec high rate thickener is the patented “Autodil” self diluting high rate feedwell, which incorporates the following benefits:

• Improved floc formation due to optimum mixing and retention time
• Maintains density in the feedwell for optimum flocculation, irrespective of feed flow rate or density
• Consistent feed distribution to thickener tank
• No short-circuiting of slurry
• Effective dilution due to Autodil
• No separate de-aeration tank needed

One stage thickening

After installation, the new 53m Outotec high rate thickener receives flow from tailings stream No.1 plus all flow from the plant expansion. Tailings streams No.2-5 from the old system still go into two shallow tank wall thickeners. The 53m Outotec thickener was required to produce a nominal capacity of 3.67Mtpa and maximum throughput of 4.65Mtpa, but in practice, the throughput has reached approximately 5.26Mtpa. From initial testwork, however, it is evident it should be possible that an even higher throughput of more than 7.00Mtpa could be achieved. The new Outotec thickener has been successful in overcoming the previous process limitations. In the former tailings circuit, two stages of the thickening process were needed to treat tailings at 3.3Mtpa. Now just one high rate thickener can treat more than 5.2Mtpa, to produce 53-60%w/w solids underflow density, which represents an improvement of between 9-15% in pumping efficiency and tailings system capacity, according to Tisco. As underflow is now at a higher % solids, this reduction in pumping volume results in improved pumping efficiency.

Operational savings

Significant operational savings have resulted after the installation. Power consumption has dropped as a result of fewer and more efficient thickener drives and a lower underflow pumping rate. According to a spokesperson from TISCO, “The electricity cost saved in one year was around RMB 1.5 million”.

Overflow clarity was dramatically improved from 8.0% w/w to consistently less than 150ppm. The much-improved overflow clarity has allowed process water to be captured more efficiently and reused as process water. This had a compounding benefit to the process, which saw improvements in flotation, and less iron fines lost to tailings.

Less maintenance

In addition to the high throughput and clear overflow liquor, the Outotec high rate thickener has also been a success with the TISCO operations and maintenance staff, as it provides consistent trouble-free operation and maintenance. The thickener utilizes the Outotec central column drive system, incorporating a hydraulic power pack, multi-pinion, and slew ring drive. It also offers three levels of drive safety protection, i.e. rake lift, electrical trip protection and hydraulic relief valve physical protection, which result in a safer and more reliable tailings disposal system.

The future

The new 53m thickener has successfully achieved both higher throughput and underflow density and also simplified the tailings circuit.

With clean overflow liquor and higher density underflow solids, this thickener is consistently effective in handling the required duty. Outotec’s process technology experience has again proven to be of a high standard, with the solution for the Jianshan tailings system producing many benefits for the plant process.

TISCO Jianshan and Outotec have both been very satisfied with the performance of this 53m high rate thickener and both groups are looking forward to future thickening projects.

*This case study has been written prior to the merger of Metso and Outotec.