Design Considerations
The following sections describe the major design decisions that were made during the course of the project.

Installation Type – Retrofit or Standalone?
The iron ore concentrate slurry is stored in a 15m diameter, 15m high steel tank equipped with a 4.25m diameter single pitched blade turbine impeller agitator, as shown in Figure 3. The slurry is pumped from the tank to the vacuum disc filters. The slurry heating system needed to be installed either in one of the existing slurry storage tanks, or in a separate system in-between the storage tanks and filters. This was the first main design consideration of the project. Retrofitting the existing tank would require sealing the top of the tank to be able to direct the exhaust gases to the external Heat Recovery Unit (HRU), while constructing a standalone system would require a new tank to house the five burners required and scarce real estate on which to place the tank.

Chamber Cooling
The top portion of the combustion chambers, above the static liquid level, are normally cooled by the wave action and turbulence caused by sparging the combustion gases through the liquid. In this case, the specific gas release area (area of the tank to volume of gas released) was considerably higher than traditional design for SubCom, the specific gravity (SG) of the liquid was greater than two and the viscosity was 5cP. It was the highest SG and most viscous liquid Inproheat had heated. Inproheat was concerned that the high SG and viscosity, and lower than usual specific gas release area might reduce the wave action enough to limit the cooling effect on the combustion chambers.

A slurry jet cooling system was devised by Inproheat and tested at at the University of British Columbia’s (UBC’s) NBK Institute of Mining Engineering. The slurry jet cooling system comprised four nozzles that sprayed a slipstream of slurry, pumped from the storage tank, onto the top portion of a full scale mock-up the upper section of a combustion chamber. The flow rates, nozzle design, and nozzle arrangement were developed from the test work.

Chamber Design
The SubCom combustion chambers normally have an enclosed bottom to minimise or eliminate liquid level fluctuations inside the chambers due to hydraulic wave action created by the gas sparging. The combustion chambers fill with liquid when the combustion air blower is shutdown. The blower then evacuates the chambers by pressuring them during start-up.

There is concern that settled slurry inside the chamber may be difficult to displace. Therefore, Inproheat designed the chambers with a larger than normal drainage orifice to reduce the amount of solids retained inside the chambers and for the slurry to be forced out of the when positive pressure is applied by the blower. In addition, Inproheat also designed and incorporated a new bottomless chamber as a trial to assess the performance characteristics between the two chamber designs.

System Components Description
The Inproheat SubCom system supplied to the customer was designed to heat 403 m3/h of iron ore concentrate slurry from 25°C to 60°C. The SubCom system consisted of five independent burner systems, each with a heating capacity of 12 GJ/h. The burners were designed to be natural gas fuelled.

The SubCom combustion chambers were suspended in the top of a slurry storage tank, shown below in Figure 3. The blowers and fuel train frames with Burner Panel were located on the tank top beside the combustion chambers.

A Heat Recovery Unit (HRU) preheats the incoming iron ore concentrate slurry by contacting SubCom exhaust gas with the feed slurry.

Figure 3 – Slurry Storage Tank Heater Elevation Layout

General Process Description
Slurry, at 25°C, is pumped to the HRU where it contacts exhaust gas from the slurry storage tank. The gas is hot (about 60°C) and saturated with water vapour. When the slurry and gas mix, sensible heat and latent heat from the condensing of water vapour are transferred to the slurry.

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The cooled gas is exhausted out the HRU stack. The heated slurry flows by gravity from the HRU and drops into the slurry storage tank.

The five SubCom burners heat the slurry in the slurry storage tank to 60°C. The temperature of the slurry in the tank is monitored by a thermocouple in the tank. The slurry level in the tank is controlled by a level control loop that modulates the pumping rate of slurry out of the tank. A software low level switch, backed up by a hardwired mechanical float-type level switch, ensure that the burners will not operate if there is not enough slurry in the tank to cover the combustion chambers. If the level is too low, the fuel supply to the burners is disabled by closing of the main gas shutoff valves.

If the design heating capacity is required, all five burners will be running. If conditions change that lowers the heating input requirement, some burners can be put into the low-fire mode, standby mode or completely shutdown. Some conditions that would reduce heat input requirements are: reduced feed flow rate; higher than design (25°C) feed temperature; and lower than design (60°C) output temperature. In low-fire mode, the burner output is about 30% of the maximum heat output. In the standby mode, the fuel to the burner is shutoff but the blower continues to run. This prevents the combustion chamber from filling with slurry and maintains system readiness for immediate re-firing of the burner.

A stream of slurry taken from the incoming feed line is distributed to each burner chamber. A series of four nozzles sprays slurry onto the upper region of the combustion chamber to keep it cool.

Heat Recovery Unit
The exhaust gases from the submerged combustion process, at 60°C, contain a significant quantity of recoverable heat. The Heat Recovery Unit (HRU) transfers some of that heat into the slurry that feeds into the slurry storage tank.

The HRU is a 2.4m diameter vertical vessel with a cone bottom. Cold (25°C) slurry feeds by a 200mm diameter line into the apex of the cone bottom. The slurry flows upward inside the HRU tank. SubCom exhaust gas from the slurry storage tank is forced through a rectangular duct that feeds into the side of the HRU vessel. A 1.8m diameter gas chamber, similar to the combustion chambers, conducts the SubCom exhaust gases into the HRU vessel and sparges the gas into the slurry through a series of holes in the side of the chamber. Heat transfer and condensation of water vapour occurs on contact between the slurry and the gas. The heated slurry then overflows over an internal weir, and drops through a discharge launder that transitions into a 250mm diameter pipe into the slurry storage tank. The discharge pipe outlet is submerged in the slurry so that it acts as a seal leg at all times.

Cooling Slurry System
With conventional SubCom systems, the chambers are mounted in small tanks. In the confined space, the high amount of turbulence created by the release of the combustion gas from the liquid causes splashing of liquid onto the chamber cone. This fluid helps to cool the cone. In the large 15m diameter slurry storage tank there may be insufficient liquid turbulence on the surface of the slurry around the combustion chambers to provide the necessary amount of cooling. Therefore, a system to spray slurry onto the upper cone of the chambers has been provided. The cooling system consists of four spray nozzles that direct a stream of slurry onto the discharge throat of the burner. The slurry then flows down the throat and onto the chamber cone.

Conclusion

The project is currently in the final stages of construction, with start-up expected this summer. Everyone is excited and confident about the commissioning. However, several questions about how the system will function in a dense slurry are unknown. Questions such as:

• How will the slurry react to being forced out of the combustion chambers on start-up? Will the combustion chambers be required to be bottomless to evacuate the chambers during start-up? The blowers have been oversized to ensure success, but nevertheless it is an unknown.
• Will the cooling slurry system be required or will the bubbling action of the system provide enough wave action to cool the upper sections of the combustion chambers?

The customer expects a greater than 30% efficiency gain at the vacuum disc filters and fuel cost savings of nearly $1M at the pelletising furnace by heating the iron ore slurry.

References
Häkkinen, Antti and Ekberg, Bjarne, 2009, Dewatering of iron ore slurry by a ceramic vacuum disc filter, p. 6, ICheaP-9; The Ninth International Conference on Chemical & Process Engineering.

Anecdotal information from AHMSA.