Typical process and common problems of slag vertical mill grinding

In recent years, the slag grinding process and technology using vertical mill have been developed rapidly in domestic. The process is mainly composed of vertical mill, hot air furnace, bag filter, return elevator, feeding system, main exhaust fan and chimney.

Process introduction:

The blast furnace slag is conveyed to the pneumatic double flap valve through the belt conveyor, and then enters the vertical mill. The slag is ground into fine powder inside the vertical mill, and at the same time, the slag is sent to the hot air by the hot blast furnace for drying.

The fine powder separated by the powder concentrator is transported by hot air to the main dust collector for collection, and the collected fine powder is transported to the finished product warehouse through the air conveying chute.

Some of the fine powder and metal particles that cannot pass through the powder concentrator enter the material external circulation system of the mill through the return pneumatic double flap valve. The return material is transported to the return bucket elevator by the return belt conveyor. The outlet of the bucket elevator is equipped with a pneumatic two-way valve. Under normal production conditions, the return material enters the magnetic drum separator for iron removal, and then enters the vertical mill again through the rotary air lock valve.

Under special circumstances, the pneumatic two-way valve can discharge materials directly to realize the unloading of the mill.

2. Common problems in slag grinding process

(1) Selection of slag yard

Slag yard includes three forms: open yard fed by forklift, slag shed fed by grab and stacker reclaimer. For the production line with the annual output of slag powder less than 2.4 million tons, the first two types of storage yards can be selected. For the production line with the annual output of more than 2.4 million tons, the stacker reclaimer feeding mode should be selected as far as possible.

For the slag powder production lines with the first two types of storage yards, if the plant site is located in a rainy area, the slag storage shed fed by the grab shall be used as far as possible; On the contrary, the open-air storage yard with forklift feeding can be used to save the cost of civil engineering.

(2) Selection of installation position of metal detector

In some slag grinding process designs, metal detectors are usually installed on the grinding belt conveyor, but this design method has the following problems: if the accuracy of the metal detector is adjusted higher, the grinding belt conveyor will frequently remove the slag containing metal iron, making the thickness of the material layer inside the vertical mill unstable; On the contrary, the metal detector basically loses the function of monitoring the metal containing iron slag, making a large number of iron containing slag enter the vertical mill. The above two conditions have adverse effects on the stable operation of the vertical mill.

The metal detector can be installed on the belt conveyor in front of the batching bin to ensure that the slag entering the batching bin is qualified raw materials. The slag in the batching bin is stably fed to the vertical mill through the quantitative feeder and belt conveyor to ensure the stable operation of the vertical mill.

(3) Selection of hot blast stove

The hot blast stove of slag powder production line includes gas stove and coal fired stove: the gas stove is the most commonly used blast furnace gas stove, and the coal fired stove is the most commonly used fluidized bed stove and pulverized coal stove.

If blast furnace gas can be used in the plant area, it is recommended to use blast furnace gas stove, which can greatly reduce equipment, civil engineering costs and operating costs; If there is no condition to supply blast furnace gas in the plant area, it is recommended to use pulverized coal boiler. Although the one-time equipment investment of the plant using fluidized bed furnace is low, from the perspective of long-term operation, it has the disadvantages of high coal consumption, poor working environment for workers, and inability to achieve remote control.

(4) Selection of dust collector

There are two types of dust collectors: single air inlet and double air inlet. It is recommended to use the single air inlet type dust collector. Although the double air inlet type dust collector is shorter than the single air inlet type dust collector, it can reduce the civil construction cost of the dust collector workshop. However, most users who use double inlet dust collectors report that the two air inlets of this type of dust collectors are prone to unequal wind pressures.

(5) Selection of main exhaust fan

The main exhaust fan has two forms: single air inlet and double air inlet. The specific form used in the design shall refer to the recommendations of the vertical mill manufacturer, and the performance curve of the fan shall also be confirmed by the vertical mill manufacturer.

In most grinding systems at present. Generally, the opening of the valve in front of the exhaust fan is used to adjust the air volume, and the air volume is increased by adjusting the cold air valve in front of the mill inlet without significantly reducing the temperature of the mill outlet gas, but this often leads to a decrease in the negative pressure at the mill inlet. Practice shows that it is the most economical way to directly adjust the fan ventilation by adjusting the fan speed.

(6) Selection of vertical mill

At present, there are many manufacturers at home and abroad that manufacture vertical mills for grinding slag, but the vertical mills produced are similar in form and can be generally divided into vertical mills with hydraulic roll turning device and vertical mills without hydraulic roll turning device. The main difference between the two is the surfacing cycle and the surfacing time. The former is shorter than the latter, but the time required for each surfacing is also shorter than the latter. Therefore, when users choose the vertical mill of the main equipment, they can determine it according to the sales rules of local slag powder products and their own operating characteristics.

(7) Selection of level gauge for slag powder silo

There are two kinds of level indicators used at the top of slag powder silo: radar level indicator and heavy hammer level indicator. Some users reported that the radar level gauge is not easy to be damaged, but it is easy to be mismeasured; The heavy hammer level gauge is accurate, but the heavy hammer chain is easy to break.

In view of the above situations, radar and heavy hammer can be used at the same time for the material level gauge on the top of the silo. Both of them can learn from each other and make up for each other. If one of the material level gauges fails, the operator in the central control room can find it as soon as possible.

(8) Selection of bulk dust collector at the bottom of slag powder silo

The space height between the bottom of slag powder silo and bulk platform is limited, so it is difficult to meet the requirements for replacing filter bags at the top of a single dust collector here. Therefore, the filter cartridge type dust collector can be selected or the equipment manufacturer can be required to make a dust collector for replacing filter bags at the side. However, both have their own shortcomings. The former has an unsatisfactory dust collection effect, while the latter cannot add an external insulating layer to the dust collector.

As a one-stop grinding equipment provider, SMMVIK can also be customized and configured flexibly according to your requirements. Generally, multiple machines are used, including coarse crushing, fine crushing, sand making and screening. It can be customized for you according to different needs. Interested friends can consult online or visit in person.