Lime Slaking and Wet Limestone Ball Mill Grinding Process overflow with a New Approach for Particle Size Reduction

Abstrac

Being an integral part of the typical FGD system, the design and performance of the reagent preparation system will influence overall process effectiveness. In providing the most cost effective and energy efficient FGD system design, the options and advancements in size reduction processes urge consideration. Presented is an analysis of an attrition mill option, which can be used to demonstrate the opportunities available when applying comminution processes to the FGD industry.

Introductio

Horizontal ball mills have dominated as the equipment of choice for limestone grinding functions within wet flue gas desulfurization (WFGD) systems. Recent years have led to a more wide spread investigation and acceptance of alternative mill system designs. Mill types considered within the industry have ranged from a variety of media mills to dry roller mills. The science and application of size reduction processes is a complex subject. Similarly, the design of the FGD system is also complex and can be influenced by many factors. Determining the most cost effective and energy efficient grinding method for each application can require an exhausting study.

The detailing of optional designs with significant capital cost, power and operational information is time consuming to produce and evaluate. Presented is a review of limestone grinding for FGD systems, with an attrition mill option exemplifying how advancements in comminution processes influence overall FGD system design. Relative grinding energy efficiencies are presented to assess the impact of operational power consumption and installed power. The goal is to shed light on overall objectives and design path alternatives.

Wet FGD Limestone Grinding

The wet horizontal ball mill has typically been the workhorse for limestone slurry preparation within a wet FGD system. Undoubtedly, the horizontal ball mill has certain characteristics that suit it well to FGD service, these primarily being:
Wet grinding capability
Large reduction ratio capability (ability to take a 1" feed size to a micron range product)
Resistance to abrasion
Relatively low operation, control and maintenance requirements.

Wet grinding is usually preferred over dry grinding. When looking at the grinding process in isolation, there are pros and cons of wet versus dry. Dry grinding requires more power, yet has less ball and liner consumption that in wet grinding. Additionally, the capacity of a dry mill is typically less per unit mill volume than it is with wet milling.However, it is the overall FGD system design and use requirements that push the selection in the direction of wet grinding. The two main reasons are that the ground product is utilized in a wet process and that the received condition of the stone can be one of relatively high moisture.

In addition to the higher dry milling energy required, when feed stone surface moisture rises above 2-3% by weight, a heated air source to accommodate drying is typically required. This usually leads to unacceptably high energy costs. Indeed, in most industries, the increased energy consumption and the capital cost of the equipment can swiftly move the overall economics away from dry grinding when a wet slurry is a desired end product.A commonly specified FGD limestone size for feed to the mill system is 3/4” x 0”. This is a loose description of a size distribution which can be thought of as all of the particles smaller than 3/4” with the distribution extending into a fines range below 20 mesh. Size ranges described in this manner are variable in practice and can have occasional 1 to 3” tramp material and/ or higher percentages of fines. The horizontal ball mill is capable of accepting this stone size direct, without any pre-crushing function, thus improving system flexibility, simplicity and reliability. Abrasion is controlled with wear resistant mill liners and by the inherent concentration of the wear in the media.

Alternative Design

An alternate wet ball mill design that has gained acceptance in the FGD industry is the tower mill or vertical ball mill. Vertical ball mills are a form of ‘stirred’ ball mill and can be more energy efficient than typical horizontal ball mills. Their increased efficiency is partially explained by placing the grinding action in solids beds that are of a more optimum thickness or porosity. The variation in the transport of the feed through the mill, the distribution of feed in the ball charge media, and the method of energy transfer to the media are also factors which help explain the difference in efficiencies.

Furthermore, tower mills are typically fed with a 1/4” x 0” feed size. While this necessitates the use of a pre-crusher when a larger sized stone is received, it allows for the mill to be sized for a smaller reduction ratio and a more optimum media charge size distribution.

Wet FGD Limestone Attrition Milling

Attrition milling is another approach beginning to gain acceptance in FGD. Attrition milling is also not new. Various forms of attrition milling have been used across a wide range of industries and applications, such as paints, pigments, pharmaceuticals, ceramics and coal slurries. The attrition mill could be considered on the other end of the loose generic description of stirred media or stirred ball mills. Similarly, some have classified the vertical ball mill as being in the family of attrition mills.

The tailoring of the mill design to the specific application can be critical with attrition mills, as they operate at higher speeds, smaller volumes and higher localized energy concentrations. This mill type was chosen as an example of how mill system advancement, optional mill system arrangements and sparing philosophies can impact overall system design and process effectiveness. The mill was designed specifically for limestone grinding with a reduction ratio equivalent to that typically applied to the vertical ball mill in FGD service. This is taken to be the comminution of 1/4” x 0” mill feed size to a 95% less than 325 mesh product size. Since the design of the attrition mill is suited to obtaining extremely fine grinding, finer product sizes are possible and are evaluated to some extent later.

Limestone Storage & Handling

The moisture content of the stone can be controlled somewhat by the upstream handling system design. Covered storage, protecting against direct rainfall, can prove economical if significant drying and/or handling system costs are saved. Purchasing larger ‘run-of-crusher’ stone can reduce the moisture levels in the limestone, as the maximum amount of moisture bulk stone can contain is a function of the fines content. Larger stone sizes with less surface area allow for more natural draining and reduced moisture content. The drawback is the need for an additional pre-crusher. However, pre-crushing operations can improve overall energy efficiency and apply well to the tower mill and attrition mill system designs.

The storage requirements for the ground product can impact the system design capital costs, energy consumption and arrangements. One cubic foot of limestone slurry at 30% suspended solids weighs approximately 77 pounds and contains only 23 pounds of limestone. Whereas, one cubic foot of 3/4” x 0” stone weights about 90 pounds and contains 90 pounds of limestone. This reiterates the knowledge that dry storage is more efficient use of storage volume compared to that with a wet milled product.

A multiple attrition mill system can be more ideally suited to system designs where the limestone is bulk stored on the ground and reclaimed to individual surge bins upstream of each mill. However, incorporation of larger day silos it certainly possible as it is with the typical horizontal ball mill system. In general, the entire system from limestone unloading to the final milled product should be evaluated to ensure possible synergies are identified and taken advantage of. In this manner, the overall plant storage, processing efficiency and reliability are maximized.