Permission to use the attached copyrighted document provided to the U.S. EPA by email dated 

November 3, 2009 from J. Sloesen, Weir Minerals Netherlands b.v., to Steve Souders, U.S. EPA. 
GEHO® Info for sales representatives 

Volume 2, No. 5, May 1992 

Economical ash handling system 

Ash production 

Coal-fired power plants produce large quantities of fly and bottom ash. The ash content 

of coal burned varies from 5 to 35%. In particular power plants in coal producing countries burn coal with a minimum of washing, 

hence with a large ash content. A 1000 mw 

power plant may produce as much as 3,000 

tons of ash per day. 80 to 90% of the ash in 

the coal is recovered as fly ash from the flue 

gas by electrostatic precipitators. 10 to 20% 

remains in the bottom of the combustion 

chamber as bottom ash. Some of the ash is 

sold (construction industry) depending on the 

quality, whilst the remainder has to be disposed of in the most economical way, consistent with local geographic, statutory and 

environmental considerations. 

Ash disposal 

The ash is generally transported from the 

power plant, hydraulically, pneumatically or 

by mechanical means (trucks, conveyors) to 

form: 

1. Ash dumps: mostly existing of a combination of bottom ash and fly ash. 

2. Natural ash dams: fly ash is pumped at 

low concentrations into natural geographic areas, probably locked by a dam. 

3. Artificial ash dams: using bottom ash to 

form dykes. 

4. Backfill: used to fill stopes in underground 

mines or to fill old open mine pits. 

The hydraulic transport of fly ash has been 

largely in the form of low concentration slurries i.e. 10-30% solids by weight. 

However, continued use of dilute slurry 

pumping has been questioned in view of: 

• large quantities of contaminated water 

being recirculated, 

• problems of controlling scale build-up and 

wear, 

• potential for leakage/discharge of ash 

water from the dams and for ground water 

pollution, 

• danger of ash dam wall failure, 

• pollution of surrounding terrain by windborn dust, 

• high cost of large volume dam construction, 

• cost and difficulty associated with stabilizing and restoration of the site, 

• use of multiple, energy-inefficient and 

expensively maintained centrifugal pumps 

for transport of large volumes of abrasive 

diluted slurry and contaminated return water. 

Dense ash slurry disposal 

Environmental constraints on ash disposal 

have limited the options and have increased 

costs. The problems are substantially eliminated by preparing ash slurry at high density 

i.e. low water content for discharge onto a 

sloped disposal where the deposit stabilizes. 

• Dense fly ash slurries use less water than 

dilute slurries. Typically, the volume flow 

decreases by a factor 6. 

• With the natural retention of slurry water 

by dense ash slurry the need for an ash 

dam water reclaim system and return water lines can often be avoided. 

• When used as mine backfill it does not 

add to the mine dewatering capacity and 

solids loading of the mine water. 

• Dense fly ash slurries do not flow out to a 

horizontal surface but form a shallow 

cone from the point of discharge. A typical slope of 2-6% will be generated in the 

ash dam. 

• The ash will dry out to form a fairly hard 

surface, unaffected by rainfall and not 

prone to dusting in windy conditions. 

• Savings in pumping-energy can amount 

to 50% or more. The thickened disposal 

method is also applied to tailings from 

minerals processing plants, such as red 
mud, where operations are limited by water supply, environmental, or operating 

cost constraints. 

Typical slurry behaviour 

Dense ash slurries exhibit typical properties, 

which are to be accounted for. The ratio of 

coarse and fine material in a slurry generally 

is critical for the pumpability. Experience has 

taught that dense fly ash slurries are capable 

of supporting crushed bottom ash in their 

naturally produced ratio. However, different 

ashes will produce similar rheological properties at different solids ratios. 

• Relatively low settling occurs which allows variable pumping rates. Above a 

critical concentration no particle settlement occurs. 

• Wear behaviour of abrasive particles is 

less because of turbulent flow and low 

flow velocity. 

• Scaling problems are largely eliminated. 

• Dense fly ash/bottom ash slurries can be 

transported at low flow velocities in mild 

steel piping without much wear. 

• Care must be exercised in that viscosity 

and pipe friction show exponential increase with increase in solids concentrations beyond a critical value. 

• Addition of bottom ash lowers pipe friction loss at the same solids concentration 

by weight. 

• Fly ash chemically reacts with water. 

Proper wetting and mixing fly ash with 

water is complicated. 

• Positive displacement pumps are required to transport dense ash slurries. 

GEHO® technology and equipment 

The preparation, pumping, pipeline transport 

and disposal of dense ash slurry is a challenge, which requires best of technologies 

and equipment. As a leader of high pressure 

slurry, sludge and paste pumps throughout 

the world, GEHO® PUMPS has the knowhow, experience, equipment and partnerships 

to handle complete power plant ash projects. 

Based on slurry sample testing and experience either a GEHO® crankshaft driven piston diaphragm pump or hydraulically driven 

piston pump is selected. The preference for 

diaphragm pumps is based on their high 

pressure capability, high efficiency and the 

fact that the valves only are subject to wear 

from the highly abrasive ash. These pumps 

allow slurry transport over a long distance, 

whilst they have a proven high up-time and 

low parts and maintenance needs. The ash 

slurry preparation plant is designed for continuous through-put and uses special equipment, instrumentation and process controls. 

The dry fly ash and bottom ash are unloaded 

from the silos and individually fed into a high 

density mixer by special-design conveyors. 

The mass flows are controlled to a predefined 

ratio. The water flow to the mixer is adjusted 

to control viscosity of the dense slurry mixture. An intimately mixed, homogeneous and 

stable slurry is obtained by additional agitation. 

The GEHO® scope of supply will vary from 

basic engineering to a complete and operational preparation plant depending on individual project and customer requirements. 

 

Weir Minerals Netherlands b.v. 

P.O. Box 249, NL-5900 AE Venlo 

Phone: +31(0)77-3895200 

Fax: +31(0)77-3824844 

Email: weir@weirminerals.com

Website: www.weirminerals.com

Copyright © 1992, Weir Minerals Netherlands b.v. All 

rights reserved.