FIELD OF THE INVENTION

The present invention relates to mine dewatering operation, more particularly to a method and a system for mine dewatering operation through integrated and optimal operation.

BACKGROUND

Mining involves extraction of valuable minerals from the earth by excavation. The depth of the mine can vary between few hundreds of meters to few kilometers. The development of mine generally involves penetrating through the local or regional water table causing inflow of ground water into the mine resulting in unsafe operating conditions. Dry working conditions are preferred in order to reduce wear and tear of the machine and maintain safe working environment. Therefore, water from the mine is removed by pumping it to the surface through mine dewatering operation.

Generally, the system for mine dewatering operation includes pumping stations, intermediate storage tanks, energy recovery devices etc. Sometimes, the system is coupled with refrigeration unit or plant in order to reuse water from the mine to maintain ambient working temperature in the mine.

Typically, the system for mine dewatering operation comprises of multiple pumping stations, pipes and storage dams at different depth or level of the mine. Each pumping station has different number of pumps, around 4 to 10 pumps that run parallel. The water from each pumping station is pumped to intermediate storage tanks, and is subsequently pumped to the surface. Also, the intermediate storage tanks may receive water from sideways of the mine, in varying quantity.

In some regions, the temperature inside the mine will be varying as high as 45 °C to 50°C. In order to provide ambient working temperature in the mine, ventilation fans are operated inside the mine to maintain the ambient working temperature. In some mines, part of the mine water pumped to the surface is refrigerated, cooled and recycled back to the mine in order to provide ambient working temperature in the mine, besides using the ventilation fans for the purpose. The refrigerated water flowing from the surface into the mine has lot of potential energy that is extractable. This is achieved using different types of energy recovery devices such as Three Chamber Pipe System (3-CPS), Turbine Pump or Turbine Generator or the like.

A mine dewatering system controls water levels in the storage tanks by manipulating the pump speeds of the pumps corresponding to the storage tanks using simple control algorithms such as PID (proportional, integral and derivative) controller. This approach does not consider the interaction between different pump stations and storage tanks. For example, a change in water level in a particular storage tank due to sideways water in the mine is controlled only by controlling the pump speeds of upstream pump station only using PID controller though the level in that storage tank is dependent both on the upstream flow (going out of the tank) and downstream flow (coming into the tank). Similarly, ventilation system is also operated independently by manipulating fan speeds to provide optimal air circulation inside the underground mines. In addition, if the mine has water re-circulation units for cooling air in the underground mine, the interaction between fan ventilation and water cooling may not be considered. For example, in order to control the temperature of air in the underground mine, one can use both refrigerated water that is being re-circulated, and also ventilation fans. In some of the prior-art practices, the amount of cooling load taken by refrigeration and ventilation units are fixed and may not be optimal. Therefore, operating the pumps and fans independently, without considering the interaction between dewatering and ventilation units may results in higher energy consumption. Hence, there is a need for a solution that is more efficient and cost effective in dewatering operations considering interaction with ventilation system.

SUMMARY

In one aspect of the invention, a method for mine dewatering operation with a mine dewatering operation management system in an underground mine is provided. The method considers the interaction between dewatering and ventilation operations.

The method comprises steps of obtaining plant data in relation to mine dewatering and ventilation operation in the mine dewatering operation management system through measurements using sensors in the plant units and devices used in the mines. The dewatering operation management system includes a plurality of process models (integrated process model) for plant units and devices used in the mine dewatering and ventilation operation and some of the plant data may as well be estimated using the models. Model parameters (coefficients) for the plurality of process models for the plant units and devices used in the mine dewatering and ventilation operation are determined using minimization of error techniques to have the plurality of the models represent the mining system (model tuning). An objective function is formulated for minimization of energy consumption in the mine dewatering and ventilation operation using the tuned plurality of process models and the objective function is solved. Optimization using the objective function for minimization of energy consumption in the mine dewatering and ventilation operation is performed by providing determined set points through optimization (solution) for optimized mine dewatering and ventilation operation to the DCS controllers and other controllers used in the mines for controlling the plant units and devices around the set points for minimized energy consumption in the mine dewatering and ventilation operation.

The objective function used for performing optimization includes terms for energy consumption for each of the plant units and devices in the dewatering and ventilation operation and the set points for minimized energy consumption is provided to control speed of pumps used in the dewatering operation, speed of fans used in the ventilation operation, re-circulated water flow rate used for refrigeration used in dewatering operation and air flow rate used for cooling in the ventilation operation. The plant data includes one or more of plant data pertaining to water flow in the dewatering unit, air flow in the ventilation unit, pump pressure in the dewatering unit, air pressure in the ventilation unit, water level in storage tanks in the dewatering units, valve openings for re-circulation in the dewatering units, pump speed in the dewatering units, temperature of air in the underground mine, and fan speeds in the ventilation units.

In one embodiment, the plurality of process models includes models pertaining to pumps, storage tanks , pipes, ventilation unit, energy recovery device or suitable combination thereof.

In another embodiment, the method includes performing optimization to provide optimal set points includes providing optimal set points for pumps in the dewatering operation, fans used in the ventilation operation, recirculated water flow rate in the dewatering operation and air flow rate in the ventilation operation considering operational constraints such as water level in the storage tanks, speed of the pumps, head of the pumps, speed of the fans, temperature of air in the underground mine or combination thereof.

In another embodiment, the method for optimization includes an objective function for minimizing cost by operating the plant with optimal set points considering one or more power tariff varying over time. Also accordingly, the plant units and devices may be scheduled.

In another embodiment, the plurality of process model captures the interaction between the mine dewatering operations and ventilation operations by controlling air temperature of the underground mine through control of re-circulated water flow rate in the dewatering operation and air flow rate in the ventilation operation.

In another aspect of the invention a mine dewatering operation management system for mine dewatering operation in an underground mine is provided. The system comprises a measurement and control units for obtaining plant data in relation to mine dewatering and ventilation operation including a plurality of process models for plant units and devices used in the mine dewatering and ventilation operation, and an advanced optimization module for tuning the plurality of process models and optimization of dewatering and ventilation operation to minimize energy consumption in the dewatering and ventilation operation by providing set points for optimal control of plant units and devices used in the mine dewatering and ventilation operation.

In yet another embodiment, the advanced optimization module uses the plurality of process models for plant units and devices used in the mine dewatering and ventilation operation for minimizing energy consumption in the dewatering and ventilation operation by controlling the air temperature in the underground mine for optimized operation of dewatering operation, ventilation operation and the interaction between the dewatering operation and ventilation operation. BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the accompanying drawings in which:

Fig. 1 shows a schematic representation of mine dewatering plant including ventilation system in accordance with the invention;

Fig. 2 illustrates a mine dewatering system in accordance with the mine dewatering plant of Fig. l;

Fig. 3 illustrates input and output parameters associated with models used in a mine dewatering system; and

Fig. 4 illustrates exemplary benefits in energy consumption achieved through use of the invention.

DETAILED DESCRIPTION

The invention is described further herein after with reference to Figs. 1 to 4, and with reference to non-exhaustive exemplary embodiment.

In Fig. 1, a mine dewatering plant (100) is shown having a plurality of pump stations (101a, 101b, ... , 101η). Each of the pump station has a plurality of pumps working in parallel, for pumping water in the mine. Each pump station (101a, 101b, 101 n) pumps water into its respective or corresponding storage or underground dam (102a, 102b, 102n). The water from sideways in the mine also gets collected into one or more of these storage or underground dams.

The water so pumped at different stage or levels in the mine, into the corresponding storage or underground dam is pumped into the surface dam (103).

The water from the surface dam (103) may be used by a refrigeration unit (104) for cooling the water and circulated back into the mine for providing ambient working condition in the mine, may also be provided by the cooling fans of the ventilation unit besides the refrigeration unit.

The water from the refrigeration unit has considerable potential energy that can be effectively used towards operating the mine dewatering plant. Accordingly, an energy recovery device (105) is provided therein to recover the said energy from the water coming out of refrigeration unit (104). The water from the refrigeration unit (104) is also used in maintaining ambient working temperature in the mine. Also, in general, a ventilation system (106) that consists of fans can be used in order to maintain sufficient air with ambient temperature in underground mines.

Referring to Fig. 1 , the system has measurements from the plant includes water flow rates from the pump station, air flow rates from ventilation unit, water levels in the storage tanks, head developed by the pumps and fans, and temperatures in the underground mine which are respectively indicated as FTs, LTs, PTs and TTs. Further, sideways water coming into each storage tank can be measured. Incase the sideway water is not measured, the sideways water flow can be estimated using other measurements like inflows and outflows to the tank and the water level measurement.

Referring to Fig. 2, a mine dewatering operation management system (200) for mine dewatering operation is provided. The system comprises a control and scheduling component (201) for model tuning so as to obtain the model parameters for integrated process model. This model tuning involves optimization approach where the error between the plant data (process data collected in the mine through measurement using the mine dewatering operation management system) and outputs (estimates) from the plurality of process models (integrated process model) are minimized. The model outputs from the integrated process model includes water flows, water levels, air flows, air and water pressure, air temperature in the underground mine. The integrated process model (202) referred herein includes models of each of the components in the mine dewatering operation such as pumps, storage dams, pipes, refrigeration unit, ventilation fans, energy recovery device in suitable combination integrated therein. The data required in relation to mine dewatering operation is obtained from the user (203) and / or form the plant database (204). The plant data include pump flows, head developed by the pumps, water levels in the storage tanks, air flow rate from ventilation fans and temperature inside the mine. These data are acquired from the plant by a Distributed Control System (205), and is transferred to system (200) via OPC protocol or any other such means. The data thus obtained is used to calculate energy consumption with the help of model equations, and is used in the optimization as described below. In this example, though a specific control system architecture is illustrated, the mining operation management system can be illustrated with equivalent other control system architectures using advanced control system supporting model based control and optimization.

The model of each of the components involved in mine dewatering operation given below pertains to one of the several other forms available, and is not limited to the below referred model in respect of each of the component:

Pump

Head developed by 'n' number of variable speed pumps running in parallel is

Where, a, b and c are constants of pump characteristic equations;

Q is Flow; speed of the pumps

The energy consumption of the pumps in a pump station (E^ _f . ) is given

Pipe

The flow rate across the pipe can be given as

Where, A _s is Pipe cross sectional area;

Length of the pipe;

& H is Static head difference; c is Pipe coefficient; D is Pipe inner diameter Storage Dam

The change of head in a reservoir is

¾c - Q _o (0 + d] (4)

Where, A is Area of the reservoir, Qi is inflow to the tank; Qo is outflow from the tank; d is the additional sideways water into tank from the mine; ft (f) is head in the reservoir Refrigeration Unit

The model for refrigeration units will include the amount of energy required to cool the water to the temperature required in the underground mines

Where, Q. _rei -,. is the refrigerated water flow rate that needs to be re-circulated to cool the underground mines; ΔΤ is the temperature difference of the recycled water between inlet and outlet of the refrigeration system. CL^ is a constant that depends on heat transfer coefficient.

Energy Recovery Device

The model for energy recovery devices includes amount of energy recovered from recycled refrigerated water to the mines and depth at which energy recovery unit is installed in the mine. One such model equation is given below. (6)

Where Q _ef is the re-circulation water flow rate and _srd is the depth at which ERD is placed in the underground mine. ¾, ¾ and I½ are parameters of the model.

Ventilation Fan

The model for energy consumption by fans to provide air circulation and cooling of the environment is a function of air flow rate and speed of the fan. One such model equation is given below.

E _fM = ¾ + Qfan + Cz Q an + d _z N _f + <¾ i¥ _f ² ; (7)

Where, Q _faa is the air flow rate and N _t is speed of the fans. _f d. _3s s ₃ are parameters of the model.

Further, mine temperature (air temperature of the underground mine) is calculated from Q _faa (eq. 7) and Q _rsf (eq. 6) as given below.

Where s ₃ , ki and k ₂ are parameters of the model.

The integrated process model (202) for mine dewatering operation include the model of all the above referred components like pumps, pipes, storage dams, refrigeration unit, energy recovery device and ventilation unit. The integrated process model (202) is a differential algebraic equation representing the entire mine dewatering operation and its relevant system thereof.

The integrated process model (202) is thereby used for developing optimization solution for optimal operation of mines with dewatering and ventilation systems, as described herein after.

Optimal operation involves formulating the optimization problem, which finds (i)speed of the pumps for optimal operation in relation to the mine dewatering operation, (ii) speed of the fans in ventilation system, and (iii) percentage of load between dewatering and ventilation systems with the objective of minimizing the total energy consumption. The total energy consumption is calculated by summing of (i) energy consumption of each pump in each pumping station, (ii) energy consumption by refrigeration units (in case of water recycle system) (iii) energy extracted from energy recovery devices (iv) energy consumption by fans (in case of air circulation and cooling).

Objective

Minimize Total Power

I Nts berof

Where TolalPowertKWh) = fan.}

- (9)

Where speed of the pumps (N _P ), speed of the fans (Nf), re-circulated water flow( Q _r÷i -) ,and air flow rate (Qf _3K .) are the decision variables of the optimization. Q _ref - and Q _faa variables capture the interaction between refrigeration and ventilation units, and also decide the load that is to be shared between them for cooling purposes to maintain ambient temperature (T) in the mine within limits.

Subject to the following constraints

LB < Water level in the reserv&irst < U8; i = 1, 2, Number a f LB < Flow of the ptmtpse < US;, i = 1, 2, , Number of pumps LB < Head of the pumps, < UBi ϊ = 1, 2, Number of um s LB < Speed of the ριιπιρε _έ < UB-, i— I, 2, N mber of pumps LB < Speed of the fans < UB; j= 1 _» 2,, ... , Number of ferns L3 < Temperature in tks ine (T} _; < UB- _f i = I, 2 _t „, ,Mine levels

The objective function (eq. 9) is valid when the power tariff is constant. In situations where power tariff varies over a period of time, the objective function can be re-formulated as total power cost as given below subjected to the above same constraints.

Objective

Minimize∑ _{= n} Power cost (t) (10)

Where Power cost (t) in $ = TotatlPower(£) * Power tariff (t) t is the time horizon over which load is distributed optimally.

The optimization problems (eq. 9 or 10) can be solved as a traditional non-linear programming problem or model predictive control formulation.

The optimal set points for control of various devices so determined using eq. 9 or 10 are sent to mine dewatering plant (207) through the Distributed Control System (DCS) (205) and the controllers (206) such as PID controllers. The effect of these optimal set points in the plant can be observed by change of water and air flows, levels in the tanks, head developed at different pump stations and also temperature of the underground mine.

In current practice, dewatering and ventilation systems are operated independently for which the energy consumption is calculated as given in eq. 9 where Q... _ef and Q _fa are constant values.

This conventional way of operation does not consider interaction between dewatering and ventilation systems. These interactions include the effect of water re-circulation flow rate, and air flow rate on mine temperature (eq. 8). Further, these interactions change with time due to following reasons.

(i) Change in efficiency of dewatering system due to

a. Variable amount of water to be pumped because of the additional flow from sideways of the mine b. Change in performance of pumps, energy recovery device and refrigeration system due to aging

(ii) Change in efficiency of ventilation system due to

a. Change in performance of fans with time due to aging

b. Change in mine working conditions due to change in number of working people, mine equipment and weather conditions

(iii) Change in variable power tariff

The above dynamics in the interactions makes the problem more challenging and this invention tries to address the dynamics in interactions between dewatering and ventilation systems in order to minimize the overall energy consumption. The energy consumption calculation in proposed invention is given in eq. 9 or 10 where Q _g ^ and <?,-..,, are calculated as part of optimization problem.

The invention therefore provides a method and a system for mine dewatering operation that can be effected optimally duly considering the various components involved in the operation and thereby minimizing the energy consumption so as to improve cost effectiveness in the mine dewatering operation. In summary, the method comprises steps of obtaining plant data in relation to mine dewatering and ventilation operation for model based control and optimization. A plurality of process models of plant units and devices are used in the mine dewatering and ventilation operation. An objective function is formulated for minimization of energy consumption in the mine dewatering and ventilation operation using the tuned plurality of process models and solved for determining set points for optimized mine dewatering and ventilation operation. The mining plant units and devices are controlled with DCS controllers and other controllers around the set points for minimized energy consumption in the mine dewatering and ventilation operation.

An example demonstrating some aspects of the proposed invention is presented below for better understanding. In the example description, the term normal operation refers to conventional way of operation without the use of proposed invention whereas the term optimal operation refers to operation with use of the invention. Further, the values used in the example are for illustration purpose.

Example:

Referring back to Fig. 1 , the underground mine has dewatering system that consists of 3 pump stations (PI, P2 and P3) each with four pumps connected in parallel. These pump stations are located at different levels of the mine with a total depth of ~lkm. The total power consumption of pumping system is considered as 1500KW (using eq. 2) with each pump station consuming around 500KW. The total amount of water that is removed from the mine is assumed as 350 m3 h, out of which around 70% comes from the lowest level of the mine while 30% of the flow comes from sideways of the mine. The example considered consists of recycling part (-20% of the total flow) of mine water pumped to the surface for cooling purposes. The recycled water is cooled using refrigeration unit at surface which consumes around 350KW (using eq. 5). This cooled water is then passed through energy recovery unit that is installed in the underground mine in order to recover potential energy before using it for cooling air in the mine. The recovered potential energy is - 150KW (eq. 6). As shown in Fig. 3, the energy consumption of pumping system, refrigeration system and energy extracted from the energy recovery unit are calculated by the dewatering model (302) which takes the inputs that include speeds of pumps in a pump station, sideways water that comes into the mine, depth of the mine and amount of pumped water that is re-cycled for cooling purposes. The integrated model (301) shown in Fig. 3 is equivalent to 201 in Fig. 2. As given in Fig. 1 , ventilation system that consists of multiple fans is used to provide air circulation and also to maintain ambient temperature in the underground mines. In this case, the total energy consumed by the fans of ventilation system is considered as 3000KW (using eq. 7; in general it will be minimum of 2 to 3 times the energy consumption of dewatering system). As shown in Fig. 3, in the proposed invention, the energy consumption of ventilation system is calculated by the ventilation model (303) which takes the inputs that include fan speeds, air flow required, depth of the mine, number of people and equipment involved in the mine. In some mine configurations such as the one described in this example, both re-circulated water and air flow rate are used to calculate temperature of the air in the underground mines which is to be controlled to maintain ambient working conditions.

Fig. 4 shows the energy consumption profiles of dewatering and ventilation systems under normal and optimal operating scenarios. S _sw profile shows the energy consumption of dewatering system (includes energy consumed by pumping system and refrigeration unit, energy produced by energy recovery unit) under normal operations which is between 1600 to 1800 KW. By optimally operating the pump system this energy consumption can be reduced by -5% which is represented by ¾ _ffM , _ffi t _,asji in Fig.4. This optimization involves manipulation of pump speeds to minimize energy consumption while handling additional sideways water coming into the tanks. In addition, one can also optimally operate refrigeration and energy recovery units in order to reduce the energy consumption further. Similarly, energy consumption profile of ventilation system for normal operation (E _vg ) is in the range of 2850 KW to 3150 KW which can be reduced further by 5% using optimal operation of fan speeds (given as _ε in Fig. 4).

^is rsi ^an d £tasai _, s _t st profiles represent the total energy consumption of dewatering and ventilation system under normal and optimal scenarios. The energy savings obtained in optimal operations (¾o«.e.i∞s ) i ^{s not om} y due to the optimal operation of dewatering and ventilation systems but also optimizing the airflow rate (Q _fsn ) and re-circulated water flow rate (Q^ _sf ) to maintain the ambient working temperature (eq. 8) in the mine. For example, the energy required to cool a given quantity of air using refrigeration and ventilation system shall be varying over a period of time due to change in efficiencies because of various factors which include change in performance and availability of the equipment etc. The proposed solution estimates the load sharing between refrigeration (Q _{rs f} ) and ventilation (Q _an ) system considering this dynamic changes in order to reduce overall energy consumption assumption further as given by ^wfei. _8TO(® s«i in Fig.4. These energy savings (E _t4-fi!i-e .^) will be further reduced for the situation

(eq. 10) where the power tariff is varying over a period of time. In the proposed invention, the optimal set points (pump speeds, fan speeds, re -circulated water flow and air flow) thus calculated using 201 in Fig. 3 are sent to the plant (207) through DCS (205) and low level controllers (206).

Only certain features of the invention have been specifically illustrated and described herein, and many modifications and changes will occur to those skilled in the art. The invention is not restricted by the preferred embodiment described herein in the description. It is to be noted that the invention is explained by way of exemplary embodiment and is neither exhaustive nor limiting. Certain aspects of the invention that not been elaborated herein in the description are well understood by one skilled in the art. Also, the terms relating to singular form used herein in the description also include its plurality and vice versa, wherever applicable. Any relevant modification or variation, which is not described specifically in the specification are in fact to be construed of being well within the scope of the invention. The appended claims are intended to cover all such modifications and changes which fall within the spirit of the invention.

Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.