Data Archive Script Is Taking Too Long To Delete A Large Table

We have data archive scripts, these scripts move data for a date range to a different table. so the script has two parts first copy data from original table to archive table; and second delete copied rows from the original table. The first part is executing very fast but the deletion is taking too long i.e. around 2-3 hours. The customer analysed the delete query and are saying the script is not using index and is going into full table scan. but the predicate itself is the primary key,More info below

CREATE TABLE "APP"."MON_TXNS"    (    "ID_TXN" NUMBER(12,0) NOT NULL ENABLE,     "BOL_IS_CANCELLED" VARCHAR2(1 BYTE) DEFAULT 'N' NOT NULL ENABLE,     "ID_PAYER" NUMBER(12,0),     "ID_PAYER_PI" NUMBER(12,0),     "ID_PAYEE" NUMBER(12,0),     "ID_PAYEE_PI" NUMBER(12,0),     "ID_CURRENCY" CHAR(3 BYTE) NOT NULL ENABLE,     "STR_TEXT" VARCHAR2(60 CHAR),     "DAT_MERCHANT_TIMESTAMP" DATE,     "STR_MERCHANT_ORDER_ID" VARCHAR2(30 BYTE),     "DAT_EXPIRATION" DATE,     "DAT_CREATION" DATE,     "STR_USER_CREATION" VARCHAR2(30 CHAR),     "DAT_LAST_UPDATE"

[Code]...

 Data is first moved to table in schema3.OTW. and then we are deleting all the rows in otw from original table. below is the explain plan for delete  

SQL> explain plan for  2  delete from schema1.mon_txns where id_txn in (select id_txn from schema3.OTW); 

Explained. SQL> select * from table(dbms_xplan.display); 

PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------

Plan hash value: 2798378986
 -------------------------------------------------------------------------------------
| Id  | Operation              | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------|   0 | DELETE STATEMENT       |            |  2520 |   233K|    87   (2)| 00:00:02 ||   1 |  DELETE                | MON_TXNS   |       |       |            |          ||*  2 |   HASH JOIN RIGHT SEMI |            |  2520 |   233K|    87   (2)| 00:00:02 ||   3 |    INDEX FAST FULL SCAN| OTW_ID_TXN |  2520 | 15120 |     3   (0)| 00:00:01 ||   4 |    TABLE ACCESS FULL   | MON_TXNS   | 14260 |  1239K|    83   (0)| 00:00:02 |

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 PLAN_TABLE_OUTPUT
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Predicate Information (identified by operation id):
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